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NAME
dets - A Disk Based Term Storage
DESCRIPTION
The module dets provides a term storage on file. The stored terms, in this module called
objects, are tuples such that one element is defined to be the key. A Dets table is a
collection of objects with the key at the same position stored on a file.
Dets is used by the Mnesia application, and is provided as is for users who are interested
in an efficient storage of Erlang terms on disk only. Many applications just need to store
some terms in a file. Mnesia adds transactions, queries, and distribution. The size of
Dets files cannot exceed 2 GB. If larger tables are needed, Mnesia's table fragmentation
can be used.
There are three types of Dets tables: set, bag and duplicate_bag. A table of type set has
at most one object with a given key. If an object with a key already present in the table
is inserted, the existing object is overwritten by the new object. A table of type bag has
zero or more different objects with a given key. A table of type duplicate_bag has zero or
more possibly matching objects with a given key.
Dets tables must be opened before they can be updated or read, and when finished they must
be properly closed. If a table has not been properly closed, Dets will automatically
repair the table. This can take a substantial time if the table is large. A Dets table is
closed when the process which opened the table terminates. If several Erlang processes
(users) open the same Dets table, they will share the table. The table is properly closed
when all users have either terminated or closed the table. Dets tables are not properly
closed if the Erlang runtime system is terminated abnormally.
Note:
A ^C command abnormally terminates an Erlang runtime system in a Unix environment with a
break-handler.
Since all operations performed by Dets are disk operations, it is important to realize
that a single look-up operation involves a series of disk seek and read operations. For
this reason, the Dets functions are much slower than the corresponding Ets functions,
although Dets exports a similar interface.
Dets organizes data as a linear hash list and the hash list grows gracefully as more data
is inserted into the table. Space management on the file is performed by what is called a
buddy system. The current implementation keeps the entire buddy system in RAM, which
implies that if the table gets heavily fragmented, quite some memory can be used up. The
only way to defragment a table is to close it and then open it again with the repair
option set to force.
It is worth noting that the ordered_set type present in Ets is not yet implemented by
Dets, neither is the limited support for concurrent updates which makes a sequence of
first and next calls safe to use on fixed Ets tables. Both these features will be
implemented by Dets in a future release of Erlang/OTP. Until then, the Mnesia application
(or some user implemented method for locking) has to be used to implement safe
concurrency. Currently, no library of Erlang/OTP has support for ordered disk based term
storage.
Two versions of the format used for storing objects on file are supported by Dets. The
first version, 8, is the format always used for tables created by OTP R7 and earlier. The
second version, 9, is the default version of tables created by OTP R8 (and later OTP
releases). OTP R8 can create version 8 tables, and convert version 8 tables to version 9,
and vice versa, upon request.
All Dets functions return {error, Reason} if an error occurs (first/1 and next/2 are
exceptions, they exit the process with the error tuple). If given badly formed arguments,
all functions exit the process with a badarg message.
DATA TYPES
access() = read | read_write
auto_save() = infinity | integer() >= 0
bindings_cont()
Opaque continuation used by match/1 and match/3.
cont()
Opaque continuation used by bchunk/2.
keypos() = integer() >= 1
match_spec() = ets:match_spec()
Match specifications, see the match specification documentation in the ERTS User's
Guide and ms_transform(3erl).
no_slots() = integer() >= 0 | default
object() = tuple()
object_cont()
Opaque continuation used by match_object/1 and match_object/3.
pattern() = atom() | tuple()
See ets:match/2 for a description of patterns.
select_cont()
Opaque continuation used by select/1 and select/3.
tab_name() = term()
type() = bag | duplicate_bag | set
version() = 8 | 9 | default
EXPORTS
all() -> [tab_name()]
Returns a list of the names of all open tables on this node.
bchunk(Name, Continuation) ->
{Continuation2, Data} |
'$end_of_table' |
{error, Reason}
Types:
Name = tab_name()
Continuation = start | cont()
Continuation2 = cont()
Data = binary() | tuple()
Reason = term()
Returns a list of objects stored in a table. The exact representation of the
returned objects is not public. The lists of data can be used for initializing a
table by giving the value bchunk to the format option of the init_table/3 function.
