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NAME
qlc - Query interface to Mnesia, ETS, Dets, and so on.
DESCRIPTION
This module provides a query interface to Mnesia, ETS, Dets, and other data structures
that provide an iterator style traversal of objects.
OVERVIEW
This module provides a query interface to QLC tables. Typical QLC tables are Mnesia, ETS,
and Dets tables. Support is also provided for user-defined tables, see section
Implementing a QLC Table. A query is expressed using Query List Comprehensions (QLCs). The
answers to a query are determined by data in QLC tables that fulfill the constraints
expressed by the QLCs of the query. QLCs are similar to ordinary list comprehensions as
described in Erlang Reference Manual and Programming Examples, except that variables
introduced in patterns cannot be used in list expressions. In the absence of optimizations
and options such as cache and unique (see section Common Options, every QLC free of QLC
tables evaluates to the same list of answers as the identical ordinary list comprehension.
While ordinary list comprehensions evaluate to lists, calling q/1,2 returns a query
handle. To obtain all the answers to a query, eval/1,2 is to be called with the query
handle as first argument. Query handles are essentially functional objects (funs) created
in the module calling q/1,2. As the funs refer to the module code, be careful not to keep
query handles too long if the module code is to be replaced. Code replacement is described
in section Compilation and Code Loading in the Erlang Reference Manual. The list of
answers can also be traversed in chunks by use of a query cursor. Query cursors are
created by calling cursor/1,2 with a query handle as first argument. Query cursors are
essentially Erlang processes. One answer at a time is sent from the query cursor process
to the process that created the cursor.
SYNTAX
Syntactically QLCs have the same parts as ordinary list comprehensions:
[Expression || Qualifier1, Qualifier2, ...]
Expression (the template) is any Erlang expression. Qualifiers are either filters or
generators. Filters are Erlang expressions returning boolean(). Generators have the form
Pattern <- ListExpression, where ListExpression is an expression evaluating to a query
handle or a list. Query handles are returned from append/1,2, keysort/2,3, q/1,2,
sort/1,2, string_to_handle/1,2,3, and table/2.
EVALUATION
A query handle is evaluated in the following order:
* Inspection of options and the collection of information about tables. As a result,
qualifiers are modified during the optimization phase.
* All list expressions are evaluated. If a cursor has been created, evaluation takes
place in the cursor process. For list expressions that are QLCs, the list expressions
of the generators of the QLCs are evaluated as well. Be careful if list expressions
have side effects, as list expressions are evaluated in unspecified order.
* The answers are found by evaluating the qualifiers from left to right, backtracking
when some filter returns false, or collecting the template when all filters return
true.
Filters that do not return boolean() but fail are handled differently depending on their
syntax: if the filter is a guard, it returns false, otherwise the query evaluation fails.
This behavior makes it possible for the qlc module to do some optimizations without
affecting the meaning of a query. For example, when testing some position of a table and
one or more constants for equality, only the objects with equal values are candidates for
further evaluation. The other objects are guaranteed to make the filter return false, but
never fail. The (small) set of candidate objects can often be found by looking up some key
values of the table or by traversing the table using a match specification. It is
necessary to place the guard filters immediately after the table generator, otherwise the
candidate objects are not restricted to a small set. The reason is that objects that could
make the query evaluation fail must not be excluded by looking up a key or running a match
specification.
JOIN
The qlc module supports fast join of two query handles. Fast join is possible if some
position P1 of one query handler and some position P2 of another query handler are tested
for equality. Two fast join methods are provided:
* Lookup join traverses all objects of one query handle and finds objects of the other
handle (a QLC table) such that the values at P1 and P2 match or compare equal. The qlc
module does not create any indexes but looks up values using the key position and the
indexed positions of the QLC table.
* Merge join sorts the objects of each query handle if necessary and filters out objects
where the values at P1 and P2 do not compare equal. If many objects with the same
value of P2 exist, a temporary file is used for the equivalence classes.
The qlc module warns at compile time if a QLC combines query handles in such a way that
more than one join is possible. That is, no query planner is provided that can select a
good order between possible join operations. It is up to the user to order the joins by
introducing query handles.
The join is to be expressed as a guard filter. The filter must be placed immediately after
the two joined generators, possibly after guard filters that use variables from no other
generators but the two joined generators. The qlc module inspects the operands of =:=/2,
==/2, is_record/2, element/2, and logical operators (and/2, or/2, andalso/2, orelse/2,
xor/2) when determining which joins to consider.
COMMON OPTIONS
The following options are accepted by cursor/2, eval/2, fold/4, and info/2:
* {cache_all, Cache}, where Cache is equal to ets or list adds a {cache, Cache} option
to every list expression of the query except tables and lists. Defaults to {cache_all,
no}. Option cache_all is equivalent to {cache_all, ets}.
* {max_list_size, MaxListSize}, where MaxListSize is the size in bytes of terms on the
external format. If the accumulated size of collected objects exceeds MaxListSize, the
objects are written onto a temporary file. This option is used by option {cache, list}
and by the merge join method. Defaults to 512*1024 bytes.
* {tmpdir_usage, TmpFileUsage} determines the action taken when qlc is about to create
temporary files on the directory set by option tmpdir. If the value is not_allowed, an
error tuple is returned, otherwise temporary files are created as needed. Default is
allowed, which means that no further action is taken. The values info_msg,
warning_msg, and error_msg mean that the function with the corresponding name in
module error_logger is called for printing some information (currently the
stacktrace).
