path: root/lib/stdlib/doc/src/qlc.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2004</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>qlc</title>
    <prepared>Hans Bolinder</prepared>
    <responsible>nobody</responsible>
    <docno></docno>
    <approved>nobody</approved>
    <checked>no</checked>
    <date>2004-08-25</date>
    <rev>PA1</rev>
    <file>qlc.sgml</file>
  </header>
  <module>qlc</module>
  <modulesummary>Query Interface to Mnesia, ETS, Dets, etc</modulesummary>
  <description>
    <p>The <c>qlc</c> module provides a query interface to Mnesia, ETS,
      Dets and other data structures that implement an iterator style
      traversal of objects. </p>
  </description>

  <section><title>Overview</title>

    <p>The <c>qlc</c> module implements a query interface to <em>QLC
      tables</em>. Typical QLC tables are ETS, Dets, and Mnesia
      tables. There is also support for user defined tables, see the
      <seealso marker="#implementing_a_qlc_table">Implementing a QLC
      table</seealso> section. <marker
      id="query_list_comprehension"></marker>
      A <em>query</em> is stated using
      <em>Query List Comprehensions</em> (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 the Erlang
      Reference Manual and Programming Examples except that variables
      introduced in patterns cannot be used in list expressions. In
      fact, in the absence of optimizations and options such as
      <c>cache</c> and <c>unique</c> (see below), every QLC free of
      QLC tables evaluates to the same list of answers as the
      identical ordinary list comprehension.</p>

    <p>While ordinary list comprehensions evaluate to lists, calling
      <seealso marker="#q">qlc:q/1,2</seealso> returns a <marker
      id="query_handle"></marker><em> Query
      Handle</em>. To obtain all the answers to a query, <seealso
      marker="#eval">qlc:eval/1,2</seealso> should be called with the
      query handle as first argument. Query handles are essentially
      functional objects ("funs") created in the module calling <c>q/1,2</c>.
      As the funs refer to the module's code, one should
      be careful not to keep query handles too long if the module's
      code is to be replaced.
      Code replacement is described in the <seealso
      marker="doc/reference_manual:code_loading">Erlang Reference
      Manual</seealso>. The list of answers can also be traversed in
      chunks by use of a <marker
      id="query_cursor"></marker><em>Query Cursor</em>. Query cursors are
      created by calling <seealso
      marker="#cursor">qlc:cursor/1,2</seealso> 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.</p>

  </section>

  <section><title>Syntax</title>

    <p>Syntactically QLCs have the same parts as ordinary list
      comprehensions:</p>

    <code type="none">[Expression || Qualifier1, Qualifier2, ...]</code>

    <p><c>Expression</c> (the <em>template</em>) is an arbitrary
      Erlang expression. Qualifiers are either <em>filters</em> or
      <em>generators</em>. Filters are Erlang expressions returning
      <c>bool()</c>. Generators have the form
      <c><![CDATA[Pattern <- ListExpression]]></c>, where
      <c>ListExpression</c> is an expression evaluating to a query
      handle or a list. Query handles are returned from
      <c>qlc:table/2</c>, <c>qlc:append/1,2</c>, <c>qlc:sort/1,2</c>,
      <c>qlc:keysort/2,3</c>, <c>qlc:q/1,2</c>, and
      <c>qlc:string_to_handle/1,2,3</c>.</p>

  </section>

  <section><title>Evaluation</title>

    <p>The evaluation of a query handle begins by the inspection of
      options and the collection of information about tables. As a
      result qualifiers are modified during the optimization phase.
      Next all list expressions are evaluated. If a cursor has been
      created evaluation takes place in the cursor process. For those
      list expressions that are QLCs, the list expressions of the
      QLCs' generators are evaluated as well. One has to be careful if
      list expressions have side effects since the order in which list
      expressions are evaluated is unspecified. Finally the answers
      are found by evaluating the qualifiers from left to right,
      backtracking when some filter returns <c>false</c>, or
      collecting the template when all filters return <c>true</c>.</p>

    <p>Filters that do not return <c>bool()</c> but fail are handled
      differently depending on their syntax: if the filter is a guard
      it returns <c>false</c>, otherwise the query evaluation fails.
      This behavior makes it possible for the <c>qlc</c> 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 <c>false</c>, 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's generator, otherwise the candidate
      objects will not be 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.</p>