The Mnesia application uses this function for copying open tables.
Unless the table is protected using safe_fixtable/2, calls to bchunk/2 may not work
as expected if concurrent updates are made to the table.
The first time bchunk/2 is called, an initial continuation, the atom start, must be
provided.
The bchunk/2 function returns a tuple {Continuation2, Data}, where Data is a list
of objects. Continuation2 is another continuation which is to be passed on to a
subsequent call to bchunk/2. With a series of calls to bchunk/2 it is possible to
extract all objects of the table.
bchunk/2 returns '$end_of_table' when all objects have been returned, or {error,
Reason} if an error occurs.
close(Name) -> ok | {error, Reason}
Types:
Name = tab_name()
Reason = term()
Closes a table. Only processes that have opened a table are allowed to close it.
All open tables must be closed before the system is stopped. If an attempt is made
to open a table which has not been properly closed, Dets automatically tries to
repair the table.
delete(Name, Key) -> ok | {error, Reason}
Types:
Name = tab_name()
Key = Reason = term()
Deletes all objects with the key Key from the table Name.
delete_all_objects(Name) -> ok | {error, Reason}
Types:
Name = tab_name()
Reason = term()
Deletes all objects from a table in almost constant time. However, if the table if
fixed, delete_all_objects(T) is equivalent to match_delete(T, '_').
delete_object(Name, Object) -> ok | {error, Reason}
Types:
Name = tab_name()
Object = object()
Reason = term()
Deletes all instances of a given object from a table. If a table is of type bag or
duplicate_bag, the delete/2 function cannot be used to delete only some of the
objects with a given key. This function makes this possible.
first(Name) -> Key | '$end_of_table'
Types:
Name = tab_name()
Key = term()
Returns the first key stored in the table Name according to the table's internal
order, or '$end_of_table' if the table is empty.
Unless the table is protected using safe_fixtable/2, subsequent calls to next/2 may
not work as expected if concurrent updates are made to the table.
Should an error occur, the process is exited with an error tuple {error, Reason}.
The reason for not returning the error tuple is that it cannot be distinguished
from a key.
There are two reasons why first/1 and next/2 should not be used: they are not very
efficient, and they prevent the use of the key '$end_of_table' since this atom is
used to indicate the end of the table. If possible, the match, match_object, and
select functions should be used for traversing tables.
foldl(Function, Acc0, Name) -> Acc | {error, Reason}
foldr(Function, Acc0, Name) -> Acc | {error, Reason}
Types:
Name = tab_name()
Function = fun((Object :: object(), AccIn) -> AccOut)
Acc0 = Acc = AccIn = AccOut = Reason = term()
Calls Function on successive elements of the table Name together with an extra
argument AccIn. The order in which the elements of the table are traversed is
unspecified. Function must return a new accumulator which is passed to the next
call. Acc0 is returned if the table is empty.
from_ets(Name, EtsTab) -> ok | {error, Reason}
Types:
Name = tab_name()
EtsTab = ets:tab()
Reason = term()
Deletes all objects of the table Name and then inserts all the objects of the Ets
table EtsTab. The order in which the objects are inserted is not specified. Since
ets:safe_fixtable/2 is called the Ets table must be public or owned by the calling
process.
info(Name) -> InfoList | undefined
Types:
Name = tab_name()
InfoList = [InfoTuple]
InfoTuple = {file_size, integer() >= 0}
| {filename, file:name()}
| {keypos, keypos()}
| {size, integer() >= 0}
| {type, type()}
Returns information about the table Name as a list of tuples:
* {file_size, integer() >= 0}, the size of the file in bytes.
* {filename, file:name()}, the name of the file where objects are stored.
