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NAME
ets - Built-in term storage.
DESCRIPTION
This module is an interface to the Erlang built-in term storage BIFs. These provide the ability to store
very large quantities of data in an Erlang runtime system, and to have constant access time to the data.
(In the case of ordered_set, see below, access time is proportional to the logarithm of the number of
stored objects.)
Data is organized as a set of dynamic tables, which can store tuples. Each table is created by a process.
When the process terminates, the table is automatically destroyed. Every table has access rights set at
creation.
Tables are divided into four different types, set, ordered_set, bag, and duplicate_bag. A set or
ordered_set table can only have one object associated with each key. A bag or duplicate_bag table can
have many objects associated with each key.
Note:
The number of tables stored at one Erlang node used to be limited. This is no longer the case (except by
memory usage). The previous default limit was about 1400 tables and could be increased by setting the
environment variable ERL_MAX_ETS_TABLES or the command line option +e before starting the Erlang runtime
system. This hard limit has been removed, but it is currently useful to set the ERL_MAX_ETS_TABLES
anyway. It should be set to an approximate of the maximum amount of tables used. This since an internal
table for named tables is sized using this value. If large amounts of named tables are used and
ERL_MAX_ETS_TABLES hasn't been increased, the performance of named table lookup will degrade.
Notice that there is no automatic garbage collection for tables. Even if there are no references to a
table from any process, it is not automatically destroyed unless the owner process terminates. To destroy
a table explicitly, use function delete/1. The default owner is the process that created the table. To
transfer table ownership at process termination, use option heir or call give_away/3.
Some implementation details:
* In the current implementation, every object insert and look-up operation results in a copy of the
object.
* '$end_of_table' is not to be used as a key, as this atom is used to mark the end of the table when
using functions first/1 and next/2.
Notice the subtle difference between matching and comparing equal, which is demonstrated by table types
set and ordered_set:
* Two Erlang terms match if they are of the same type and have the same value, so that 1 matches 1, but
not 1.0 (as 1.0 is a float() and not an integer()).
* Two Erlang terms compare equal if they either are of the same type and value, or if both are numeric
types and extend to the same value, so that 1 compares equal to both 1 and 1.0.
* The ordered_set works on the Erlang term order and no defined order exists between an integer() and a
float() that extends to the same value. Hence the key 1 and the key 1.0 are regarded as equal in an
ordered_set table.
FAILURE
The functions in this module exits with reason badarg if any argument has the wrong format, if the table
identifier is invalid, or if the operation is denied because of table access rights (protected or
private).
CONCURRENCY
This module provides some limited support for concurrent access. All updates to single objects are
guaranteed to be both atomic and isolated. This means that an updating operation to a single object
either succeeds or fails completely without any effect (atomicity) and that no intermediate results of
the update can be seen by other processes (isolation). Some functions that update many objects state that
they even guarantee atomicity and isolation for the entire operation. In database terms the isolation
level can be seen as "serializable", as if all isolated operations are carried out serially, one after
the other in a strict order.
TABLE TRAVERSAL
There are different ways to traverse through the objects of a table.
* Single-step traversal one key at at time, using first/1, next/2, last/1 and prev/2.
* Search with simple match patterns, using match/1/2/3, match_delete/2 and match_object/1/2/3.
* Search with more powerful match specifications, using select/1/2/3, select_count/2, select_delete/2,
select_replace/2 and select_reverse/1/2/3.
* Table conversions, using tab2file/2/3 and tab2list/1.
None of these ways of table traversal will guarantee a consistent table snapshot if the table is also
updated during the traversal. Moreover, traversals not done in a safe way, on tables where keys are
inserted or deleted during the traversal, may yield the following undesired effects:
* Any key may be missed.
* Any key may be found more than once.
* The traversal may fail with badarg exception if keys are deleted.
A table traversal is safe if either
* the table is of type ordered_set.
* the entire table traversal is done within one ETS function call.
* function safe_fixtable/2 is used to keep the table fixated during the entire traversal.
Note:
Even though the access of a single object is always guaranteed to be atomic and isolated, each traversal
through a table to find the next key is not done with such guarantees. This is often not a problem, but
may cause rare subtle "unexpected" effects if a concurrent process inserts objects during a traversal.
For example, consider one process doing
ets:new(t, [ordered_set, named_table]),
ets:insert(t, {1}),
ets:insert(t, {2}),
ets:insert(t, {3}),
A concurrent call to ets:first(t), done by another process, may then in rare cases return 2 even though 2
has never existed in the table ordered as the first key. In the same way, a concurrent call to
ets:next(t, 1) may return 3 even though 3 never existed in the table ordered directly after 1.
Effects like this are improbable but possible. The probability will further be reduced (if not vanish) if
table option write_concurrency is not enabled. This can also only be a potential concern for ordered_set
where the traversal order is defined.
Traversals using match and select functions may not need to scan the entire table depending on how the
key is specified. A match pattern with a fully bound key (without any match variables) will optimize the
operation to a single key lookup without any table traversal at all. For ordered_set a partially bound
key will limit the traversal to only scan a subset of the table based on term order. A partially bound
key is either a list or a tuple with a prefix that is fully bound. Example:
1> T = ets:new(t,[ordered_set]), ets:insert(T, {"555-1234", "John Smith"}).
true
2> %% Efficient search of all with area code 555
2> ets:match(T,{[$5,$5,$5,$- |'$1'],'$2'}).
[["1234","John Smith"]]
MATCH SPECIFICATIONS
Some of the functions use a match specification, match_spec. For a brief explanation, see select/2. For a
detailed description, see section Match Specifications in Erlang in ERTS User's Guide.
DATA TYPES
access() = public | protected | private
continuation()
Opaque continuation used by select/1,3, select_reverse/1,3, match/1,3, and match_object/1,3.
match_spec() = [{match_pattern(), [term()], [term()]}]
A match specification, see above.
comp_match_spec()
A compiled match specification.
match_pattern() = atom() | tuple()
tab() = atom() | tid()
tid()
A table identifier, as returned by new/2.
type() = set | ordered_set | bag | duplicate_bag
EXPORTS
all() -> [Tab]
Types:
Tab = tab()
Returns a list of all tables at the node. Named tables are specified by their names, unnamed
tables are specified by their table identifiers.
There is no guarantee of consistency in the returned list. Tables created or deleted by other
processes "during" the ets:all() call either are or are not included in the list. Only tables
created/deleted before ets:all() is called are guaranteed to be included/excluded.
delete(Tab) -> true
Types:
Tab = tab()
Deletes the entire table Tab.
delete(Tab, Key) -> true
Types:
Tab = tab()
Key = term()
Deletes all objects with key Key from table Tab.
delete_all_objects(Tab) -> true
Types:
Tab = tab()
Delete all objects in the ETS table Tab. The operation is guaranteed to be atomic and isolated.
delete_object(Tab, Object) -> true
Types:
Tab = tab()
Object = tuple()
Delete the exact object Object from the ETS table, leaving objects with the same key but other
differences (useful for type bag). In a duplicate_bag table, all instances of the object are
deleted.
file2tab(Filename) -> {ok, Tab} | {error, Reason}
Types:
Filename = file:name()
Tab = tab()
Reason = term()
Reads a file produced by tab2file/2 or tab2file/3 and creates the corresponding table Tab.