  </section>

  <section><title>Join</title>

    <p>The <c>qlc</c> module supports fast join of two query handles.
      Fast join is possible if some position <c>P1</c> of one query
      handler and some position <c>P2</c> of another query handler are
      tested for equality. Two fast join methods have been
      implemented:</p>

    <list type="bulleted">
      <item>Lookup join traverses all objects of one query handle and
         finds objects of the other handle (a QLC table) such that the
         values at <c>P1</c> and <c>P2</c> match or compare equal.
         The <c>qlc</c> module does not create
         any indices but looks up values using the key position and
         the indexed positions of the QLC table.
      </item>
      <item>Merge join sorts the objects of each query handle if
         necessary and filters out objects where the values at
         <c>P1</c> and <c>P2</c> do not compare equal. If there are
         many objects with the same value of <c>P2</c> a temporary
         file will be used for the equivalence classes.
      </item>
    </list>

    <p>The <c>qlc</c> module warns at compile time if a QLC
      combines query handles in such a way that more than one join is
      possible. In other words, there is no query planner that can
      choose a good order between possible join operations. It is up
      to the user to order the joins by introducing query handles.</p>

    <p>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 <c>qlc</c> module inspects 
      the operands of
      <c>=:=/2</c>, <c>==/2</c>, <c>is_record/2</c>, <c>element/2</c>,
      and logical operators (<c>and/2</c>, <c>or/2</c>,
      <c>andalso/2</c>, <c>orelse/2</c>, <c>xor/2</c>) when
      determining which joins to consider.</p>

  </section>

  <section><title>Common options</title>

    <p>The following options are accepted by <c>cursor/2</c>,
      <c>eval/2</c>, <c>fold/4</c>, and <c>info/2</c>:</p>

    <list type="bulleted">
      <item><c>{cache_all, Cache}</c> where <c>Cache</c> is
        equal to <c>ets</c> or <c>list</c> adds a
        <c>{cache,&nbsp;Cache}</c> option to every list expression
        of the query except tables and lists. Default is
        <c>{cache_all,&nbsp;no}</c>. The option <c>cache_all</c> is
        equivalent to <c>{cache_all,&nbsp;ets}</c>.
      </item>
      <item><c>{max_list_size, MaxListSize}</c> <marker
        id="max_list_size"></marker> where <c>MaxListSize</c> is the
        size in bytes of terms on the external format. If the
        accumulated size of collected objects exceeds
        <c>MaxListSize</c> the objects are written onto a temporary
        file. This option is used by the <c>{cache,&nbsp;list}</c>
        option as well as by the merge join method. Default is
        512*1024 bytes.
      </item>
      <item><c>{tmpdir_usage, TmpFileUsage}</c> determines the
        action taken when <c>qlc</c> is about to create temporary
        files on the directory set by the <c>tmpdir</c> option. If the
        value is <c>not_allowed</c> an error tuple is returned,
        otherwise temporary files are created as needed. Default is
        <c>allowed</c> which means that no further action is taken.
        The values <c>info_msg</c>, <c>warning_msg</c>, and
        <c>error_msg</c> mean that the function with the corresponding
        name in the module <c>error_logger</c> is called for printing
        some information (currently the stacktrace).
      </item>
      <item><c>{tmpdir, TempDirectory}</c> sets the directory used by
        merge join for temporary files and by the
        <c>{cache,&nbsp;list}</c> option. The option also overrides
        the <c>tmpdir</c> option of <c>keysort/3</c> and
        <c>sort/2</c>. The default value is <c>""</c> which means that
        the directory returned by <c>file:get_cwd()</c> is used.
      </item>
      <item><c>{unique_all, true}</c> adds a
        <c>{unique,&nbsp;true}</c> option to every list expression of
        the query. Default is <c>{unique_all,&nbsp;false}</c>. The
        option <c>unique_all</c> is equivalent to
        <c>{unique_all,&nbsp;true}</c>.
      </item>
    </list>