* {keypos, keypos()} , the position of the key.
* {size, integer() >= 0}, the number of objects stored in the table.
* {type, type()}, the type of the table.
info(Name, Item) -> Value | undefined
Types:
Name = tab_name()
Item = access
| auto_save
| bchunk_format
| hash
| file_size
| filename
| keypos
| memory
| no_keys
| no_objects
| no_slots
| owner
| ram_file
| safe_fixed
| size
| type
| version
Value = term()
Returns the information associated with Item for the table Name. In addition to the
{Item, Value} pairs defined for info/1, the following items are allowed:
* {access, access()} , the access mode.
* {auto_save, auto_save()}, the auto save interval.
* {bchunk_format, binary()}, an opaque binary describing the format of the
objects returned by bchunk/2. The binary can be used as argument to
is_compatible_chunk_format/2. Only available for version 9 tables.
* {hash, Hash}. Describes which BIF is used to calculate the hash values of the
objects stored in the Dets table. Possible values of Hash are hash, which
implies that the erlang:hash/2 BIF is used, phash, which implies that the
erlang:phash/2 BIF is used, and phash2, which implies that the erlang:phash2/1
BIF is used.
* {memory, integer() >= 0}, the size of the file in bytes. The same value is
associated with the item file_size.
* {no_keys, integer >= 0()}, the number of different keys stored in the table.
Only available for version 9 tables.
* {no_objects, integer >= 0()}, the number of objects stored in the table.
* {no_slots, {Min, Used, Max}}, the number of slots of the table. Min is the
minimum number of slots, Used is the number of currently used slots, and Max is
the maximum number of slots. Only available for version 9 tables.
* {owner, pid()}, the pid of the process that handles requests to the Dets table.
* {ram_file, boolean()}, whether the table is kept in RAM.
* {safe_fixed, SafeFixed}. If the table is fixed, SafeFixed is a tuple
{FixedAtTime, [{Pid,RefCount}]}. FixedAtTime is the time when the table was
first fixed, and Pid is the pid of the process that fixes the table RefCount
times. There may be any number of processes in the list. If the table is not
fixed, SafeFixed is the atom false.
* {version, integer(), the version of the format of the table.
init_table(Name, InitFun) -> ok | {error, Reason}
init_table(Name, InitFun, Options) -> ok | {error, Reason}
Types:
Name = tab_name()
InitFun = fun((Arg) -> Res)
Arg = read | close
Res = end_of_input
| {[object()], InitFun}
| {Data, InitFun}
| term()
Options = Option | [Option]
Option = {min_no_slots, no_slots()} | {format, term | bchunk}
Reason = term()
Data = binary() | tuple()
Replaces the existing objects of the table Name with objects created by calling the
input function InitFun, see below. The reason for using this function rather than
calling insert/2 is that of efficiency. It should be noted that the input functions
are called by the process that handles requests to the Dets table, not by the
calling process.
When called with the 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 will be called exactly
once, and should an error occur, the last function is called with the argument
close, the reply of which is ignored.
If the type of the table is set and there is more than one object 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. Duplicate
keys should be avoided, or the file will be unnecessarily fragmented. This holds
also for duplicated objects stored in tables of type bag.
It is important that the table has a sufficient number of slots for the objects. If
not, the hash list will start to grow when init_table/2 returns which will
significantly slow down access to the table for a period of time. The minimum
number of slots is set by the open_file/2 option min_no_slots and returned by the
info/2 item no_slots. See also the min_no_slots option below.
The Options argument is a list of {Key, Val} tuples where the following values are
allowed:
* {min_no_slots, no_slots()}. Specifies the estimated number of different keys
that will be stored in the table. The open_file option with the same name is
ignored unless the table is created, and in that case performance can be
enhanced by supplying an estimate when initializing the table.