Equivalent to file2tab(Filename, []).
file2tab(Filename, Options) -> {ok, Tab} | {error, Reason}
Types:
Filename = file:name()
Tab = tab()
Options = [Option]
Option = {verify, boolean()}
Reason = term()
Reads a file produced by tab2file/2 or tab2file/3 and creates the corresponding table Tab.
The only supported option is {verify,boolean()}. If verification is turned on (by specifying
{verify,true}), the function uses whatever information is present in the file to assert that the
information is not damaged. How this is done depends on which extended_info was written using
tab2file/3.
If no extended_info is present in the file and {verify,true} is specified, the number of objects
written is compared to the size of the original table when the dump was started. This can make
verification fail if the table was public and objects were added or removed while the table was
dumped to file. To avoid this problem, either do not verify files dumped while updated
simultaneously or use option {extended_info, [object_count]} to tab2file/3, which extends the
information in the file with the number of objects written.
If verification is turned on and the file was written with option {extended_info, [md5sum]},
reading the file is slower and consumes radically more CPU time than otherwise.
{verify,false} is the default.
first(Tab) -> Key | '$end_of_table'
Types:
Tab = tab()
Key = term()
Returns the first key Key in table Tab. For an ordered_set table, the first key in Erlang term
order is returned. For other table types, the first key according to the internal order of the
table is returned. If the table is empty, '$end_of_table' is returned.
To find subsequent keys in the table, use next/2.
foldl(Function, Acc0, Tab) -> Acc1
Types:
Function = fun((Element :: term(), AccIn) -> AccOut)
Tab = tab()
Acc0 = Acc1 = AccIn = AccOut = term()
Acc0 is returned if the table is empty. This function is similar to lists:foldl/3. The table
elements are traversed in an unspecified order, except for ordered_set tables, where they are
traversed first to last.
If Function inserts objects into the table, or another process inserts objects into the table,
those objects can (depending on key ordering) be included in the traversal.
foldr(Function, Acc0, Tab) -> Acc1
Types:
Function = fun((Element :: term(), AccIn) -> AccOut)
Tab = tab()
Acc0 = Acc1 = AccIn = AccOut = term()
Acc0 is returned if the table is empty. This function is similar to lists:foldr/3. The table
elements are traversed in an unspecified order, except for ordered_set tables, where they are
traversed last to first.
If Function inserts objects into the table, or another process inserts objects into the table,
those objects can (depending on key ordering) be included in the traversal.
from_dets(Tab, DetsTab) -> true
Types:
Tab = tab()
DetsTab = dets:tab_name()
Fills an already created ETS table with the objects in the already opened Dets table DetsTab.
Existing objects in the ETS table are kept unless overwritten.
If any of the tables does not exist or the Dets table is not open, a badarg exception is raised.
fun2ms(LiteralFun) -> MatchSpec
Types:
LiteralFun = function()
MatchSpec = match_spec()
Pseudo function that by a parse_transform translates LiteralFun typed as parameter in the function
call to a match specification. With "literal" is meant that the fun must textually be written as
the parameter of the function, it cannot be held in a variable that in turn is passed to the
function.
The parse transform is provided in the ms_transform module and the source must include file
ms_transform.hrl in STDLIB for this pseudo function to work. Failing to include the hrl file in
the source results in a runtime error, not a compile time error. The include file is easiest
included by adding line -include_lib("stdlib/include/ms_transform.hrl"). to the source file.
The fun is very restricted, it can take only a single parameter (the object to match): a sole
variable or a tuple. It must use the is_ guard tests. Language constructs that have no
representation in a match specification (if, case, receive, and so on) are not allowed.
The return value is the resulting match specification.
Example:
1> ets:fun2ms(fun({M,N}) when N > 3 -> M end).
[{{'$1','$2'},[{'>','$2',3}],['$1']}]
Variables from the environment can be imported, so that the following works:
2> X=3.
3
3> ets:fun2ms(fun({M,N}) when N > X -> M end).
[{{'$1','$2'},[{'>','$2',{const,3}}],['$1']}]
The imported variables are replaced by match specification const expressions, which is consistent
with the static scoping for Erlang funs. However, local or global function calls cannot be in the
guard or body of the fun. Calls to built-in match specification functions is of course allowed:
4> ets:fun2ms(fun({M,N}) when N > X, my_fun(M) -> M end).
Error: fun containing local Erlang function calls
('my_fun' called in guard) cannot be translated into match_spec
{error,transform_error}
5> ets:fun2ms(fun({M,N}) when N > X, is_atom(M) -> M end).
[{{'$1','$2'},[{'>','$2',{const,3}},{is_atom,'$1'}],['$1']}]
As shown by the example, the function can be called from the shell also. The fun must be literally
in the call when used from the shell as well.
Warning:
If the parse_transform is not applied to a module that calls this pseudo function, the call fails
in runtime (with a badarg). The ets module exports a function with this name, but it is never to
be called except when using the function in the shell. If the parse_transform is properly applied
by including header file ms_transform.hrl, compiled code never calls the function, but the
function call is replaced by a literal match specification.
For more information, see ms_transform(3erl).
give_away(Tab, Pid, GiftData) -> true
Types:
Tab = tab()
Pid = pid()
GiftData = term()
Make process Pid the new owner of table Tab. If successful, message {'ETS-
TRANSFER',Tab,FromPid,GiftData} is sent to the new owner.
The process Pid must be alive, local, and not already the owner of the table. The calling process
must be the table owner.
Notice that this function does not affect option heir of the table. A table owner can, for
example, set heir to itself, give the table away, and then get it back if the receiver terminates.
i() -> ok
Displays information about all ETS tables on a terminal.
i(Tab) -> ok
Types:
Tab = tab()
Browses table Tab on a terminal.
info(Tab) -> InfoList | undefined
Types:
Tab = tab()
InfoList = [InfoTuple]
InfoTuple =
{compressed, boolean()} |
{heir, pid() | none} |
{id, tid()} |
{keypos, integer() >= 1} |
{memory, integer() >= 0} |
{name, atom()} |
{named_table, boolean()} |
{node, node()} |
{owner, pid()} |
{protection, access()} |
{size, integer() >= 0} |
{type, type()} |
{write_concurrency, boolean()} |
{read_concurrency, boolean()}
Returns information about table Tab as a list of tuples. If Tab has the correct type for a table
identifier, but does not refer to an existing ETS table, undefined is returned. If Tab is not of
the correct type, a badarg exception is raised.
{compressed, boolean()}:
Indicates if the table is compressed.
{heir, pid() | none}:
The pid of the heir of the table, or none if no heir is set.
{id,tid()}:
The table identifier.
{keypos, integer() >= 1}:
The key position.
{memory, integer() >= 0:
The number of words allocated to the table.
{name, atom()}:
The table name.
{named_table, boolean()}:
Indicates if the table is named.