  </section>

  <section><title>Getting started</title>

    <p><marker id="getting_started"></marker> As already mentioned
      queries are stated in the list comprehension syntax as described
      in the <seealso marker="doc/reference_manual:expressions">Erlang
      Reference Manual</seealso>. In the following some familiarity
      with list comprehensions is assumed. There are examples in
      <seealso
      marker="doc/programming_examples:list_comprehensions">Programming
      Examples</seealso> that can get you started. It should be
      stressed 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 a
      syntax for expressing simple search problems which is compact
      and clear once you get used to it.</p>

    <p>Many list comprehension expressions can be evaluated by the
      <c>qlc</c> 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): 
      <c><![CDATA[[X ||Y <- L, X <- Y]]]></c>. 
      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
      <c>qlc:eval/1,2</c> collects answers in a list which is finally
      reversed, while list comprehensions collect answers on the stack
      which is finally unwound.</p>

    <p>What the <c>qlc</c> 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 <c>qlc:table/2</c>.
      Usually <c>qlc:table/2</c> is not called directly from the query
      but via an interface function of some data structure. There are
      a few examples of such functions in Erlang/OTP:
      <c>mnesia:table/1,2</c>, <c>ets:table/1,2</c>, and
      <c>dets:table/1,2</c>. For a given data structure there can be
      several functions that create QLC tables, but common for all
      these functions is that they return a query handle created by
      <c>qlc:table/2</c>. Using the QLC tables provided by OTP is
      probably sufficient in most cases, but for the more advanced
      user the section <seealso
      marker="#implementing_a_qlc_table">Implementing a QLC
      table</seealso> describes the implementation of a function
      calling <c>qlc:table/2</c>.</p>

    <p>Besides <c>qlc:table/2</c> there are other functions that
      return query handles. They might not be used as often as tables,
      but are useful from time to time. <c>qlc:append</c> traverses
      objects from several tables or lists after each other. If, for
      instance, you want to traverse all answers to a query QH and
      then finish off by a term <c>{finished}</c>, you can do that by
      calling <c>qlc:append(QH, [{finished}])</c>. <c>append</c> first
      returns all objects of QH, then <c>{finished}</c>. If there is
      one tuple <c>{finished}</c> among the answers to QH it will be
      returned twice from <c>append</c>.</p>

    <p>As another example, consider concatenating the answers to two
      queries QH1 and QH2 while removing all duplicates. The means to
      accomplish this is to use the <c>unique</c> option:</p> 

    <code type="none"><![CDATA[
qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})]]></code>

    <p>The cost is substantial: every returned answer will be 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 <c>unique</c> options all answers to QH1 would be returned
      followed by all answers to QH2. The <c>unique</c> options keeps
      the order between the remaining answers.</p>

    <p>If the order of the answers is not important there is the
      alternative to sort the answers uniquely:</p>

    <code type="none"><![CDATA[
qlc:sort(qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})).]]></code>

    <p>This query also removes duplicates but the answers will be
      sorted. If there are many answers temporary files will be used.
      Note that in order to get the first unique answer all answers
      have to 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.</p>

    <p>To return just 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:</p>

    <code type="none"><![CDATA[
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.]]></code>

    <p>Query list comprehensions are convenient for stating
      constraints on data from two or more tables. An example that
      does a natural join on two query handles on position 2:</p>

    <code type="none"><![CDATA[
qlc:q([{X1,X2,X3,Y1} || 
          {X1,X2,X3} <- QH1, 
          {Y1,Y2} <- QH2, 
          X2 =:= Y2])]]></code>

    <p>The <c>qlc</c> module will evaluate this differently depending on 
      the query
      handles <c>QH1</c> and <c>QH2</c>. If, for example, <c>X2</c> is
      matched against the key of a QLC table the lookup join method
      will traverse the objects of <c>QH2</c> while looking up key
      values in the table. On the other hand, if neither <c>X2</c> nor
      <c>Y2</c> is matched against the key or an indexed position of a
      QLC table, the merge join method will make sure that <c>QH1</c>
      and <c>QH2</c> are both sorted on position 2 and next do the
      join by traversing the objects one by one.</p>

    <p>The <c>join</c> option can be used to force the <c>qlc</c> 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:</p>