* {tmpdir, TempDirectory} sets the directory used by merge join for temporary files and
by option {cache, list}. The option also overrides option tmpdir of keysort/3 and
sort/2. Defaults to "", which means that the directory returned by file:get_cwd() is
used.
* {unique_all, true} adds a {unique, true} option to every list expression of the query.
Defaults to {unique_all, false}. Option unique_all is equivalent to {unique_all,
true}.
GETTING STARTED
As mentioned earlier, queries are expressed in the list comprehension syntax as described
in section Expressions in Erlang Reference Manual. In the following, some familiarity with
list comprehensions is assumed. The examples in section List Comprehensions in
Programming Examples can get you started. Notice that list comprehensions do not add any
computational power to the language; anything that can be done with list comprehensions
can also be done without them. But they add syntax for expressing simple search problems,
which is compact and clear once you get used to it.
Many list comprehension expressions can be evaluated by the qlc module. Exceptions are
expressions, such that variables introduced in patterns (or filters) are used in some
generator later in the list comprehension. As an example, consider an implementation of
lists:append(L): [X ||Y <- L, X <- Y]. Y is introduced in the first generator and used in
the second. The ordinary list comprehension is normally to be preferred when there is a
choice as to which to use. One difference is that eval/1,2 collects answers in a list that
is finally reversed, while list comprehensions collect answers on the stack that is
finally unwound.
What the qlc module primarily adds to list comprehensions is that data can be read from
QLC tables in small chunks. A QLC table is created by calling qlc:table/2. Usually
qlc:table/2 is not called directly from the query but through an interface function of
some data structure. Erlang/OTP includes a few examples of such functions:
mnesia:table/1,2, ets:table/1,2, and dets:table/1,2. For a given data structure, many
functions can create QLC tables, but common for these functions is that they return a
query handle created by qlc:table/2. Using the QLC tables provided by Erlang/OTP is
usually probably sufficient, but for the more advanced user section Implementing a QLC
Table describes the implementation of a function calling qlc:table/2.
Besides qlc:table/2, other functions return query handles. They are used more seldom than
tables, but are sometimes useful. qlc:append/1,2 traverses objects from many tables or
lists after each other. If, for example, you want to traverse all answers to a query QH
and then finish off by a term {finished}, you can do that by calling qlc:append(QH,
[{finished}]). append/2 first returns all objects of QH, then {finished}. If a tuple
{finished} exists among the answers to QH, it is returned twice from append/2.
As another example, consider concatenating the answers to two queries QH1 and QH2 while
removing all duplicates. This is accomplished by using option unique:
qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})
The cost is substantial: every returned answer is stored in an ETS table. Before returning
an answer, it is looked up in the ETS table to check if it has already been returned.
Without the unique option, all answers to QH1 would be returned followed by all answers to
QH2. The unique option keeps the order between the remaining answers.
If the order of the answers is not important, there is an alternative to the unique
option, namely to sort the answers uniquely:
qlc:sort(qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})).
This query also removes duplicates but the answers are sorted. If there are many answers,
temporary files are used. Notice that to get the first unique answer, all answers must be
found and sorted. Both alternatives find duplicates by comparing answers, that is, if A1
and A2 are answers found in that order, then A2 is a removed if A1 == A2.
To return only a few answers, cursors can be used. The following code returns no more than
five answers using an ETS table for storing the unique answers:
C = qlc:cursor(qlc:q([X || X <- qlc:append(QH1, QH2)],{unique,true})),
R = qlc:next_answers(C, 5),
ok = qlc:delete_cursor(C),
R.
QLCs are convenient for stating constraints on data from two or more tables. The following
example does a natural join on two query handles on position 2:
qlc:q([{X1,X2,X3,Y1} ||
{X1,X2,X3} <- QH1,
{Y1,Y2} <- QH2,
X2 =:= Y2])
The qlc module evaluates this differently depending on the query handles QH1 and QH2. If,
for example, X2 is matched against the key of a QLC table, the lookup join method
traverses the objects of QH2 while looking up key values in the table. However, if not X2
or Y2 is matched against the key or an indexed position of a QLC table, the merge join
method ensures that QH1 and QH2 are both sorted on position 2 and next do the join by
traversing the objects one by one.
Option join can be used to force the qlc module to use a certain join method. For the rest
of this section it is assumed that the excessively slow join method called "nested loop"
has been chosen:
qlc:q([{X1,X2,X3,Y1} ||
{X1,X2,X3} <- QH1,
{Y1,Y2} <- QH2,
X2 =:= Y2],
{join, nested_loop})
In this case the filter is applied to every possible pair of answers to QH1 and QH2, one
at a time. If there are M answers to QH1 and N answers to QH2, the filter is run M*N
times.
If QH2 is a call to the function for gb_trees, as defined in section Implementing a QLC
Table, then gb_table:table/1, the iterator for the gb-tree is initiated for each answer to
QH1. The objects of the gb-tree are then returned one by one. This is probably the most
efficient way of traversing the table in that case, as it takes minimal computational
power to get the following object. But if QH2 is not a table but a more complicated QLC,
it can be more efficient to use some RAM memory for collecting the answers in a cache,
particularly if there are only a few answers. It must then be assumed that evaluating QH2
has no side effects so that the meaning of the query does not change if QH2 is evaluated
only once. One way of caching the answers is to evaluate QH2 first of all and substitute
the list of answers for QH2 in the query. Another way is to use option cache. It is
expressed like this:
QH2' = qlc:q([X || X <- QH2], {cache, ets})
or only
QH2' = qlc:q([X || X <- QH2], cache)
The effect of option cache is that when generator QH2' is run the first time, every answer
is stored in an ETS table. When the next answer of QH1 is tried, answers to QH2' are
copied from the ETS table, which is very fast. As for option unique the cost is a possibly
substantial amount of RAM memory.