{node, node()}:
The node where the table is stored. This field is no longer meaningful, as tables cannot be
accessed from other nodes.
{owner, pid()}:
The pid of the owner of the table.
{protection,access()}:
The table access rights.
{size, integer() >= 0:
The number of objects inserted in the table.
{type,type()}:
The table type.
{read_concurrency, boolean()}:
Indicates whether the table uses read_concurrency or not.
{write_concurrency, boolean()}:
Indicates whether the table uses write_concurrency.
info(Tab, Item) -> Value | undefined
Types:
Tab = tab()
Item =
binary | compressed | fixed | heir | id | keypos | memory |
name | named_table | node | owner | protection | safe_fixed |
safe_fixed_monotonic_time | size | stats | type |
write_concurrency | read_concurrency
Value = term()
Returns the information associated with Item for table Tab, or returns undefined if Tab does not
refer an existing ETS table. If Tab is not of the correct type, or if Item is not one of the
allowed values, a badarg exception is raised.
In addition to the {Item,Value} pairs defined for info/1, the following items are allowed:
* Item=binary, Value=BinInfo
BinInfo is a list containing miscellaneous information about binaries kept by the table. This
Item can be changed or removed without prior notice. In the current implementation BinInfo is
a list of tuples {BinaryId,BinarySize,BinaryRefcCount}.
* Item=fixed, Value=boolean()
Indicates if the table is fixed by any process.
*
Item=safe_fixed|safe_fixed_monotonic_time, Value={FixationTime,Info}|false
If the table is fixed using safe_fixtable/2, the call returns a tuple where FixationTime is
the last time when the table changed from unfixed to fixed.
The format and value of FixationTime depends on Item:
safe_fixed:
FixationTime corresponds to the result returned by erlang:timestamp/0 at the time of
fixation. Notice that when the system uses single or multi time warp modes this can produce
strange results, as the use of safe_fixed is not time warp safe. Time warp safe code must
use safe_fixed_monotonic_time instead.
safe_fixed_monotonic_time:
FixationTime corresponds to the result returned by erlang:monotonic_time/0 at the time of
fixation. The use of safe_fixed_monotonic_time is time warp safe.
Info is a possibly empty lists of tuples {Pid,RefCount}, one tuple for every process the table
is fixed by now. RefCount is the value of the reference counter and it keeps track of how many
times the table has been fixed by the process.
Table fixations are not limited to safe_fixtable/2. Temporary fixations may also be done by
for example traversing functions like select and match. Such table fixations are automatically
released before the corresponding functions returns, but they may be seen by a concurrent call
to ets:info(T,safe_fixed|safe_fixed_monotonic_time).
If the table is not fixed at all, the call returns false.
* Item=stats, Value=tuple()
Returns internal statistics about tables on an internal format used by OTP test suites. Not
for production use.
init_table(Tab, InitFun) -> true
Types:
Tab = tab()
InitFun = fun((Arg) -> Res)
Arg = read | close
Res = end_of_input | {Objects :: [term()], InitFun} | term()
Replaces the existing objects of table Tab with objects created by calling the input function
InitFun, see below. This function is provided for compatibility with the dets module, it is not
more efficient than filling a table by using insert/2.
When called with argument read, the function InitFun is assumed to return end_of_input when there
is no more input, or {Objects, Fun}, where Objects is a list of objects and Fun is a new input
function. Any other value Value is returned as an error {error, {init_fun, Value}}. Each input
function is called exactly once, and if an error occur, the last function is called with argument
close, the reply of which is ignored.
If the table type is set and more than one object exists with a given key, one of the objects is
chosen. This is not necessarily the last object with the given key in the sequence of objects
returned by the input functions. This holds also for duplicated objects stored in tables of type
bag.
insert(Tab, ObjectOrObjects) -> true
Types:
Tab = tab()
ObjectOrObjects = tuple() | [tuple()]
Inserts the object or all of the objects in list ObjectOrObjects into table Tab.
* If the table type is set and the key of the inserted objects matches the key of any object in
the table, the old object is replaced.
* If the table type is ordered_set and the key of the inserted object compares equal to the key
of any object in the table, the old object is replaced.
* If the list contains more than one object with matching keys and the table type is set, one is
inserted, which one is not defined. The same holds for table type ordered_set if the keys
compare equal.
The entire operation is guaranteed to be atomic and isolated, even when a list of objects is
inserted.
insert_new(Tab, ObjectOrObjects) -> boolean()
Types:
Tab = tab()
ObjectOrObjects = tuple() | [tuple()]
Same as insert/2 except that instead of overwriting objects with the same key (for set or
ordered_set) or adding more objects with keys already existing in the table (for bag and
duplicate_bag), false is returned.
If ObjectOrObjects is a list, the function checks every key before inserting anything. Nothing is
inserted unless all keys present in the list are absent from the table. Like insert/2, the entire
operation is guaranteed to be atomic and isolated.
is_compiled_ms(Term) -> boolean()
Types:
Term = term()
Checks if a term represent a valid compiled match specification. A compiled match specifications
is only valid on the Erlang node where it was compiled by calling match_spec_compile/1.
Note:
Before STDLIB 3.4 (OTP 20.0) compiled match specifications did not have an external
representation. If passed through binary_to_term(term_to_binary(CMS)) or sent to another node and
back, the result was always an empty binary <<>>.
After STDLIB 3.4 (OTP 20.0) compiled match specifications have an external representation as a
node specific reference to the original compiled match specification. If passed through
binary_to_term(term_to_binary(CMS)) or sent to another node and back, the result may or may not be
a valid compiled match specification depending on if the original compiled match specification was
still alive.
last(Tab) -> Key | '$end_of_table'
Types:
Tab = tab()
Key = term()
Returns the last key Key according to Erlang term order in table Tab of type ordered_set. For
other table types, the function is synonymous to first/1. If the table is empty, '$end_of_table'
is returned.
To find preceding keys in the table, use prev/2.
lookup(Tab, Key) -> [Object]
Types:
Tab = tab()
Key = term()
Object = tuple()
Returns a list of all objects with key Key in table Tab.
* For tables of type set, bag, or duplicate_bag, an object is returned only if the specified key
matches the key of the object in the table.
* For tables of type ordered_set, an object is returned if the specified key compares equal to
the key of an object in the table.
The difference is the same as between =:= and ==.
As an example, one can insert an object with integer() 1 as a key in an ordered_set and get the
object returned as a result of doing a lookup/2 with float() 1.0 as the key to search for.
For tables of type set or ordered_set, the function returns either the empty list or a list with
one element, as there cannot be more than one object with the same key. For tables of type bag or
duplicate_bag, the function returns a list of arbitrary length.
Notice that the time order of object insertions is preserved; the first object inserted with the
specified key is the first in the resulting list, and so on.
Insert and lookup times in tables of type set, bag, and duplicate_bag are constant, regardless of
the table size. For the ordered_set datatype, time is proportional to the (binary) logarithm of
the number of objects.
lookup_element(Tab, Key, Pos) -> Elem
Types:
Tab = tab()
Key = term()
Pos = integer() >= 1
Elem = term() | [term()]
For a table Tab of type set or ordered_set, the function returns the Pos:th element of the object
with key Key.