    <code type="none"><![CDATA[
qlc:q([{X1,X2,X3,Y1} || 
          {X1,X2,X3} <- QH1, 
          {Y1,Y2} <- QH2, 
          X2 =:= Y2],
      {join, nested_loop})]]></code>

    <p>In this case the filter will be 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 will be run M*N
      times.</p>

    <p>If QH2 is a call to the function for <c>gb_trees</c> as defined
      in the <seealso marker="#implementing_a_qlc_table">Implementing
      a QLC table</seealso> section, <c>gb_table:table/1</c>, the
      iterator for the gb-tree will be initiated for each answer to
      QH1 after which the objects of the gb-tree will be returned one
      by one. This is probably the most efficient way of traversing
      the table in that case since 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 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 the <c>cache</c> option. It is stated like this:</p>

    <code type="none"><![CDATA[
QH2' = qlc:q([X || X <- QH2], {cache, ets})]]></code>

    <p>or just</p> 

    <code type="none"><![CDATA[
QH2' = qlc:q([X || X <- QH2], cache)]]></code>

    <p>The effect of the <c>cache</c> option is that when the
      generator QH2' is run the first time every answer is stored in
      an ETS table. When next answer of QH1 is tried, answers to QH2'
      are copied from the ETS table which is very fast. As for the
      <c>unique</c> option the cost is a possibly substantial amount
      of RAM memory. The <c>{cache,&nbsp;list}</c> option offers the
      possibility to store the answers in a list on the process heap.
      While this has the potential of being faster than ETS tables
      since there is no need to copy answers from the table it can
      often result in slower evaluation due to more garbage
      collections of the process' heap as well as increased RAM memory
      consumption due to larger heaps. Another drawback with cache
      lists is that if the size of the list exceeds a limit a
      temporary file will be used. Reading the answers from a file is
      very much slower than copying them from an ETS table. But if the
      available RAM memory is scarce setting the <seealso
      marker="#max_list_size">limit</seealso> to some low value is an
      alternative.</p>

    <p>There is an option <c>cache_all</c> that can be set to
      <c>ets</c> or <c>list</c> when evaluating a query. It adds a
      <c>cache</c> or <c>{cache,&nbsp;list}</c> 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 should be cached.</p>

  </section>

  <section><title>Implementing a QLC table</title>

    <p><marker id="implementing_a_qlc_table"></marker>As an example of
      how to use the <seealso marker="#q">qlc:table/2</seealso>
      function the implementation of a QLC table for the <seealso
      marker="gb_trees">gb_trees</seealso> module is given:</p>

    <code type="none"><![CDATA[
-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.]]></code>

    <p><c>TF</c> is the traversal function. The <c>qlc</c> module
      requires that there is a way of traversing all objects of the
      data structure; in <c>gb_trees</c> there is an iterator function
      suitable for that purpose. Note that for each object returned a
      new fun is created. As long as the list is not terminated by
      <c>[]</c> it is assumed that the tail of the list is a nullary
      function and that calling the function returns further objects
      (and functions).</p>

    <p>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. Since <c>qlc_next</c> returns the objects as
      {Key,&nbsp;Value} pairs the position is 1. Note that the lookup
      function should return {Key,&nbsp;Value} pairs, just as the
      traversal function does.</p>

    <p>The format function is also optional. It is called by
      <c>qlc:info</c> to give feedback at runtime of how the query
      will be evaluated. One should try to give as good feedback as
      possible without showing too much details. In the example at
      most 7 objects of the table are shown. The format function
      handles two cases: <c>all</c> means that all objects of the
      table will be traversed; <c>{lookup,&nbsp;1,&nbsp;KeyValues}</c>
      means that the lookup function will be used for looking up key
      values.</p>

    <p>Whether the whole table will be traversed or just some keys
      looked up depends on how the query is stated. If the query has
      the form</p>

    <code type="none"><![CDATA[
qlc:q([T || P <- LE, F])]]></code>

    <p>and P is a tuple, the <c>qlc</c> module analyzes P and F in
      compile time to find positions of the 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 have to be traversed. It is
      the info function <c>InfoFun</c> that 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 indices upon request,
      thereby introducing so called secondary keys. The <c>qlc</c>
      module prefers to look up objects using the key before secondary
      keys regardless of the number of constants to look up.</p>