Option {cache, list} offers the possibility to store the answers in a list on the process
heap. This has the potential of being faster than ETS tables, as there is no need to copy
answers from the table. However, it can often result in slower evaluation because of more
garbage collections of the process heap and increased RAM memory consumption because of
larger heaps. Another drawback with cache lists is that if the list size exceeds a limit,
a temporary file is used. Reading the answers from a file is much slower than copying them
from an ETS table. But if the available RAM memory is scarce, setting the limit to some
low value is an alternative.
Option cache_all can be set to ets or list when evaluating a query. It adds a cache or
{cache, list} option to every list expression except QLC tables and lists on all levels of
the query. This can be used for testing if caching would improve efficiency at all. If the
answer is yes, further testing is needed to pinpoint the generators that are to be cached.
IMPLEMENTING A QLC TABLE
As an example of how to use function table/2, the implementation of a QLC table for the
gb_trees module is given:
-module(gb_table).
-export([table/1]).
table(T) ->
TF = fun() -> qlc_next(gb_trees:next(gb_trees:iterator(T))) end,
InfoFun = fun(num_of_objects) -> gb_trees:size(T);
(keypos) -> 1;
(is_sorted_key) -> true;
(is_unique_objects) -> true;
(_) -> undefined
end,
LookupFun =
fun(1, Ks) ->
lists:flatmap(fun(K) ->
case gb_trees:lookup(K, T) of
{value, V} -> [{K,V}];
none -> []
end
end, Ks)
end,
FormatFun =
fun({all, NElements, ElementFun}) ->
ValsS = io_lib:format("gb_trees:from_orddict(~w)",
[gb_nodes(T, NElements, ElementFun)]),
io_lib:format("gb_table:table(~s)", [ValsS]);
({lookup, 1, KeyValues, _NElements, ElementFun}) ->
ValsS = io_lib:format("gb_trees:from_orddict(~w)",
[gb_nodes(T, infinity, ElementFun)]),
io_lib:format("lists:flatmap(fun(K) -> "
"case gb_trees:lookup(K, ~s) of "
"{value, V} -> [{K,V}];none -> [] end "
"end, ~w)",
[ValsS, [ElementFun(KV) || KV <- KeyValues]])
end,
qlc:table(TF, [{info_fun, InfoFun}, {format_fun, FormatFun},
{lookup_fun, LookupFun},{key_equality,'=='}]).
qlc_next({X, V, S}) ->
[{X,V} | fun() -> qlc_next(gb_trees:next(S)) end];
qlc_next(none) ->
[].
gb_nodes(T, infinity, ElementFun) ->
gb_nodes(T, -1, ElementFun);
gb_nodes(T, NElements, ElementFun) ->
gb_iter(gb_trees:iterator(T), NElements, ElementFun).
gb_iter(_I, 0, _EFun) ->
'...';
gb_iter(I0, N, EFun) ->
case gb_trees:next(I0) of
{X, V, I} ->
[EFun({X,V}) | gb_iter(I, N-1, EFun)];
none ->
[]
end.
TF is the traversal function. The qlc module requires that there is a way of traversing
all objects of the data structure. gb_trees has an iterator function suitable for that
purpose. Notice that for each object returned, a new fun is created. As long as the list
is not terminated by [], it is assumed that the tail of the list is a nullary function and
that calling the function returns further objects (and functions).
The lookup function is optional. It is assumed that the lookup function always finds
values much faster than it would take to traverse the table. The first argument is the
position of the key. As qlc_next/1 returns the objects as {Key, Value} pairs, the position
is 1. Notice that the lookup function is to return {Key, Value} pairs, as the traversal
function does.
The format function is also optional. It is called by info/1,2 to give feedback at runtime
of how the query is to be evaluated. Try to give as good feedback as possible without
showing too much details. In the example, at most seven objects of the table are shown.
The format function handles two cases: all means that all objects of the table are
traversed; {lookup, 1, KeyValues} means that the lookup function is used for looking up
key values.
Whether the whole table is traversed or only some keys looked up depends on how the query
is expressed. If the query has the form
qlc:q([T || P <- LE, F])
and P is a tuple, the qlc module analyzes P and F in compile time to find positions of
tuple P that are tested for equality to constants. If such a position at runtime turns out
to be the key position, the lookup function can be used, otherwise all objects of the
table must be traversed. The info function InfoFun returns the key position. There can be
indexed positions as well, also returned by the info function. An index is an extra table
that makes lookup on some position fast. Mnesia maintains indexes upon request, and
introduces so called secondary keys. The qlc module prefers to look up objects using the
key before secondary keys regardless of the number of constants to look up.
KEY EQUALITY
Erlang/OTP has two operators for testing term equality: ==/2 and =:=/2. The difference is
all about the integers that can be represented by floats. For example, 2 == 2.0 evaluates
to true while 2 =:= 2.0 evaluates to false. Normally this is a minor issue, but the qlc
module cannot ignore the difference, which affects the user's choice of operators in QLCs.