For tables of type bag or duplicate_bag, the functions returns a list with the Pos:th element of
every object with key Key.
If no object with key Key exists, the function exits with reason badarg.
The difference between set, bag, and duplicate_bag on one hand, and ordered_set on the other,
regarding the fact that ordered_set view keys as equal when they compare equal whereas the other
table types regard them equal only when they match, holds for lookup_element/3.
match(Continuation) -> {[Match], Continuation} | '$end_of_table'
Types:
Match = [term()]
Continuation = continuation()
Continues a match started with match/3. The next chunk of the size specified in the initial
match/3 call is returned together with a new Continuation, which can be used in subsequent calls
to this function.
When there are no more objects in the table, '$end_of_table' is returned.
match(Tab, Pattern) -> [Match]
Types:
Tab = tab()
Pattern = match_pattern()
Match = [term()]
Matches the objects in table Tab against pattern Pattern.
A pattern is a term that can contain:
* Bound parts (Erlang terms)
* '_' that matches any Erlang term
* Pattern variables '$N', where N=0,1,...
The function returns a list with one element for each matching object, where each element is an
ordered list of pattern variable bindings, for example:
6> ets:match(T, '$1'). % Matches every object in table
[[{rufsen,dog,7}],[{brunte,horse,5}],[{ludde,dog,5}]]
7> ets:match(T, {'_',dog,'$1'}).
[[7],[5]]
8> ets:match(T, {'_',cow,'$1'}).
[]
If the key is specified in the pattern, the match is very efficient. If the key is not specified,
that is, if it is a variable or an underscore, the entire table must be searched. The search time
can be substantial if the table is very large.
For tables of type ordered_set, the result is in the same order as in a first/next traversal.
match(Tab, Pattern, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
Pattern = match_pattern()
Limit = integer() >= 1
Match = [term()]
Continuation = continuation()
Works like match/2, but returns only a limited (Limit) number of matching objects. Term
Continuation can then be used in subsequent calls to match/1 to get the next chunk of matching
objects. This is a space-efficient way to work on objects in a table, which is faster than
traversing the table object by object using first/1 and next/2.
If the table is empty, '$end_of_table' is returned.
Use safe_fixtable/2 to guarantee safe traversal for subsequent calls to match/1.
match_delete(Tab, Pattern) -> true
Types:
Tab = tab()
Pattern = match_pattern()
Deletes all objects that match pattern Pattern from table Tab. For a description of patterns, see
match/2.
match_object(Continuation) ->
{[Object], Continuation} | '$end_of_table'
Types:
Object = tuple()
Continuation = continuation()
Continues a match started with match_object/3. The next chunk of the size specified in the initial
match_object/3 call is returned together with a new Continuation, which can be used in subsequent
calls to this function.
When there are no more objects in the table, '$end_of_table' is returned.
match_object(Tab, Pattern) -> [Object]
Types:
Tab = tab()
Pattern = match_pattern()
Object = tuple()
Matches the objects in table Tab against pattern Pattern. For a description of patterns, see
match/2. The function returns a list of all objects that match the pattern.
If the key is specified in the pattern, the match is very efficient. If the key is not specified,
that is, if it is a variable or an underscore, the entire table must be searched. The search time
can be substantial if the table is very large.
For tables of type ordered_set, the result is in the same order as in a first/next traversal.
match_object(Tab, Pattern, Limit) ->
{[Object], Continuation} | '$end_of_table'
Types:
Tab = tab()
Pattern = match_pattern()
Limit = integer() >= 1
Object = tuple()
Continuation = continuation()
Works like match_object/2, but only returns a limited (Limit) number of matching objects. Term
Continuation can then be used in subsequent calls to match_object/1 to get the next chunk of
matching objects. This is a space-efficient way to work on objects in a table, which is faster
than traversing the table object by object using first/1 and next/2.
If the table is empty, '$end_of_table' is returned.
Use safe_fixtable/2 to guarantee safe traversal for subsequent calls to match_object/1.
match_spec_compile(MatchSpec) -> CompiledMatchSpec
Types:
MatchSpec = match_spec()
CompiledMatchSpec = comp_match_spec()
Transforms a match specification into an internal representation that can be used in subsequent
calls to match_spec_run/2. The internal representation is opaque. To check the validity of a
compiled match specification, use is_compiled_ms/1.
If term MatchSpec cannot be compiled (does not represent a valid match specification), a badarg
exception is raised.
Note:
This function has limited use in normal code. It is used by the dets module to perform the
dets:select() operations.
match_spec_run(List, CompiledMatchSpec) -> list()
Types:
List = [term()]
CompiledMatchSpec = comp_match_spec()
Executes the matching specified in a compiled match specification on a list of terms. Term
CompiledMatchSpec is to be the result of a call to match_spec_compile/1 and is hence the internal
representation of the match specification one wants to use.
The matching is executed on each element in List and the function returns a list containing all
results. If an element in List does not match, nothing is returned for that element. The length of
the result list is therefore equal or less than the length of parameter List.
Example:
The following two calls give the same result (but certainly not the same execution time):
Table = ets:new...
MatchSpec = ...
% The following call...
ets:match_spec_run(ets:tab2list(Table),
ets:match_spec_compile(MatchSpec)),
% ...gives the same result as the more common (and more efficient)
ets:select(Table, MatchSpec),
Note:
This function has limited use in normal code. It is used by the dets module to perform the
dets:select() operations and by Mnesia during transactions.
member(Tab, Key) -> boolean()
Types:
Tab = tab()
Key = term()
Works like lookup/2, but does not return the objects. Returns true if one or more elements in the
table has key Key, otherwise false.
new(Name, Options) -> tid() | atom()
Types:
Name = atom()
Options = [Option]
Option =
Type | Access | named_table |
{keypos, Pos} |
{heir, Pid :: pid(), HeirData} |
{heir, none} |
Tweaks
Type = type()
Access = access()
Tweaks =
{write_concurrency, boolean()} |
{read_concurrency, boolean()} |
compressed
Pos = integer() >= 1
HeirData = term()
Creates a new table and returns a table identifier that can be used in subsequent operations. The
table identifier can be sent to other processes so that a table can be shared between different
processes within a node.
Parameter Options is a list of options that specifies table type, access rights, key position, and
whether the table is named. Default values are used for omitted options. This means that not
specifying any options ([]) is the same as specifying [set, protected, {keypos,1}, {heir,none},
{write_concurrency,false}, {read_concurrency,false}].
set:
The table is a set table: one key, one object, no order among objects. This is the default
table type.
ordered_set:
The table is a ordered_set table: one key, one object, ordered in Erlang term order, which is
the order implied by the < and > operators. Tables of this type have a somewhat different
behavior in some situations than tables of other types. Most notably, the ordered_set tables
regard keys as equal when they compare equal, not only when they match. This means that to an
ordered_set table, integer() 1 and float() 1.0 are regarded as equal. This also means that the
key used to lookup an element not necessarily matches the key in the returned elements, if
float()'s and integer()'s are mixed in keys of a table.
bag:
The table is a bag table, which can have many objects, but only one instance of each object,
per key.
duplicate_bag:
The table is a duplicate_bag table, which can have many objects, including multiple copies of
the same object, per key.
public:
Any process can read or write to the table.
protected:
The owner process can read and write to the table. Other processes can only read the table.