  </section>

  <section><title>Key equality</title>

    <p>In Erlang there are two operators for testing term equality,
      namely <c>==/2</c> and <c>=:=/2</c>. The difference between them
      is all about the integers that can be represented by floats. For
      instance, <c>2 == 2.0</c> evaluates to
      <c>true</c> while <c>2 =:= 2.0</c> evaluates to <c>false</c>.
      Normally this is a minor issue, but the <c>qlc</c> module cannot
      ignore the difference, which affects the user's choice of
      operators in QLCs.</p>

    <p>If the <c>qlc</c> module can find out at compile time that some
      constant is free of integers, it does not matter which one of
      <c>==/2</c> or <c>=:=/2</c> is used:</p>

    <pre>
1> <input>E1 = ets:new(t, [set]), % uses =:=/2 for key equality</input>
<input>Q1 = qlc:q([K ||</input>
<input>{K} &lt;- ets:table(E1),</input>
<input>K == 2.71 orelse K == a]),</input>
<input>io:format("~s~n", [qlc:info(Q1)]).</input>
ets:match_spec_run(lists:flatmap(fun(V) ->
                                        ets:lookup(20493, V)
                                 end,
                                 [a,2.71]),
                   ets:match_spec_compile([{{'$1'},[],['$1']}]))</pre>

    <p>In the example the <c>==/2</c> operator has been handled
      exactly as <c>=:=/2</c> would have been handled. On the other
      hand, if it cannot be determined at compile time that some
      constant is free of integers and the table uses <c>=:=/2</c>
      when comparing keys for equality (see the option <seealso
      marker="#key_equality">key_equality</seealso>), the
      <c>qlc</c> module will 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 has to be
      looked up:</p>

    <pre>
2> <input>E2 = ets:new(t, [set]),</input>
<input>true = ets:insert(E2, [{{2,2},a},{{2,2.0},b},{{2.0,2},c}]),</input>
<input>F2 = fun(I) -></input>
<input>qlc:q([V || {K,V} &lt;- ets:table(E2), K == I])</input>
<input>end,</input>
<input>Q2 = F2({2,2}),</input>
<input>io:format("~s~n", [qlc:info(Q2)]).</input>
ets:table(53264,
          [{traverse,
            {select,[{{'$1','$2'},[{'==','$1',{const,{2,2}}}],['$2']}]}}])
3> <input>lists:sort(qlc:e(Q2)).</input>
[a,b,c]</pre>

    <p>Looking up just <c>{2,2}</c> would not return <c>b</c> and
      <c>c</c>.</p>

    <p>If the table uses <c>==/2</c> when comparing keys for equality,
      the <c>qlc</c> module will look up the constant regardless of
      which operator is used in the QLC. However, <c>==/2</c> is to
      be preferred:</p>

    <pre>
4> <input>E3 = ets:new(t, [ordered_set]), % uses ==/2 for key equality</input>
<input>true = ets:insert(E3, [{{2,2.0},b}]),</input>
<input>F3 = fun(I) -></input>
<input>qlc:q([V || {K,V} &lt;- ets:table(E3), K == I])</input>
<input>end,</input>
<input>Q3 = F3({2,2}),</input>
<input>io:format("~s~n", [qlc:info(Q3)]).</input>
ets:match_spec_run(ets:lookup(86033, {2,2}),
                   ets:match_spec_compile([{{'$1','$2'},[],['$2']}]))
5> <input>qlc:e(Q3).</input>
[b]</pre>

    <p>Lookup join is handled analogously to lookup of constants in a
      table: if the join operator is <c>==/2</c> and the table where
      constants are to be looked up uses <c>=:=/2</c> when testing
      keys for equality, the <c>qlc</c> module will not consider
      lookup join for that table.</p>