* {format, Format}. Specifies the format of the objects returned by the function
InitFun. If Format is term (the default), InitFun is assumed to return a list
of tuples. If Format is bchunk, InitFun is assumed to return Data as returned
by bchunk/2. This option overrides the min_no_slots option.
insert(Name, Objects) -> ok | {error, Reason}
Types:
Name = tab_name()
Objects = object() | [object()]
Reason = term()
Inserts one or more objects into the table Name. If there already exists an object
with a key matching the key of some of the given objects and the table type is set,
the old object will be replaced.
insert_new(Name, Objects) -> boolean()
Types:
Name = tab_name()
Objects = object() | [object()]
Inserts one or more objects into the table Name. If there already exists some
object with a key matching the key of any of the given objects the table is not
updated and false is returned, otherwise the objects are inserted and true
returned.
is_compatible_bchunk_format(Name, BchunkFormat) -> boolean()
Types:
Name = tab_name()
BchunkFormat = binary()
Returns true if it would be possible to initialize the table Name, using
init_table/3 with the option {format, bchunk}, with objects read with bchunk/2 from
some table T such that calling info(T, bchunk_format) returns BchunkFormat.
is_dets_file(Filename) -> boolean() | {error, Reason}
Types:
Filename = file:name()
Reason = term()
Returns true if the file Filename is a Dets table, false otherwise.
lookup(Name, Key) -> Objects | {error, Reason}
Types:
Name = tab_name()
Key = term()
Objects = [object()]
Reason = term()
Returns a list of all objects with the key Key stored in the table Name. For
example:
2> dets:open_file(abc, [{type, bag}]).
{ok,abc}
3> dets:insert(abc, {1,2,3}).
ok
4> dets:insert(abc, {1,3,4}).
ok
5> dets:lookup(abc, 1).
[{1,2,3},{1,3,4}]
If the table is of type set, the function returns either the empty list or a list
with one object, as there cannot be more than one object with a given key. If the
table is of type bag or duplicate_bag, the function returns a list of arbitrary
length.
Note that the order of objects returned is unspecified. In particular, the order in
which objects were inserted is not reflected.
match(Continuation) ->
{[Match], Continuation2} |
'$end_of_table' |
{error, Reason}
Types:
Continuation = Continuation2 = bindings_cont()
Match = [term()]
Reason = term()
Matches some objects stored in a table and returns a non-empty list of the bindings
that match a given pattern in some unspecified order. The table, the pattern, and
the number of objects that are matched are all defined by Continuation, which has
been returned by a prior call to match/1 or match/3.
When all objects of the table have been matched, '$end_of_table' is returned.
match(Name, Pattern) -> [Match] | {error, Reason}
Types:
Name = tab_name()
Pattern = pattern()
Match = [term()]
Reason = term()
Returns for each object of the table Name that matches Pattern a list of bindings
in some unspecified order. See ets:match/2 for a description of patterns. If the
keypos'th element of Pattern is unbound, all objects of the table are matched. If
the keypos'th element is bound, only the objects with the right key are matched.
match(Name, Pattern, N) ->
{[Match], Continuation} |
'$end_of_table' |
{error, Reason}
Types:
Name = tab_name()
Pattern = pattern()
N = default | integer() >= 0
Continuation = bindings_cont()
Match = [term()]
Reason = term()
Matches some or all objects of the table Name and returns a non-empty list of the
bindings that match Pattern in some unspecified order. See ets:match/2 for a
description of patterns.
A tuple of the bindings and a continuation is returned, unless the table is empty,
in which case '$end_of_table' is returned. The continuation is to be used when
matching further objects by calling match/1.
If the keypos'th element of Pattern is bound, all objects of the table are matched.
If the keypos'th element is unbound, all objects of the table are matched, N
objects at a time, until at least one object matches or the end of the table has
been reached. The default, indicated by giving N the value default, is to let the
number of objects vary depending on the sizes of the objects. If Name is a version
9 table, all objects with the same key are always matched at the same time which
implies that more than N objects may sometimes be matched.