If the qlc module at compile time can determine that some constant is free of integers, it
does not matter which one of ==/2 or =:=/2 is used:
1> E1 = ets:new(t, [set]), % uses =:=/2 for key equality
Q1 = qlc:q([K ||
{K} <- ets:table(E1),
K == 2.71 orelse K == a]),
io:format("~s~n", [qlc:info(Q1)]).
ets:match_spec_run(
lists:flatmap(fun(V) ->
ets:lookup(#Ref<0.3098908599.2283929601.256025>,
V)
end,
[a, 2.71]),
ets:match_spec_compile([{{'$1'}, [], ['$1']}]))
In the example, operator ==/2 has been handled exactly as =:=/2 would have been handled.
However, if it cannot be determined at compile time that some constant is free of
integers, and the table uses =:=/2 when comparing keys for equality (see option
key_equality), then the qlc module does not try to look up the constant. The reason is
that there is in the general case no upper limit on the number of key values that can
compare equal to such a constant; every combination of integers and floats must be looked
up:
2> E2 = ets:new(t, [set]),
true = ets:insert(E2, [{{2,2},a},{{2,2.0},b},{{2.0,2},c}]),
F2 = fun(I) ->
qlc:q([V || {K,V} <- ets:table(E2), K == I])
end,
Q2 = F2({2,2}),
io:format("~s~n", [qlc:info(Q2)]).
ets:table(#Ref<0.3098908599.2283929601.256125>,
[{traverse,
{select,
[{{'$1', '$2'}, [{'==', '$1', {const, {2, 2}}}], ['$2']}]}}])
3> lists:sort(qlc:e(Q2)).
[a,b,c]
Looking up only {2,2} would not return b and c.
If the table uses ==/2 when comparing keys for equality, the qlc module looks up the
constant regardless of which operator is used in the QLC. However, ==/2 is to be
preferred:
4> E3 = ets:new(t, [ordered_set]), % uses ==/2 for key equality
true = ets:insert(E3, [{{2,2.0},b}]),
F3 = fun(I) ->
qlc:q([V || {K,V} <- ets:table(E3), K == I])
end,
Q3 = F3({2,2}),
io:format("~s~n", [qlc:info(Q3)]).
ets:match_spec_run(ets:lookup(#Ref<0.3098908599.2283929601.256211>,
{2, 2}),
ets:match_spec_compile([{{'$1', '$2'}, [], ['$2']}]))
5> qlc:e(Q3).
[b]
Lookup join is handled analogously to lookup of constants in a table: if the join operator
is ==/2, and the table where constants are to be looked up uses =:=/2 when testing keys
for equality, then the qlc module does not consider lookup join for that table.
DATA TYPES
abstract_expr() = erl_parse:abstract_expr()
Parse trees for Erlang expression, see section The Abstract Format in the ERTS
User's Guide.
answer() = term()
answers() = [answer()]
cache() = ets | list | no
match_expression() = ets:match_spec()
Match specification, see section Match Specifications in Erlang in the ERTS User's
Guide and ms_transform(3erl).
no_files() = integer() >= 1
An integer > 1.
key_pos() = integer() >= 1 | [integer() >= 1]
max_list_size() = integer() >= 0
order() = ascending | descending | order_fun()
order_fun() = fun((term(), term()) -> boolean())
query_cursor()
A query cursor.
query_handle()
A query handle.
query_handle_or_list() = query_handle() | list()
query_list_comprehension() = term()
A literal query list comprehension.
spawn_options() = default | [proc_lib:spawn_option()]
sort_options() = [sort_option()] | sort_option()
sort_option() =
{compressed, boolean()} |
{no_files, no_files()} |
{order, order()} |
{size, integer() >= 1} |
{tmpdir, tmp_directory()} |
{unique, boolean()}
See file_sorter(3erl).
tmp_directory() = [] | file:name()
tmp_file_usage() =
allowed | not_allowed | info_msg | warning_msg | error_msg
EXPORTS
append(QHL) -> QH
Types:
QHL = [query_handle_or_list()]
QH = query_handle()
Returns a query handle. When evaluating query handle QH, all answers to the first
query handle in QHL are returned, followed by all answers to the remaining query
handles in QHL.
append(QH1, QH2) -> QH3
Types:
QH1 = QH2 = query_handle_or_list()
QH3 = query_handle()
Returns a query handle. When evaluating query handle QH3, all answers to QH1 are
returned, followed by all answers to QH2.
append(QH1, QH2) is equivalent to append([QH1, QH2]).
cursor(QH) -> Cursor
cursor(QH, Options) -> Cursor
Types:
QH = query_handle_or_list()
Options = [Option] | Option
Option =
{cache_all, cache()} |
cache_all |
{max_list_size, max_list_size()} |
{spawn_options, spawn_options()} |
{tmpdir_usage, tmp_file_usage()} |
{tmpdir, tmp_directory()} |
{unique_all, boolean()} |
unique_all
Cursor = query_cursor()
Creates a query cursor and makes the calling process the owner of the cursor. The
cursor is to be used as argument to next_answers/1,2 and (eventually)
delete_cursor/1. Calls erlang:spawn_opt/2 to spawn and link to a process that
evaluates the query handle. The value of option spawn_options is used as last
argument when calling spawn_opt/2. Defaults to [link].
Example:
1> QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),
QC = qlc:cursor(QH),
qlc:next_answers(QC, 1).
[{a,1}]
2> qlc:next_answers(QC, 1).
[{a,2}]
3> qlc:next_answers(QC, all_remaining).