This is the default setting for the access rights.
private:
Only the owner process can read or write to the table.
named_table:
If this option is present, the table is registered under its Name which can then be used
instead of the table identifier in subsequent operations.
The function will also return the Name instead of the table identifier. To get the table
identifier of a named table, use whereis/1.
{keypos,Pos}:
Specifies which element in the stored tuples to use as key. By default, it is the first
element, that is, Pos=1. However, this is not always appropriate. In particular, we do not
want the first element to be the key if we want to store Erlang records in a table.
Notice that any tuple stored in the table must have at least Pos number of elements.
{heir,Pid,HeirData} | {heir,none}:
Set a process as heir. The heir inherits the table if the owner terminates. Message {'ETS-
TRANSFER',tid(),FromPid,HeirData} is sent to the heir when that occurs. The heir must be a
local process. Default heir is none, which destroys the table when the owner terminates.
{write_concurrency,boolean()}:
Performance tuning. Defaults to false, in which case an operation that mutates (writes to) the
table obtains exclusive access, blocking any concurrent access of the same table until
finished. If set to true, the table is optimized to concurrent write access. Different objects
of the same table can be mutated (and read) by concurrent processes. This is achieved to some
degree at the expense of memory consumption and the performance of sequential access and
concurrent reading.
Option write_concurrency can be combined with option read_concurrency. You typically want to
combine these when large concurrent read bursts and large concurrent write bursts are common;
for more information, see option read_concurrency.
Notice that this option does not change any guarantees about atomicity and isolation.
Functions that makes such promises over many objects (like insert/2) gain less (or nothing)
from this option.
The memory consumption inflicted by both write_concurrency and read_concurrency is a constant
overhead per table for set, bag and duplicate_bag. For ordered_set the memory overhead depends
on the number of inserted objects and the amount of actual detected concurrency in runtime.
The memory overhead can be especially large when both options are combined.
Note:
Prior to stdlib-3.7 (OTP-22.0) write_concurrency had no effect on ordered_set.
{read_concurrency,boolean()}:
Performance tuning. Defaults to false. When set to true, the table is optimized for concurrent
read operations. When this option is enabled on a runtime system with SMP support, read
operations become much cheaper; especially on systems with multiple physical processors.
However, switching between read and write operations becomes more expensive.
You typically want to enable this option when concurrent read operations are much more
frequent than write operations, or when concurrent reads and writes comes in large read and
write bursts (that is, many reads not interrupted by writes, and many writes not interrupted
by reads).
You typically do not want to enable this option when the common access pattern is a few read
operations interleaved with a few write operations repeatedly. In this case, you would get a
performance degradation by enabling this option.
Option read_concurrency can be combined with option write_concurrency. You typically want to
combine these when large concurrent read bursts and large concurrent write bursts are common.
compressed:
If this option is present, the table data is stored in a more compact format to consume less
memory. However, it will make table operations slower. Especially operations that need to
inspect entire objects, such as match and select, get much slower. The key element is not
compressed.
next(Tab, Key1) -> Key2 | '$end_of_table'
Types:
Tab = tab()
Key1 = Key2 = term()
Returns the next key Key2, following key Key1 in table Tab. For table type ordered_set, the next
key in Erlang term order is returned. For other table types, the next key according to the
internal order of the table is returned. If no next key exists, '$end_of_table' is returned.
To find the first key in the table, use first/1.
Unless a table of type set, bag, or duplicate_bag is fixated using safe_fixtable/2, a call to
next/2 will fail if Key1 no longer exists in the table. For table type ordered_set, the function
always returns the next key after Key1 in term order, regardless whether Key1 ever existed in the
table.
prev(Tab, Key1) -> Key2 | '$end_of_table'
Types:
Tab = tab()
Key1 = Key2 = term()
Returns the previous key Key2, preceding key Key1 according to Erlang term order in table Tab of
type ordered_set. For other table types, the function is synonymous to next/2. If no previous key
exists, '$end_of_table' is returned.
To find the last key in an ordered_set table, use last/1.
rename(Tab, Name) -> Name
Types:
Tab = tab()
Name = atom()
Renames the named table Tab to the new name Name. Afterwards, the old name cannot be used to
access the table. Renaming an unnamed table has no effect.
repair_continuation(Continuation, MatchSpec) -> Continuation
Types:
Continuation = continuation()
MatchSpec = match_spec()
Restores an opaque continuation returned by select/3 or select/1 if the continuation has passed
through external term format (been sent between nodes or stored on disk).
The reason for this function is that continuation terms contain compiled match specifications and
may therefore be invalidated if converted to external term format. Given that the original match
specification is kept intact, the continuation can be restored, meaning it can once again be used
in subsequent select/1 calls even though it has been stored on disk or on another node.
Examples:
The following sequence of calls may fail:
T=ets:new(x,[]),
...
MS = ets:fun2ms(fun({N,_}=A) when (N rem 10) =:= 0 -> A end),
{_,C} = ets:select(T, MS, 10),
MaybeBroken = binary_to_term(term_to_binary(C)),
ets:select(MaybeBroken).
The following sequence works, as the call to repair_continuation/2 reestablishes the MaybeBroken
continuation.
T=ets:new(x,[]),
...
MS = ets:fun2ms(fun({N,_}=A) when (N rem 10) =:= 0 -> A end),
{_,C} = ets:select(T,MS,10),
MaybeBroken = binary_to_term(term_to_binary(C)),
ets:select(ets:repair_continuation(MaybeBroken,MS)).
Note:
This function is rarely needed in application code. It is used by Mnesia to provide distributed
select/3 and select/1 sequences. A normal application would either use Mnesia or keep the
continuation from being converted to external format.
The actual behavior of compiled match specifications when recreated from external format has
changed and may change in future releases, but this interface remains for backward compatibility.
See is_compiled_ms/1.
safe_fixtable(Tab, Fix) -> true
Types:
Tab = tab()
Fix = boolean()
Fixes a table of type set, bag, or duplicate_bag for safe traversal using first/1 & next/2,
match/3 & match/1, match_object/3 & match_object/1, or select/3 & select/1.
A process fixes a table by calling safe_fixtable(Tab, true). The table remains fixed until the
process releases it by calling safe_fixtable(Tab, false), or until the process terminates.
If many processes fix a table, the table remains fixed until all processes have released it (or
terminated). A reference counter is kept on a per process basis, and N consecutive fixes requires
N releases to release the table.
When a table is fixed, a sequence of first/1 and next/2 calls are guaranteed to succeed even if
keys are removed during the traversal. The keys for objects inserted or deleted during a traversal
may or may not be returned by next/2 depending on the ordering of keys within the table and if the
key exists at the time next/2 is called.