The table should always be protected using safe_fixtable/2 before calling match/3,
or errors may occur when calling match/1.
match_delete(Name, Pattern) -> ok | {error, Reason}
Types:
Name = tab_name()
Pattern = pattern()
Reason = term()
Deletes all objects that match Pattern from the table Name. See ets:match/2 for a
description of patterns.
If the keypos'th element of Pattern is bound, only the objects with the right key
are matched.
match_object(Continuation) ->
{Objects, Continuation2} |
'$end_of_table' |
{error, Reason}
Types:
Continuation = Continuation2 = object_cont()
Objects = [object()]
Reason = term()
Returns a non-empty list of some objects stored in a table that match a given
pattern in some unspecified order. The table, the pattern, and the number of
objects that are matched are all defined by Continuation, which has been returned
by a prior call to match_object/1 or match_object/3.
When all objects of the table have been matched, '$end_of_table' is returned.
match_object(Name, Pattern) -> Objects | {error, Reason}
Types:
Name = tab_name()
Pattern = pattern()
Objects = [object()]
Reason = term()
Returns a list of all objects of the table Name that match Pattern in some
unspecified order. See ets:match/2 for a description of patterns.
If the keypos'th element of Pattern is unbound, all objects of the table are
matched. If the keypos'th element of Pattern is bound, only the objects with the
right key are matched.
Using the match_object functions for traversing all objects of a table is more
efficient than calling first/1 and next/2 or slot/2.
match_object(Name, Pattern, N) ->
{Objects, Continuation} |
'$end_of_table' |
{error, Reason}
Types:
Name = tab_name()
Pattern = pattern()
N = default | integer() >= 0
Continuation = object_cont()
Objects = [object()]
Reason = term()
Matches some or all objects stored in the table Name and returns a non-empty list
of the objects that match Pattern in some unspecified order. See ets:match/2 for a
description of patterns.
A list of objects and a continuation is returned, unless the table is empty, in
which case '$end_of_table' is returned. The continuation is to be used when
matching further objects by calling match_object/1.
If the keypos'th element of Pattern is bound, all objects of the table are matched.
If the keypos'th element is unbound, all objects of the table are matched, N
objects at a time, until at least one object matches or the end of the table has
been reached. The default, indicated by giving N the value default, is to let the
number of objects vary depending on the sizes of the objects. If Name is a version
9 table, all matching objects with the same key are always returned in the same
reply which implies that more than N objects may sometimes be returned.
The table should always be protected using safe_fixtable/2 before calling
match_object/3, or errors may occur when calling match_object/1.
member(Name, Key) -> boolean() | {error, Reason}
Types:
Name = tab_name()
Key = Reason = term()
Works like lookup/2, but does not return the objects. The function returns true if
one or more elements of the table has the key Key, false otherwise.
next(Name, Key1) -> Key2 | '$end_of_table'
Types:
Name = tab_name()
Key1 = Key2 = term()
Returns the key following Key1 in the table Name according to the table's internal
order, or '$end_of_table' if there is no next key.
Should an error occur, the process is exited with an error tuple {error, Reason}.
Use first/1 to find the first key in the table.
open_file(Filename) -> {ok, Reference} | {error, Reason}
Types:
Filename = file:name()
Reference = reference()
Reason = term()
Opens an existing table. If the table has not been properly closed, it will be
repaired. The returned reference is to be used as the name of the table. This
function is most useful for debugging purposes.
open_file(Name, Args) -> {ok, Name} | {error, Reason}
Types:
Name = tab_name()
Args = [OpenArg]
OpenArg = {access, access()}
| {auto_save, auto_save()}
| {estimated_no_objects, integer() >= 0}
| {file, file:name()}
| {max_no_slots, no_slots()}
| {min_no_slots, no_slots()}
| {keypos, keypos()}
| {ram_file, boolean()}
| {repair, boolean() | force}
| {type, type()}
| {version, version()}
Reason = term()
Opens a table. An empty Dets table is created if no file exists.