[{b,1},{b,2}]
4> qlc:delete_cursor(QC).
ok
cursor(QH) is equivalent to cursor(QH, []).
delete_cursor(QueryCursor) -> ok
Types:
QueryCursor = query_cursor()
Deletes a query cursor. Only the owner of the cursor can delete the cursor.
e(QH) -> Answers | Error
e(QH, Options) -> Answers | Error
eval(QH) -> Answers | Error
eval(QH, Options) -> Answers | Error
Types:
QH = query_handle_or_list()
Answers = answers()
Options = [Option] | Option
Option =
{cache_all, cache()} |
cache_all |
{max_list_size, max_list_size()} |
{tmpdir_usage, tmp_file_usage()} |
{tmpdir, tmp_directory()} |
{unique_all, boolean()} |
unique_all
Error = {error, module(), Reason}
Reason = file_sorter:reason()
Evaluates a query handle in the calling process and collects all answers in a list.
Example:
1> QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),
qlc:eval(QH).
[{a,1},{a,2},{b,1},{b,2}]
eval(QH) is equivalent to eval(QH, []).
fold(Function, Acc0, QH) -> Acc1 | Error
fold(Function, Acc0, QH, Options) -> Acc1 | Error
Types:
QH = query_handle_or_list()
Function = fun((answer(), AccIn) -> AccOut)
Acc0 = Acc1 = AccIn = AccOut = term()
Options = [Option] | Option
Option =
{cache_all, cache()} |
cache_all |
{max_list_size, max_list_size()} |
{tmpdir_usage, tmp_file_usage()} |
{tmpdir, tmp_directory()} |
{unique_all, boolean()} |
unique_all
Error = {error, module(), Reason}
Reason = file_sorter:reason()
Calls Function on successive answers to the query handle together with an extra
argument AccIn. The query handle and the function are evaluated in the calling
process. Function must return a new accumulator, which is passed to the next call.
Acc0 is returned if there are no answers to the query handle.
Example:
1> QH = [1,2,3,4,5,6],
qlc:fold(fun(X, Sum) -> X + Sum end, 0, QH).
21
fold(Function, Acc0, QH) is equivalent to fold(Function, Acc0, QH, []).
format_error(Error) -> Chars
Types:
Error = {error, module(), term()}
Chars = io_lib:chars()
Returns a descriptive string in English of an error tuple returned by some of the
functions of the qlc module or the parse transform. This function is mainly used by
the compiler invoking the parse transform.
info(QH) -> Info
info(QH, Options) -> Info
Types:
QH = query_handle_or_list()
Options = [Option] | Option
Option = EvalOption | ReturnOption
EvalOption =
{cache_all, cache()} |
cache_all |
{max_list_size, max_list_size()} |
{tmpdir_usage, tmp_file_usage()} |
{tmpdir, tmp_directory()} |
{unique_all, boolean()} |
unique_all
ReturnOption =
{depth, Depth} |
{flat, boolean()} |
{format, Format} |
{n_elements, NElements}
Depth = infinity | integer() >= 0
Format = abstract_code | string
NElements = infinity | integer() >= 1
Info = abstract_expr() | string()
Returns information about a query handle. The information describes the
simplifications and optimizations that are the results of preparing the query for
evaluation. This function is probably mainly useful during debugging.
The information has the form of an Erlang expression where QLCs most likely occur.
Depending on the format functions of mentioned QLC tables, it is not certain that
the information is absolutely accurate.
Options:
* The default is to return a sequence of QLCs in a block, but if option {flat,
false} is specified, one single QLC is returned.
* The default is to return a string, but if option {format, abstract_code} is
specified, abstract code is returned instead. In the abstract code, port
identifiers, references, and pids are represented by strings.
* The default is to return all elements in lists, but if option {n_elements,
NElements} is specified, only a limited number of elements are returned.
* The default is to show all parts of objects and match specifications, but if
option {depth, Depth} is specified, parts of terms below a certain depth are
replaced by '...'.
info(QH) is equivalent to info(QH, []).
Examples:
In the following example two simple QLCs are inserted only to hold option {unique,
true}:
1> QH = qlc:q([{X,Y} || X <- [x,y], Y <- [a,b]]),
io:format("~s~n", [qlc:info(QH, unique_all)]).
begin
V1 =
qlc:q([
SQV ||
SQV <- [x, y]
],
[{unique, true}]),
V2 =
qlc:q([
SQV ||
SQV <- [a, b]
],
[{unique, true}]),
qlc:q([
{X,Y} ||
X <- V1,
Y <- V2
],
[{unique, true}])
end
In the following example QLC V2 has been inserted to show the joined generators and
the join method chosen. A convention is used for lookup join: the first generator
(G2) is the one traversed, the second (G1) is the table where constants are looked
up.
1> E1 = ets:new(e1, []),
E2 = ets:new(e2, []),
true = ets:insert(E1, [{1,a},{2,b}]),
true = ets:insert(E2, [{a,1},{b,2}]),
Q = qlc:q([{X,Z,W} ||
{X, Z} <- ets:table(E1),
{W, Y} <- ets:table(E2),
X =:= Y]),
io:format("~s~n", [qlc:info(Q)]).
begin
V1 =
qlc:q([
P0 ||
P0 = {W, Y} <-
ets:table(#Ref<0.3098908599.2283929601.256549>)
]),
V2 =
qlc:q([
[G1 | G2] ||
G2 <- V1,
G1 <-
ets:table(#Ref<0.3098908599.2283929601.256548>),
element(2, G1) =:= element(1, G2)
],
[{join, lookup}]),
qlc:q([
{X, Z, W} ||
[{X, Z} | {W, Y}] <- V2
])
end
keysort(KeyPos, QH1) -> QH2
keysort(KeyPos, QH1, SortOptions) -> QH2
Types:
KeyPos = key_pos()
SortOptions = sort_options()
QH1 = query_handle_or_list()
QH2 = query_handle()
Returns a query handle. When evaluating query handle QH2, the answers to query
handle QH1 are sorted by file_sorter:keysort/4 according to the options.