Example:
clean_all_with_value(Tab,X) ->
safe_fixtable(Tab,true),
clean_all_with_value(Tab,X,ets:first(Tab)),
safe_fixtable(Tab,false).
clean_all_with_value(Tab,X,'$end_of_table') ->
true;
clean_all_with_value(Tab,X,Key) ->
case ets:lookup(Tab,Key) of
[{Key,X}] ->
ets:delete(Tab,Key);
_ ->
true
end,
clean_all_with_value(Tab,X,ets:next(Tab,Key)).
Notice that deleted objects are not freed from a fixed table until it has been released. If a
process fixes a table but never releases it, the memory used by the deleted objects is never
freed. The performance of operations on the table also degrades significantly.
To retrieve information about which processes have fixed which tables, use info(Tab,
safe_fixed_monotonic_time). A system with many processes fixing tables can need a monitor that
sends alarms when tables have been fixed for too long.
Notice that safe_fixtable/2 is not necessary for table type ordered_set and for traversals done by
a single ETS function call, like select/2.
select(Continuation) -> {[Match], Continuation} | '$end_of_table'
Types:
Match = term()
Continuation = continuation()
Continues a match started with select/3. The next chunk of the size specified in the initial
select/3 call is returned together with a new Continuation, which can be used in subsequent calls
to this function.
When there are no more objects in the table, '$end_of_table' is returned.
select(Tab, MatchSpec) -> [Match]
Types:
Tab = tab()
MatchSpec = match_spec()
Match = term()
Matches the objects in table Tab using a match specification. This is a more general call than
match/2 and match_object/2 calls. In its simplest form, the match specification is as follows:
MatchSpec = [MatchFunction]
MatchFunction = {MatchHead, [Guard], [Result]}
MatchHead = "Pattern as in ets:match"
Guard = {"Guardtest name", ...}
Result = "Term construct"
This means that the match specification is always a list of one or more tuples (of arity 3). The
first element of the tuple is to be a pattern as described in match/2. The second element of the
tuple is to be a list of 0 or more guard tests (described below). The third element of the tuple
is to be a list containing a description of the value to return. In almost all normal cases, the
list contains exactly one term that fully describes the value to return for each object.
The return value is constructed using the "match variables" bound in MatchHead or using the
special match variables '$_' (the whole matching object) and '$$' (all match variables in a list),
so that the following match/2 expression:
ets:match(Tab,{'$1','$2','$3'})
is exactly equivalent to:
ets:select(Tab,[{{'$1','$2','$3'},[],['$$']}])
And that the following match_object/2 call:
ets:match_object(Tab,{'$1','$2','$1'})
is exactly equivalent to
ets:select(Tab,[{{'$1','$2','$1'},[],['$_']}])
Composite terms can be constructed in the Result part either by simply writing a list, so that the
following code:
ets:select(Tab,[{{'$1','$2','$3'},[],['$$']}])
gives the same output as:
ets:select(Tab,[{{'$1','$2','$3'},[],[['$1','$2','$3']]}])
That is, all the bound variables in the match head as a list. If tuples are to be constructed, one
has to write a tuple of arity 1 where the single element in the tuple is the tuple one wants to
construct (as an ordinary tuple can be mistaken for a Guard).
Therefore the following call:
ets:select(Tab,[{{'$1','$2','$1'},[],['$_']}])
gives the same output as:
ets:select(Tab,[{{'$1','$2','$1'},[],[{{'$1','$2','$3'}}]}])
This syntax is equivalent to the syntax used in the trace patterns (see the dbg(3erl)) module in
Runtime_Tools.
The Guards are constructed as tuples, where the first element is the test name and the remaining
elements are the test parameters. To check for a specific type (say a list) of the element bound
to the match variable '$1', one would write the test as {is_list, '$1'}. If the test fails, the
object in the table does not match and the next MatchFunction (if any) is tried. Most guard tests
present in Erlang can be used, but only the new versions prefixed is_ are allowed (is_float,
is_atom, and so on).
The Guard section can also contain logic and arithmetic operations, which are written with the
same syntax as the guard tests (prefix notation), so that the following guard test written in
Erlang:
is_integer(X), is_integer(Y), X + Y < 4711
is expressed as follows (X replaced with '$1' and Y with '$2'):
[{is_integer, '$1'}, {is_integer, '$2'}, {'<', {'+', '$1', '$2'}, 4711}]
For tables of type ordered_set, objects are visited in the same order as in a first/next
traversal. This means that the match specification is executed against objects with keys in the
first/next order and the corresponding result list is in the order of that execution.
select(Tab, MatchSpec, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
MatchSpec = match_spec()
Limit = integer() >= 1
Match = term()
Continuation = continuation()
Works like select/2, but only returns a limited (Limit) number of matching objects. Term
Continuation can then be used in subsequent calls to select/1 to get the next chunk of matching
objects. This is a space-efficient way to work on objects in a table, which is still faster than
traversing the table object by object using first/1 and next/2.
If the table is empty, '$end_of_table' is returned.
Use safe_fixtable/2 to guarantee safe traversal for subsequent calls to select/1.
select_count(Tab, MatchSpec) -> NumMatched
Types:
Tab = tab()
MatchSpec = match_spec()
NumMatched = integer() >= 0
Matches the objects in table Tab using a match specification. If the match specification returns
true for an object, that object considered a match and is counted. For any other result from the
match specification the object is not considered a match and is therefore not counted.
This function can be described as a match_delete/2 function that does not delete any elements, but
only counts them.
The function returns the number of objects matched.
select_delete(Tab, MatchSpec) -> NumDeleted
Types:
Tab = tab()
MatchSpec = match_spec()
NumDeleted = integer() >= 0
Matches the objects in table Tab using a match specification. If the match specification returns
true for an object, that object is removed from the table. For any other result from the match
specification the object is retained. This is a more general call than the match_delete/2 call.
The function returns the number of objects deleted from the table.
Note:
The match specification has to return the atom true if the object is to be deleted. No other
return value gets the object deleted. So one cannot use the same match specification for looking
up elements as for deleting them.
select_replace(Tab, MatchSpec) -> NumReplaced
Types:
Tab = tab()
MatchSpec = match_spec()
NumReplaced = integer() >= 0
Matches the objects in the table Tab using a match specification. For each matched object, the
existing object is replaced with the match specification result.
The match-and-replace operation for each individual object is guaranteed to be atomic and
isolated. The select_replace table traversal as a whole, like all other select functions, does not
give such guarantees.
The match specifiction must be guaranteed to retain the key of any matched object. If not,
select_replace will fail with badarg without updating any objects.
For the moment, due to performance and semantic constraints, tables of type bag are not yet
supported.
The function returns the total number of replaced objects.
Example
For all 2-tuples with a list in second position, add atom 'marker' first in the list:
1> T = ets:new(x,[]), ets:insert(T, {key, [1, 2, 3]}).
true
2> MS = ets:fun2ms(fun({K, L}) when is_list(L) -> {K, [marker | L]} end).
[{{'$1','$2'},[{is_list,'$2'}],[{{'$1',[marker|'$2']}}]}]
3> ets:select_replace(T, MS).