The atom Name is the name of the table. The table name must be provided in all
subsequent operations on the table. The name can be used by other processes as
well, and several process can share one table.
If two processes open the same table by giving the same name and arguments, then
the table will have two users. If one user closes the table, it still remains open
until the second user closes the table.
The Args argument is a list of {Key, Val} tuples where the following values are
allowed:
* {access, access()}. It is possible to open existing tables in read-only mode.
A table which is opened in read-only mode is not subjected to the automatic
file reparation algorithm if it is later opened after a crash. The default
value is read_write.
* {auto_save, auto_save()}, the auto save interval. If the interval is an
integer Time, the table is flushed to disk whenever it is not accessed for Time
milliseconds. A table that has been flushed will require no reparation when
reopened after an uncontrolled emulator halt. If the interval is the atom
infinity, auto save is disabled. The default value is 180000 (3 minutes).
* {estimated_no_objects, no_slots()}. Equivalent to the min_no_slots option.
* {file, file:name()}, the name of the file to be opened. The default value is
the name of the table.
* {max_no_slots, no_slots()}, the maximum number of slots that will be used. The
default value as well as the maximal value is 32 M. Note that a higher value
may increase the fragmentation of the table, and conversely, that a smaller
value may decrease the fragmentation, at the expense of execution time. Only
available for version 9 tables.
* {min_no_slots, no_slots()}. Application performance can be enhanced with this
flag by specifying, when the table is created, the estimated number of
different keys that will be stored in the table. The default value as well as
the minimum value is 256.
* {keypos, keypos()}, the position of the element of each object to be used as
key. The default value is 1. The ability to explicitly state the key position
is most convenient when we want to store Erlang records in which the first
position of the record is the name of the record type.
* {ram_file, boolean()}, whether the table is to be kept in RAM. Keeping the
table in RAM may sound like an anomaly, but can enhance the performance of
applications which open a table, insert a set of objects, and then close the
table. When the table is closed, its contents are written to the disk file. The
default value is false.
* {repair, Value}. Value can be either a boolean() or the atom force. The flag
specifies whether the Dets server should invoke the automatic file reparation
algorithm. The default is true. If false is specified, there is no attempt to
repair the file and {error, {needs_repair, FileName}} is returned if the table
needs to be repaired.
The value force means that a reparation will take place even if the table has
been properly closed. This is how to convert tables created by older versions
of STDLIB. An example is tables hashed with the deprecated erlang:hash/2 BIF.
Tables created with Dets from a STDLIB version of 1.8.2 and later use the
erlang:phash/2 function or the erlang:phash2/1 function, which is preferred.
The repair option is ignored if the table is already open.
* {type, type()}, the type of the table. The default value is set.
* {version, version()}, the version of the format used for the table. The
default value is 9. Tables on the format used before OTP R8 can be created by
giving the value 8. A version 8 table can be converted to a version 9 table by
giving the options {version,9} and {repair,force}.
pid2name(Pid) -> {ok, Name} | undefined
Types:
Pid = pid()
Name = tab_name()
Returns the name of the table given the pid of a process that handles requests to a
table, or undefined if there is no such table.
This function is meant to be used for debugging only.
repair_continuation(Continuation, MatchSpec) -> Continuation2
Types:
Continuation = Continuation2 = select_cont()
MatchSpec = match_spec()
This function can be used to restore 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 therefore will 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.
See also ets(3erl) for further explanations and examples.
Note:
This function is very rarely needed in application code. It is used by Mnesia to
implement 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 reason for not having an external representation of compiled match
specifications is performance. It may be subject to change in future releases,
while this interface will remain for backward compatibility.
safe_fixtable(Name, Fix) -> ok
Types:
Name = tab_name()
Fix = boolean()
If Fix is true, the table Name is fixed (once more) by the calling process,
otherwise the table is released. The table is also released when a fixing process
terminates.