The sorter uses temporary files only if QH1 does not evaluate to a list and the
size of the binary representation of the answers exceeds Size bytes, where Size is
the value of option size.
keysort(KeyPos, QH1) is equivalent to keysort(KeyPos, QH1, []).
next_answers(QueryCursor) -> Answers | Error
next_answers(QueryCursor, NumberOfAnswers) -> Answers | Error
Types:
QueryCursor = query_cursor()
Answers = answers()
NumberOfAnswers = all_remaining | integer() >= 1
Error = {error, module(), Reason}
Reason = file_sorter:reason()
Returns some or all of the remaining answers to a query cursor. Only the owner of
QueryCursor can retrieve answers.
Optional argument NumberOfAnswers determines the maximum number of answers
returned. Defaults to 10. If less than the requested number of answers is returned,
subsequent calls to next_answers return [].
q(QLC) -> QH
q(QLC, Options) -> QH
Types:
QH = query_handle()
Options = [Option] | Option
Option =
{max_lookup, MaxLookup} |
{cache, cache()} |
cache |
{join, Join} |
{lookup, Lookup} |
{unique, boolean()} |
unique
MaxLookup = integer() >= 0 | infinity
Join = any | lookup | merge | nested_loop
Lookup = boolean() | any
QLC = query_list_comprehension()
Returns a query handle for a QLC. The QLC must be the first argument to this
function, otherwise it is evaluated as an ordinary list comprehension. It is also
necessary to add the following line to the source code:
-include_lib("stdlib/include/qlc.hrl").
This causes a parse transform to substitute a fun for the QLC. The (compiled) fun
is called when the query handle is evaluated.
When calling qlc:q/1,2 from the Erlang shell, the parse transform is automatically
called. When this occurs, the fun substituted for the QLC is not compiled but is
evaluated by erl_eval(3erl). This is also true when expressions are evaluated by
file:eval/1,2 or in the debugger.
To be explicit, this does not work:
...
A = [X || {X} <- [{1},{2}]],
QH = qlc:q(A),
...
Variable A is bound to the evaluated value of the list comprehension ([1,2]). The
compiler complains with an error message ("argument is not a query list
comprehension"); the shell process stops with a badarg reason.
q(QLC) is equivalent to q(QLC, []).
Options:
* Option {cache, ets} can be used to cache the answers to a QLC. The answers are
stored in one ETS table for each cached QLC. When a cached QLC is evaluated
again, answers are fetched from the table without any further computations.
Therefore, when all answers to a cached QLC have been found, the ETS tables
used for caching answers to the qualifiers of the QLC can be emptied. Option
cache is equivalent to {cache, ets}.
* Option {cache, list} can be used to cache the answers to a QLC like {cache,
ets}. The difference is that the answers are kept in a list (on the process
heap). If the answers would occupy more than a certain amount of RAM memory, a
temporary file is used for storing the answers. Option max_list_size sets the
limit in bytes and the temporary file is put on the directory set by option
tmpdir.
Option cache has no effect if it is known that the QLC is to be evaluated at
most once. This is always true for the top-most QLC and also for the list
expression of the first generator in a list of qualifiers. Notice that in the
presence of side effects in filters or callback functions, the answers to QLCs
can be affected by option cache.
* Option {unique, true} can be used to remove duplicate answers to a QLC. The
unique answers are stored in one ETS table for each QLC. The table is emptied
every time it is known that there are no more answers to the QLC. Option unique
is equivalent to {unique, true}. If option unique is combined with option
{cache, ets}, two ETS tables are used, but the full answers are stored in one
table only. If option unique is combined with option {cache, list}, the answers
are sorted twice using keysort/3; once to remove duplicates and once to restore
the order.
Options cache and unique apply not only to the QLC itself but also to the results
of looking up constants, running match specifications, and joining handles.
Example:
In the following example the cached results of the merge join are traversed for
each value of A. Notice that without option cache the join would have been carried
out three times, once for each value of A.
1> Q = qlc:q([{A,X,Z,W} ||
A <- [a,b,c],
{X,Z} <- [{a,1},{b,4},{c,6}],
{W,Y} <- [{2,a},{3,b},{4,c}],
X =:= Y],
{cache, list}),
io:format("~s~n", [qlc:info(Q)]).
begin
V1 =
qlc:q([
P0 ||
P0 = {X, Z} <-
qlc:keysort(1, [{a, 1}, {b, 4}, {c, 6}], [])
]),
V2 =
qlc:q([
P0 ||
P0 = {W, Y} <-
qlc:keysort(2, [{2, a}, {3, b}, {4, c}], [])
]),
V3 =
qlc:q([
[G1 | G2] ||
G1 <- V1,
G2 <- V2,
element(1, G1) == element(2, G2)
],
[{join, merge}, {cache, list}]),
qlc:q([
{A, X, Z, W} ||
A <- [a, b, c],
[{X, Z} | {W, Y}] <- V3,
X =:= Y
])
end
sort/1,2 and keysort/2,3 can also be used for caching answers and for removing
duplicates. When sorting answers are cached in a list, possibly stored on a
temporary file, and no ETS tables are used.
Sometimes (see table/2) traversal of tables can be done by looking up key values,
which is assumed to be fast. Under certain (rare) circumstances there can be too
many key values to look up. Option {max_lookup, MaxLookup} can then be used to
limit the number of lookups: if more than MaxLookup lookups would be required, no
lookups are done but the table is traversed instead. Defaults to infinity, which
means that there is no limit on the number of keys to look up.