  </section>

  <datatypes>
    <datatype>
      <name name="abstract_expr"></name>
      <desc><p>Parse trees for Erlang expression, see the <seealso
        marker="erts:absform">abstract format</seealso>
        documentation in the ERTS User's Guide.</p></desc>
    </datatype>
    <datatype>
      <name name="answer"></name>
    </datatype>
    <datatype>
      <name name="answers"></name>
    </datatype>
    <datatype>
      <name name="cache"></name>
    </datatype>
    <datatype>
      <name name="match_expression"></name>
      <desc><p>Match&nbsp;specification, see the <seealso
          marker="erts:match_spec">match specification</seealso>
          documentation in the ERTS User's Guide and <seealso
          marker="ms_transform">ms_transform(3).</seealso></p></desc>
    </datatype>
    <datatype>
      <name name="no_files"></name>
      <desc><p>Actually an integer > 1.</p></desc>
    </datatype>
    <datatype>
      <name name="key_pos"></name>
    </datatype>
    <datatype>
      <name name="max_list_size"></name>
    </datatype>
    <datatype>
      <name name="order"></name>
    </datatype>
    <datatype>
      <name name="order_fun"></name>
    </datatype>
    <datatype>
      <name name="query_cursor"></name>
      <desc><p>A <seealso marker="#query_cursor">query cursor</seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="query_handle"></name>
      <desc><p>A <seealso marker="#query_handle">query handle</seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="query_handle_or_list"></name>
    </datatype>
    <datatype>
      <name name="query_list_comprehension"></name>
      <desc><p>A literal
        <seealso marker="#query_list_comprehension">query
         list comprehension</seealso>.</p></desc>
    </datatype>
    <datatype>
      <name name="spawn_options"></name>
    </datatype>
    <datatype>
      <name name="sort_options"></name>
    </datatype>
    <datatype>
      <name name="sort_option"></name>
      <desc><p>See <seealso
        marker="file_sorter">file_sorter(3)</seealso>.</p></desc>
    </datatype>
    <datatype>
      <name name="tmp_directory"></name>
    </datatype>
    <datatype>
      <name name="tmp_file_usage"></name>
    </datatype>
  </datatypes>

  <funcs>

    <func>
      <name name="append" arity="1"/>
      <fsummary>Return a query handle.</fsummary>
      <desc>
        <p>Returns a query handle. When evaluating the query handle
          <c><anno>QH</anno></c> all answers to the first query handle in
          <c><anno>QHL</anno></c> are returned followed by all answers
          to the rest of the query handles in <c><anno>QHL</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="append" arity="2"/>
      <fsummary>Return a query handle.</fsummary>
      <desc>
        <p>Returns a query handle. When evaluating the query handle
          <c><anno>QH3</anno></c> all answers to
          <c><anno>QH1</anno></c> are returned followed by all answers
          to <c><anno>QH2</anno></c>.</p>

        <p><c>append(QH1,&nbsp;QH2)</c> is equivalent to
          <c>append([QH1,&nbsp;QH2])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="cursor" arity="1"/>
      <name name="cursor" arity="2"/>
      <fsummary>Create a query cursor.</fsummary>
      <desc>
        <p><marker id="cursor"></marker>Creates a query cursor and
          makes the calling process the owner of the cursor. The
          cursor is to be used as argument to <c>next_answers/1,2</c>
          and (eventually) <c>delete_cursor/1</c>. Calls
          <c>erlang:spawn_opt</c> to spawn and link a process which
          will evaluate the query handle. The value of the option
          <c>spawn_options</c> is used as last argument when calling
          <c>spawn_opt</c>. The default value is <c>[link]</c>.</p>

        <pre>
1> <input>QH = qlc:q([{X,Y} || X &lt;- [a,b], Y &lt;- [1,2]]),</input>
<input>QC = qlc:cursor(QH),</input>
<input>qlc:next_answers(QC, 1).</input>
[{a,1}]
2> <input>qlc:next_answers(QC, 1).</input>
[{a,2}]
3> <input>qlc:next_answers(QC, all_remaining).</input>
[{b,1},{b,2}]
4> <input>qlc:delete_cursor(QC).</input>
ok</pre>
        <p><c>cursor(<anno>QH</anno>)</c> is equivalent to
          <c>cursor(<anno>QH</anno>, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="delete_cursor" arity="1"/>
      <fsummary>Delete a query cursor.</fsummary>
      <desc>
        <p>Deletes a query cursor. Only the owner of the cursor can
          delete the cursor.</p>
      </desc>
    </func>