If several processes fix a table, the table will remain fixed until all processes
have released it or terminated. A reference counter is kept on a per process basis,
and N consecutive fixes require N releases to release the table.
It is not guaranteed that calls to first/1, next/2, select and match functions work
as expected even if the table has been fixed; the limited support for concurrency
implemented in Ets has not yet been implemented in Dets. Fixing a table currently
only disables resizing of the hash list of the table.
If objects have been added while the table was fixed, the hash list will start to
grow when the table is released which will significantly slow down access to the
table for a period of time.
select(Continuation) ->
{Selection, Continuation2} |
'$end_of_table' |
{error, Reason}
Types:
Continuation = Continuation2 = select_cont()
Selection = [term()]
Reason = term()
Applies a match specification to some objects stored in a table and returns a non-
empty list of the results. The table, the match specification, and the number of
objects that are matched are all defined by Continuation, which has been returned
by a prior call to select/1 or select/3.
When all objects of the table have been matched, '$end_of_table' is returned.
select(Name, MatchSpec) -> Selection | {error, Reason}
Types:
Name = tab_name()
MatchSpec = match_spec()
Selection = [term()]
Reason = term()
Returns the results of applying the match specification MatchSpec to all or some
objects stored in the table Name. The order of the objects is not specified. See
the ERTS User's Guide for a description of match specifications.
If the keypos'th element of MatchSpec is unbound, the match specification is
applied to all objects of the table. If the keypos'th element is bound, the match
specification is applied to the objects with the right key(s) only.
Using the select functions for traversing all objects of a table is more efficient
than calling first/1 and next/2 or slot/2.
select(Name, MatchSpec, N) ->
{Selection, Continuation} |
'$end_of_table' |
{error, Reason}
Types:
Name = tab_name()
MatchSpec = match_spec()
N = default | integer() >= 0
Continuation = select_cont()
Selection = [term()]
Reason = term()
Returns the results of applying the match specification MatchSpec to some or all
objects stored in the table Name. The order of the objects is not specified. See
the ERTS User's Guide for a description of match specifications.
A tuple of the results of applying the match specification and a continuation is
returned, unless the table is empty, in which case '$end_of_table' is returned. The
continuation is to be used when matching further objects by calling select/1.
If the keypos'th element of MatchSpec is bound, the match specification is applied
to all objects of the table with the right key(s). If the keypos'th element of
MatchSpec is unbound, the match specification is applied to all objects of the
table, N objects at a time, until at least one object matches or the end of the
table has been reached. The default, indicated by giving N the value default, is to
let the number of objects vary depending on the sizes of the objects. If Name is a
version 9 table, all objects with the same key are always handled at the same time
which implies that the match specification may be applied to more than N objects.
The table should always be protected using safe_fixtable/2 before calling select/3,
or errors may occur when calling select/1.
select_delete(Name, MatchSpec) -> N | {error, Reason}
Types:
Name = tab_name()
MatchSpec = match_spec()
N = integer() >= 0
Reason = term()
Deletes each object from the table Name such that applying the match specification
MatchSpec to the object returns the value true. See the ERTS User's Guide for a
description of match specifications. Returns the number of deleted objects.
If the keypos'th element of MatchSpec is bound, the match specification is applied
to the objects with the right key(s) only.
slot(Name, I) -> '$end_of_table' | Objects | {error, Reason}
Types:
Name = tab_name()
I = integer() >= 0
Objects = [object()]
Reason = term()
The objects of a table are distributed among slots, starting with slot 0 and ending
with slot n. This function returns the list of objects associated with slot I. If I
is greater than n '$end_of_table' is returned.
sync(Name) -> ok | {error, Reason}
Types:
Name = tab_name()
Reason = term()
Ensures that all updates made to the table Name are written to disk. This also
applies to tables which have been opened with the ram_file flag set to true. In
this case, the contents of the RAM file are flushed to disk.