Example:
In the following example, using the gb_table module from section Implementing a QLC
Table, there are six keys to look up: {1,a}, {1,b}, {1,c}, {2,a}, {2,b}, and {2,c}.
The reason is that the two elements of key {X, Y} are compared separately.
1> T = gb_trees:empty(),
QH = qlc:q([X || {{X,Y},_} <- gb_table:table(T),
((X == 1) or (X == 2)) andalso
((Y == a) or (Y == b) or (Y == c))]),
io:format("~s~n", [qlc:info(QH)]).
ets:match_spec_run(
lists:flatmap(fun(K) ->
case
gb_trees:lookup(K,
gb_trees:from_orddict([]))
of
{value, V} ->
[{K, V}];
none ->
[]
end
end,
[{1, a},
{1, b},
{1, c},
{2, a},
{2, b},
{2, c}]),
ets:match_spec_compile([{{{'$1', '$2'}, '_'},
[],
['$1']}]))
Options:
* Option {lookup, true} can be used to ensure that the qlc module looks up
constants in some QLC table. If there are more than one QLC table among the
list expressions of the generators, constants must be looked up in at least one
of the tables. The evaluation of the query fails if there are no constants to
look up. This option is useful when it would be unacceptable to traverse all
objects in some table. Setting option lookup to false ensures that no constants
are looked up ({max_lookup, 0} has the same effect). Defaults to any, which
means that constants are looked up whenever possible.
* Option {join, Join} can be used to ensure that a certain join method is used:
* {join, lookup} invokes the lookup join method.
* {join, merge} invokes the merge join method.
* {join, nested_loop} invokes the method of matching every pair of objects from
two handles. This method is mostly very slow.
The evaluation of the query fails if the qlc module cannot carry out the chosen
join method. Defaults to any, which means that some fast join method is used if
possible.
sort(QH1) -> QH2
sort(QH1, SortOptions) -> QH2
Types:
SortOptions = sort_options()
QH1 = query_handle_or_list()
QH2 = query_handle()
Returns a query handle. When evaluating query handle QH2, the answers to query
handle QH1 are sorted by file_sorter:sort/3 according to the options.
The sorter uses temporary files only if QH1 does not evaluate to a list and the
size of the binary representation of the answers exceeds Size bytes, where Size is
the value of option size.
sort(QH1) is equivalent to sort(QH1, []).
string_to_handle(QueryString) -> QH | Error
string_to_handle(QueryString, Options) -> QH | Error
string_to_handle(QueryString, Options, Bindings) -> QH | Error
Types:
QueryString = string()
Options = [Option] | Option
Option =
{max_lookup, MaxLookup} |
{cache, cache()} |
cache |
{join, Join} |
{lookup, Lookup} |
{unique, boolean()} |
unique
MaxLookup = integer() >= 0 | infinity
Join = any | lookup | merge | nested_loop
Lookup = boolean() | any
Bindings = erl_eval:binding_struct()
QH = query_handle()
Error = {error, module(), Reason}
Reason = erl_parse:error_info() | erl_scan:error_info()
A string version of q/1,2. When the query handle is evaluated, the fun created by
the parse transform is interpreted by erl_eval(3erl). The query string is to be one
single QLC terminated by a period.
Example:
1> L = [1,2,3],
Bs = erl_eval:add_binding('L', L, erl_eval:new_bindings()),
QH = qlc:string_to_handle("[X+1 || X <- L].", [], Bs),
qlc:eval(QH).
[2,3,4]
string_to_handle(QueryString) is equivalent to string_to_handle(QueryString, []).
string_to_handle(QueryString, Options) is equivalent to
string_to_handle(QueryString, Options, erl_eval:new_bindings()).
This function is probably mainly useful when called from outside of Erlang, for
example from a driver written in C.
table(TraverseFun, Options) -> QH
Types:
TraverseFun = TraverseFun0 | TraverseFun1
TraverseFun0 = fun(() -> TraverseResult)
TraverseFun1 = fun((match_expression()) -> TraverseResult)
TraverseResult = Objects | term()
Objects = [] | [term() | ObjectList]
ObjectList = TraverseFun0 | Objects
Options = [Option] | Option
Option =
{format_fun, FormatFun} |
{info_fun, InfoFun} |
{lookup_fun, LookupFun} |
{parent_fun, ParentFun} |
{post_fun, PostFun} |
{pre_fun, PreFun} |
{key_equality, KeyComparison}
FormatFun = undefined | fun((SelectedObjects) -> FormatedTable)
SelectedObjects =
all |
{all, NElements, DepthFun} |
{match_spec, match_expression()} |
{lookup, Position, Keys} |
{lookup, Position, Keys, NElements, DepthFun}
NElements = infinity | integer() >= 1
DepthFun = fun((term()) -> term())
FormatedTable = {Mod, Fun, Args} | abstract_expr() | string()
InfoFun = undefined | fun((InfoTag) -> InfoValue)
InfoTag = indices | is_unique_objects | keypos | num_of_objects
InfoValue = undefined | term()
LookupFun = undefined | fun((Position, Keys) -> LookupResult)
LookupResult = [term()] | term()
ParentFun = undefined | fun(() -> ParentFunValue)
PostFun = undefined | fun(() -> term())
PreFun = undefined | fun((PreArgs) -> term())
PreArgs = [PreArg]
PreArg = {parent_value, ParentFunValue} | {stop_fun, StopFun}
ParentFunValue = undefined | term()
StopFun = undefined | fun(() -> term())
KeyComparison = '=:=' | '=='
Position = integer() >= 1
Keys = [term()]
Mod = Fun = atom()
Args = [term()]
QH = query_handle()
Returns a query handle for a QLC table. In Erlang/OTP there is support for ETS,
Dets, and Mnesia tables, but many other data structures can be turned into QLC
tables. This is accomplished by letting function(s) in the module implementing the
data structure create a query handle by calling qlc:table/2. The different ways to
traverse the table and properties of the table are handled by callback functions
provided as options to qlc:table/2.