    <func>
      <name name="eval" arity="1"/>
      <name name="eval" arity="2"/>
      <name name="e" arity="1"/>
      <name name="e" arity="2"/>
      <fsummary>Return all answers to a query.</fsummary>
      <desc>
        <p><marker id="eval"></marker>Evaluates a query handle in the
          calling process and collects all answers in a list.</p>

        <pre>
1> <input>QH = qlc:q([{X,Y} || X &lt;- [a,b], Y &lt;- [1,2]]),</input>
<input>qlc:eval(QH).</input>
[{a,1},{a,2},{b,1},{b,2}]</pre>
        <p><c>eval(<anno>QH</anno>)</c> is equivalent to
          <c>eval(<anno>QH</anno>, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="fold" arity="3"/>
      <name name="fold" arity="4"/>
      <fsummary>Fold a function over the answers to a query.</fsummary>
      <desc>
        <p>Calls <c><anno>Function</anno></c> on successive answers to
          the query handle together with an extra argument
          <c><anno>AccIn</anno></c>. The query handle and the function
          are evaluated in the calling process.
          <c><anno>Function</anno></c> must return a new accumulator
          which is passed to the next call.
          <c><anno>Acc0</anno></c> is returned if there are no answers
          to the query handle.</p>

        <pre>
1> <input>QH = [1,2,3,4,5,6],</input>
<input>qlc:fold(fun(X, Sum) -> X + Sum end, 0, QH).</input>
21</pre>
        <p><c>fold(<anno>Function</anno>, <anno>Acc0</anno>,
          <anno>QH</anno>)</c> is equivalent to
          <c>fold(<anno>Function</anno>, <anno>Acc0</anno>,
          <anno>QH</anno>, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="format_error" arity="1"/>
      <fsummary>Return an English description of a an error tuple.</fsummary>
      <desc>
        <p>Returns a descriptive string in English of an error tuple
          returned by some of the functions of the <c>qlc</c> module
          or the parse transform. This function is mainly used by the
          compiler invoking the parse transform.</p>
      </desc>
    </func>

    <func>
      <name name="info" arity="1"/>
      <name name="info" arity="2"/>
      <fsummary>Return code describing a query handle.</fsummary>
      <desc>
        <p><marker id="info"></marker>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 useful
          mostly during debugging.</p>

        <p>The information has the form of an Erlang expression where
          QLCs most likely occur. Depending on the format functions of
          mentioned QLC tables it may not be absolutely accurate.</p>

        <p>The default is to return a sequence of QLCs in a block, but
          if the option <c>{flat,&nbsp;false}</c> is given, one single
          QLC is returned. The default is to return a string, but if
          the option <c>{format,&nbsp;abstract_code}</c> is given,
          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 the
          <c>{n_elements,&nbsp;NElements}</c> option is given, only a
          limited number of elements are returned. The default is to
          show all of objects and match specifications, but if the
          <c>{depth,&nbsp;Depth}</c> option is given, parts of terms
          below a certain depth are replaced by <c>'...'</c>.</p>

        <pre>
1> <input>QH = qlc:q([{X,Y} || X &lt;- [x,y], Y &lt;- [a,b]]),</input>
<input>io:format("~s~n", [qlc:info(QH, unique_all)]).</input>
begin
    V1 =
        qlc:q([
               SQV ||
                   SQV &lt;- [x,y]
              ],
              [{unique,true}]),
    V2 =
        qlc:q([
               SQV ||
                   SQV &lt;- [a,b]
              ],
              [{unique,true}]),
    qlc:q([
           {X,Y} ||
               X &lt;- V1,
               Y &lt;- V2
          ],
          [{unique,true}])
end</pre>

        <p>In this example two simple QLCs have been inserted just to
          hold the <c>{unique,&nbsp;true}</c> option.</p>