Note that the space management data structures kept in RAM, the buddy system, is
also written to the disk. This may take some time if the table is fragmented.
table(Name) -> QueryHandle
table(Name, Options) -> QueryHandle
Types:
Name = tab_name()
Options = Option | [Option]
Option = {n_objects, Limit} | {traverse, TraverseMethod}
Limit = default | integer() >= 1
TraverseMethod = first_next | select | {select, match_spec()}
QueryHandle = qlc:query_handle()
Returns a QLC (Query List Comprehension) query handle. The module qlc implements a
query language aimed mainly at Mnesia but Ets tables, Dets tables, and lists are
also recognized by qlc as sources of data. Calling dets:table/1,2 is the means to
make the Dets table Name usable to qlc.
When there are only simple restrictions on the key position qlc uses dets:lookup/2
to look up the keys, but when that is not possible the whole table is traversed.
The option traverse determines how this is done:
* first_next. The table is traversed one key at a time by calling dets:first/1
and dets:next/2.
* select. The table is traversed by calling dets:select/3 and dets:select/1. The
option n_objects determines the number of objects returned (the third argument
of select/3). 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 have to be applied to all objects
returned by select/3 given a match specification that matches all objects.
* {select, match_spec()}. As for select the table is traversed by calling
dets:select/3 and dets:select/1. The difference is that the match specification
is explicitly given. This is how to state match specifications that cannot
easily be expressed within the syntax provided by qlc.
The following example uses an explicit match specification to traverse the table:
1> dets:open_file(t, []),
ok = dets:insert(t, [{1,a},{2,b},{3,c},{4,d}]),
MS = ets:fun2ms(fun({X,Y}) when (X > 1) or (X < 5) -> {Y} end),
QH1 = dets:table(t, [{traverse, {select, MS}}]).
An example with implicit match specification:
2> QH2 = qlc:q([{Y} || {X,Y} <- dets:table(t), (X > 1) or (X < 5)]).
The latter example is in fact equivalent to the former which can be verified using
the function qlc:info/1:
3> 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.
to_ets(Name, EtsTab) -> EtsTab | {error, Reason}
Types:
Name = tab_name()
EtsTab = ets:tab()
Reason = term()
Inserts the objects of the Dets table Name into the Ets table EtsTab. The order in
which the objects are inserted is not specified. The existing objects of the Ets
table are kept unless overwritten.
traverse(Name, Fun) -> Return | {error, Reason}
Types:
Name = tab_name()
Fun = fun((Object) -> FunReturn)
Object = object()
FunReturn = continue
| {continue, Val}
| {done, Value}
| OtherValue
Return = [term()] | OtherValue
Val = Value = OtherValue = Reason = term()
Applies Fun to each object stored in the table Name in some unspecified order.
Different actions are taken depending on the return value of Fun. The following Fun
return values are allowed:
continue:
Continue to perform the traversal. For example, the following function can be
used to print out the contents of a table:
fun(X) -> io:format("~p~n", [X]), continue end.
{continue, Val}:
Continue the traversal and accumulate Val. The following function is supplied
in order to collect all objects of a table in a list:
fun(X) -> {continue, X} end.
{done, Value}:
Terminate the traversal and return [Value | Acc].
Any other value OtherValue returned by Fun terminates the traversal and is
immediately returned.
update_counter(Name, Key, Increment) -> Result
Types:
Name = tab_name()
Key = term()
Increment = {Pos, Incr} | Incr
Pos = Incr = Result = integer()
Updates the object with key Key stored in the table Name of type set by adding Incr
to the element at the Pos:th position. The new counter value is returned. If no
position is specified, the element directly following the key is updated.
This functions provides a way of updating a counter, without having to look up an
object, update the object by incrementing an element and insert the resulting
object into the table again.
SEE ALSO
ets(3erl), mnesia(3erl), qlc(3erl)