1
4> ets:tab2list(T).
[{key,[marker,1,2,3]}]
A generic single object compare-and-swap operation:
[Old] = ets:lookup(T, Key),
New = update_object(Old),
Success = (1 =:= ets:select_replace(T, [{Old, [], [{const, New}]}])),
select_reverse(Continuation) ->
{[Match], Continuation} | '$end_of_table'
Types:
Continuation = continuation()
Match = term()
Continues a match started with select_reverse/3. For tables of type ordered_set, the traversal of
the table continues to objects with keys earlier in the Erlang term order. The returned list also
contains objects with keys in reverse order. For all other table types, the behavior is exactly
that of select/1.
Example:
1> T = ets:new(x,[ordered_set]).
2> [ ets:insert(T,{N}) || N <- lists:seq(1,10) ].
...
3> {R0,C0} = ets:select_reverse(T,[{'_',[],['$_']}],4).
...
4> R0.
[{10},{9},{8},{7}]
5> {R1,C1} = ets:select_reverse(C0).
...
6> R1.
[{6},{5},{4},{3}]
7> {R2,C2} = ets:select_reverse(C1).
...
8> R2.
[{2},{1}]
9> '$end_of_table' = ets:select_reverse(C2).
...
select_reverse(Tab, MatchSpec) -> [Match]
Types:
Tab = tab()
MatchSpec = match_spec()
Match = term()
Works like select/2, but returns the list in reverse order for table type ordered_set. For all
other table types, the return value is identical to that of select/2.
select_reverse(Tab, MatchSpec, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
MatchSpec = match_spec()
Limit = integer() >= 1
Match = term()
Continuation = continuation()
Works like select/3, but for table type ordered_set traversing is done starting at the last object
in Erlang term order and moves to the first. For all other table types, the return value is
identical to that of select/3.
Notice that this is not equivalent to reversing the result list of a select/3 call, as the result
list is not only reversed, but also contains the last Limit matching objects in the table, not the
first.
setopts(Tab, Opts) -> true
Types:
Tab = tab()
Opts = Opt | [Opt]
Opt = {heir, pid(), HeirData} | {heir, none}
HeirData = term()
Sets table options. The only allowed option to be set after the table has been created is heir.
The calling process must be the table owner.
slot(Tab, I) -> [Object] | '$end_of_table'
Types:
Tab = tab()
I = integer() >= 0
Object = tuple()
This function is mostly for debugging purposes, Normally first/next or last/prev are to be used
instead.
Returns all objects in slot I of table Tab. A table can be traversed by repeatedly calling the
function, starting with the first slot I=0 and ending when '$end_of_table' is returned. If
argument I is out of range, the function fails with reason badarg.
Unless a table of type set, bag, or duplicate_bag is protected using safe_fixtable/2, a traversal
can fail if concurrent updates are made to the table. For table type ordered_set, the function
returns a list containing object I in Erlang term order.
tab2file(Tab, Filename) -> ok | {error, Reason}
Types:
Tab = tab()
Filename = file:name()
Reason = term()
Dumps table Tab to file Filename.
Equivalent to tab2file(Tab, Filename,[])
tab2file(Tab, Filename, Options) -> ok | {error, Reason}
Types:
Tab = tab()
Filename = file:name()
Options = [Option]
Option = {extended_info, [ExtInfo]} | {sync, boolean()}
ExtInfo = md5sum | object_count
Reason = term()
Dumps table Tab to file Filename.
When dumping the table, some information about the table is dumped to a header at the beginning of
the dump. This information contains data about the table type, name, protection, size, version,
and if it is a named table. It also contains notes about what extended information is added to the
file, which can be a count of the objects in the file or a MD5 sum of the header and records in
the file.
The size field in the header might not correspond to the number of records in the file if the
table is public and records are added or removed from the table during dumping. Public tables
updated during dump, and that one wants to verify when reading, needs at least one field of
extended information for the read verification process to be reliable later.
Option extended_info specifies what extra information is written to the table dump:
object_count:
The number of objects written to the file is noted in the file footer, so file truncation can
be verified even if the file was updated during dump.
md5sum:
The header and objects in the file are checksummed using the built-in MD5 functions. The MD5
sum of all objects is written in the file footer, so that verification while reading detects
the slightest bitflip in the file data. Using this costs a fair amount of CPU time.
Whenever option extended_info is used, it results in a file not readable by versions of ETS before
that in STDLIB 1.15.1
If option sync is set to true, it ensures that the content of the file is written to the disk
before tab2file returns. Defaults to {sync, false}.
tab2list(Tab) -> [Object]
Types:
Tab = tab()
Object = tuple()
Returns a list of all objects in table Tab.
tabfile_info(Filename) -> {ok, TableInfo} | {error, Reason}
Types:
Filename = file:name()
TableInfo = [InfoItem]
InfoItem =
{name, atom()} |
{type, Type} |
{protection, Protection} |
{named_table, boolean()} |
{keypos, integer() >= 0} |
{size, integer() >= 0} |
{extended_info, [ExtInfo]} |
{version,
{Major :: integer() >= 0, Minor :: integer() >= 0}}
ExtInfo = md5sum | object_count
Type = bag | duplicate_bag | ordered_set | set
Protection = private | protected | public
Reason = term()
Returns information about the table dumped to file by tab2file/2 or tab2file/3.
The following items are returned:
name:
The name of the dumped table. If the table was a named table, a table with the same name
cannot exist when the table is loaded from file with file2tab/2. If the table is not saved as
a named table, this field has no significance when loading the table from file.
type:
The ETS type of the dumped table (that is, set, bag, duplicate_bag, or ordered_set). This type
is used when loading the table again.
protection:
The protection of the dumped table (that is, private, protected, or public). A table loaded
from the file gets the same protection.
named_table:
true if the table was a named table when dumped to file, otherwise false. Notice that when a
named table is loaded from a file, there cannot exist a table in the system with the same
name.
keypos:
The keypos of the table dumped to file, which is used when loading the table again.
size:
The number of objects in the table when the table dump to file started. For a public table,
this number does not need to correspond to the number of objects saved to the file, as objects
can have been added or deleted by another process during table dump.
extended_info:
The extended information written in the file footer to allow stronger verification during
table loading from file, as specified to tab2file/3. Notice that this function only tells
which information is present, not the values in the file footer. The value is a list
containing one or more of the atoms object_count and md5sum.
version:
A tuple {Major,Minor} containing the major and minor version of the file format for ETS table
dumps. This version field was added beginning with STDLIB 1.5.1. Files dumped with older
versions return {0,0} in this field.
An error is returned if the file is inaccessible, badly damaged, or not produced with tab2file/2
or tab2file/3.
table(Tab) -> QueryHandle
table(Tab, Options) -> QueryHandle
Types:
Tab = tab()
QueryHandle = qlc:query_handle()
Options = [Option] | Option
Option = {n_objects, NObjects} | {traverse, TraverseMethod}
NObjects = default | integer() >= 1
TraverseMethod =
first_next | last_prev | select |
{select, MatchSpec :: match_spec()}
Returns a Query List Comprehension (QLC) query handle. The qlc module provides a query language
aimed mainly at Mnesia, but ETS tables, Dets tables, and lists are also recognized by QLC as
sources of data. Calling table/1,2 is the means to make the ETS table Tab usable to QLC.