* Callback function TraverseFun is used for traversing the table. It is to return
a list of objects terminated by either [] or a nullary fun to be used for
traversing the not yet traversed objects of the table. Any other return value
is immediately returned as value of the query evaluation. Unary TraverseFuns
are to accept a match specification as argument. The match specification is
created by the parse transform by analyzing the pattern of the generator
calling qlc:table/2 and filters using variables introduced in the pattern. If
the parse transform cannot find a match specification equivalent to the pattern
and filters, TraverseFun is called with a match specification returning every
object.
* Modules that can use match specifications for optimized traversal of tables
are to call qlc:table/2 with an unary TraverseFun. An example is ets:table/2.
* Other modules can provide a nullary TraverseFun. An example is
gb_table:table/1 in section Implementing a QLC Table.
* Unary callback function PreFun is called once before the table is read for the
first time. If the call fails, the query evaluation fails.
Argument PreArgs is a list of tagged values. There are two tags, parent_value
and stop_fun, used by Mnesia for managing transactions.
* The value of parent_value is the value returned by ParentFun, or undefined if
there is no ParentFun. ParentFun is called once just before the call of
PreFun in the context of the process calling eval/1,2, fold/3,4, or
cursor/1,2.
* The value of stop_fun is a nullary fun that deletes the cursor if called from
the parent, or undefined if there is no cursor.
* Nullary callback function PostFun is called once after the table was last read.
The return value, which is caught, is ignored. If PreFun has been called for a
table, PostFun is guaranteed to be called for that table, even if the
evaluation of the query fails for some reason.
The pre (post) functions for different tables are evaluated in unspecified
order.
Other table access than reading, such as calling InfoFun, is assumed to be OK
at any time.
* Binary callback function LookupFun is used for looking up objects in the table.
The first argument Position is the key position or an indexed position and the
second argument Keys is a sorted list of unique values. The return value is to
be a list of all objects (tuples), such that the element at Position is a
member of Keys. Any other return value is immediately returned as value of the
query evaluation. LookupFun is called instead of traversing the table if the
parse transform at compile time can determine that the filters match and
compare the element at Position in such a way that only Keys need to be looked
up to find all potential answers.
The key position is obtained by calling InfoFun(keypos) and the indexed
positions by calling InfoFun(indices). If the key position can be used for
lookup, it is always chosen, otherwise the indexed position requiring the least
number of lookups is chosen. If there is a tie between two indexed positions,
the one occurring first in the list returned by InfoFun is chosen. Positions
requiring more than max_lookup lookups are ignored.
* Unary callback function InfoFun is to return information about the table.
undefined is to be returned if the value of some tag is unknown:
indices:
Returns a list of indexed positions, a list of positive integers.
is_unique_objects:
Returns true if the objects returned by TraverseFun are unique.
keypos:
Returns the position of the table key, a positive integer.
is_sorted_key:
Returns true if the objects returned by TraverseFun are sorted on the key.
num_of_objects:
Returns the number of objects in the table, a non-negative integer.
* Unary callback function FormatFun is used by info/1,2 for displaying the call
that created the query handle of the table. Defaults to undefined, which means
that info/1,2 displays a call to '$MOD':'$FUN'/0. It is up to FormatFun to
present the selected objects of the table in a suitable way. However, if a
character list is chosen for presentation, it must be an Erlang expression that
can be scanned and parsed (a trailing dot is added by info/1,2 though).
FormatFun is called with an argument that describes the selected objects based
on optimizations done as a result of analyzing the filters of the QLC where the
call to qlc:table/2 occurs. The argument can have the following values:
{lookup, Position, Keys, NElements, DepthFun}.:
LookupFun is used for looking up objects in the table.
{match_spec, MatchExpression}:
No way of finding all possible answers by looking up keys was found, but the
filters could be transformed into a match specification. All answers are
found by calling TraverseFun(MatchExpression).
{all, NElements, DepthFun}:
No optimization was found. A match specification matching all objects is used
if TraverseFun is unary.
NElements is the value of the info/1,2 option n_elements.
DepthFun is a function that can be used for limiting the size of terms;
calling DepthFun(Term) substitutes '...' for parts of Term below the depth
specified by the info/1,2 option depth.
If calling FormatFun with an argument including NElements and DepthFun fails,
FormatFun is called once again with an argument excluding NElements and
DepthFun ({lookup, Position, Keys} or all).
* The value of option key_equality is to be '=:=' if the table considers two keys
equal if they match, and to be '==' if two keys are equal if they compare
equal. Defaults to '=:='.
For the various options recognized by table/1,2 in respective module, see
ets(3erl), dets(3erl), and mnesia(3erl).
SEE ALSO
dets(3erl), erl_eval(3erl), erlang(3erl), error_logger(3erl), ets(3erl), file(3erl),
file_sorter(3erl), mnesia(3erl), shell(3erl), Erlang Reference Manual, Programming
Examples