When there are only simple restrictions on the key position, QLC uses lookup/2 to look up the
keys. When that is not possible, the whole table is traversed. Option traverse determines how this
is done:
first_next:
The table is traversed one key at a time by calling first/1 and next/2.
last_prev:
The table is traversed one key at a time by calling last/1 and prev/2.
select:
The table is traversed by calling select/3 and select/1. Option n_objects determines the
number of objects returned (the third argument of select/3); the default is to return 100
objects at a time. The match specification (the second argument of select/3) is assembled by
QLC: simple filters are translated into equivalent match specifications while more complicated
filters must be applied to all objects returned by select/3 given a match specification that
matches all objects.
{select, MatchSpec}:
As for select, the table is traversed by calling select/3 and select/1. The difference is that
the match specification is explicitly specified. This is how to state match specifications
that cannot easily be expressed within the syntax provided by QLC.
Examples:
An explicit match specification is here used to traverse the table:
9> true = ets:insert(Tab = ets:new(t, []), [{1,a},{2,b},{3,c},{4,d}]),
MS = ets:fun2ms(fun({X,Y}) when (X > 1) or (X < 5) -> {Y} end),
QH1 = ets:table(Tab, [{traverse, {select, MS}}]).
An example with an implicit match specification:
10> QH2 = qlc:q([{Y} || {X,Y} <- ets:table(Tab), (X > 1) or (X < 5)]).
The latter example is equivalent to the former, which can be verified using function qlc:info/1:
11> qlc:info(QH1) =:= qlc:info(QH2).
true
qlc:info/1 returns information about a query handle, and in this case identical information is
returned for the two query handles.
take(Tab, Key) -> [Object]
Types:
Tab = tab()
Key = term()
Object = tuple()
Returns and removes a list of all objects with key Key in table Tab.
The specified Key is used to identify the object by either comparing equal the key of an object in
an ordered_set table, or matching in other types of tables (for details on the difference, see
lookup/2 and new/2).
test_ms(Tuple, MatchSpec) -> {ok, Result} | {error, Errors}
Types:
Tuple = tuple()
MatchSpec = match_spec()
Result = term()
Errors = [{warning | error, string()}]
This function is a utility to test a match specification used in calls to select/2. The function
both tests MatchSpec for "syntactic" correctness and runs the match specification against object
Tuple.
If the match specification is syntactically correct, the function either returns {ok,Result},
where Result is what would have been the result in a real select/2 call, or false if the match
specification does not match object Tuple.
If the match specification contains errors, tuple {error, Errors} is returned, where Errors is a
list of natural language descriptions of what was wrong with the match specification.
This is a useful debugging and test tool, especially when writing complicated select/2 calls.
See also: erlang:match_spec_test/3.
to_dets(Tab, DetsTab) -> DetsTab
Types:
Tab = tab()
DetsTab = dets:tab_name()
Fills an already created/opened Dets table with the objects in the already opened ETS table named
Tab. The Dets table is emptied before the objects are inserted.
update_counter(Tab, Key, UpdateOp) -> Result
update_counter(Tab, Key, UpdateOp, Default) -> Result
update_counter(Tab, Key, X3 :: [UpdateOp]) -> [Result]
update_counter(Tab, Key, X3 :: [UpdateOp], Default) -> [Result]
update_counter(Tab, Key, Incr) -> Result
update_counter(Tab, Key, Incr, Default) -> Result
Types:
Tab = tab()
Key = term()
UpdateOp = {Pos, Incr} | {Pos, Incr, Threshold, SetValue}
Pos = Incr = Threshold = SetValue = Result = integer()
Default = tuple()
This function provides an efficient way to update one or more counters, without the trouble of
having to look up an object, update the object by incrementing an element, and insert the
resulting object into the table again. The operation is guaranteed to be atomic and isolated.
This function destructively update the object with key Key in table Tab by adding Incr to the
element at position Pos. The new counter value is returned. If no position is specified, the
element directly following key (<keypos>+1) is updated.
If a Threshold is specified, the counter is reset to value SetValue if the following conditions
occur:
* Incr is not negative (>= 0) and the result would be greater than (>) Threshold.
* Incr is negative (< 0) and the result would be less than (<) Threshold.
A list of UpdateOp can be supplied to do many update operations within the object. The operations
are carried out in the order specified in the list. If the same counter position occurs more than
once in the list, the corresponding counter is thus updated many times, each time based on the
previous result. The return value is a list of the new counter values from each update operation
in the same order as in the operation list. If an empty list is specified, nothing is updated and
an empty list is returned. If the function fails, no updates are done.
The specified Key is used to identify the object by either matching the key of an object in a set
table, or compare equal to the key of an object in an ordered_set table (for details on the
difference, see lookup/2 and new/2).
If a default object Default is specified, it is used as the object to be updated if the key is
missing from the table. The value in place of the key is ignored and replaced by the proper key
value. The return value is as if the default object had not been used, that is, a single updated
element or a list of them.
The function fails with reason badarg in the following situations:
* The table type is not set or ordered_set.
* No object with the correct key exists and no default object was supplied.
* The object has the wrong arity.
* The default object arity is smaller than <keypos>.
* Any field from the default object that is updated is not an integer.
* The element to update is not an integer.
* The element to update is also the key.
* Any of Pos, Incr, Threshold, or SetValue is not an integer.
update_element(Tab, Key, ElementSpec :: {Pos, Value}) -> boolean()
update_element(Tab, Key, ElementSpec :: [{Pos, Value}]) ->
boolean()
Types:
Tab = tab()
Key = term()
Value = term()
Pos = integer() >= 1
This function provides an efficient way to update one or more elements within an object, without
the trouble of having to look up, update, and write back the entire object.
This function destructively updates the object with key Key in table Tab. The element at position
Pos is given the value Value.
A list of {Pos,Value} can be supplied to update many elements within the same object. If the same
position occurs more than once in the list, the last value in the list is written. If the list is
empty or the function fails, no updates are done. The function is also atomic in the sense that
other processes can never see any intermediate results.
Returns true if an object with key Key is found, otherwise false.
The specified Key is used to identify the object by either matching the key of an object in a set
table, or compare equal to the key of an object in an ordered_set table (for details on the
difference, see lookup/2 and new/2).
The function fails with reason badarg in the following situations:
* The table type is not set or ordered_set.
* Pos < 1.
* Pos > object arity.
* The element to update is also the key.
whereis(TableName) -> tid() | undefined
Types:
TableName = atom()
This function returns the tid() of the named table identified by TableName, or undefined if no
such table exists. The tid() can be used in place of the table name in all operations, which is
slightly faster since the name does not have to be resolved on each call.
If the table is deleted, the tid() will be invalid even if another named table is created with the
same name.
Ericsson AB stdlib 3.11.2 ets(3erl)