        <pre>
1> <input>E1 = ets:new(e1, []),</input>
<input>E2 = ets:new(e2, []),</input>
<input>true = ets:insert(E1, [{1,a},{2,b}]),</input>
<input>true = ets:insert(E2, [{a,1},{b,2}]),</input>
<input>Q = qlc:q([{X,Z,W} ||</input>
<input>{X, Z} &lt;- ets:table(E1),</input>
<input>{W, Y} &lt;- ets:table(E2),</input>
<input>X =:= Y]),</input>
<input>io:format("~s~n", [qlc:info(Q)]).</input>
begin
    V1 =
        qlc:q([
               P0 ||
                   P0 = {W,Y} &lt;- ets:table(17)
              ]),
    V2 =
        qlc:q([
               [G1|G2] ||
                   G2 &lt;- V1,
                   G1 &lt;- ets:table(16),
                   element(2, G1) =:= element(1, G2)
              ],
              [{join,lookup}]),
    qlc:q([
           {X,Z,W} ||
               [{X,Z}|{W,Y}] &lt;- V2
          ])
end</pre>

        <p>In this example the query list comprehension <c>V2</c> has
          been inserted to show the joined generators and the join
          method chosen. A convention is used for lookup join: the
          first generator (<c>G2</c>) is the one traversed, the second
          one (<c>G1</c>) is the table where constants are looked up.</p>

        <p><c>info(<anno>QH</anno>)</c> is equivalent to
          <c>info(<anno>QH</anno>, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keysort" arity="2"/>
      <name name="keysort" arity="3"/>
      <fsummary>Return a query handle.</fsummary>
      <desc>
        <p>Returns a query handle. When evaluating the query handle
          <c><anno>QH2</anno></c> the answers to the query handle
          <c><anno>QH1</anno></c> are sorted by <seealso
          marker="file_sorter">file_sorter:keysort/4</seealso>
          according to the options.</p>

        <p>The sorter will use temporary files only if
          <c><anno>QH1</anno></c> does not evaluate to a list and the
          size of the binary representation of the answers exceeds
          <c>Size</c> bytes, where <c>Size</c> is the value of the
          <c>size</c> option.</p>

        <p><c>keysort(<anno>KeyPos</anno>, <anno>QH1</anno>)</c>
          is equivalent to
          <c>keysort(<anno>KeyPos</anno>, <anno>QH1</anno>, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="next_answers" arity="1"/>
      <name name="next_answers" arity="2"/>
      <fsummary>Return some or all answers to a query.</fsummary>
      <desc>
        <p>Returns some or all of the remaining answers to a query
          cursor. Only the owner of <c><anno>QueryCursor</anno></c> can
          retrieve answers.</p>
        <p>The optional argument <c>NumberOfAnswers</c>determines the
          maximum number of answers returned. The default value is
          <c>10</c>. If less than the requested number of answers is
          returned, subsequent calls to <c>next_answers</c> will
          return <c>[]</c>.</p>
      </desc>
    </func>

    <func>
      <name name="q" arity="1"/>
      <name name="q" arity="2"/>
      <fsummary>Return a handle for a query list comprehension.</fsummary>
      <desc>
        <p><marker id="q"></marker>Returns a query handle for a query
          list comprehension. The query list comprehension must be the
          first argument to <c>qlc:q/1,2</c> or it will be evaluated
          as an ordinary list comprehension. It is also necessary to
          add the line</p>

        <code type="none">
-include_lib("stdlib/include/qlc.hrl").</code>

        <p>to the source file. This causes a parse transform to
          substitute a fun for the query list comprehension. The
          (compiled) fun will be called when the query handle is
          evaluated.</p>

        <p>When calling <c>qlc:q/1,2</c> from the Erlang shell the
          parse transform is automatically called. When this happens
          the fun substituted for the query list comprehension is not
          compiled but will be evaluated by <c>erl_eval(3)</c>. This
          is also true when expressions are evaluated by means of
          <c>file:eval/1,2</c> or in the debugger.</p>

        <p>To be very explicit, this will not work:</p>

        <pre>
...
A = [X || {X} &lt;- [{1},{2}]],
QH = qlc:q(A),
...</pre>

        <p>The variable <c>A</c> will be bound to the evaluated value
          of the list comprehension (<c>[1,2]</c>). The compiler
          complains with an error message ("argument is not a query
          list comprehension"); the shell process stops with a
          <c>badarg</c> reason.</p>

        <p><c>q(<anno>QLC</anno>)</c> is equivalent to
          <c>q(<anno>QLC</anno>, [])</c>.</p>

        <p>The <c>{cache,&nbsp;ets}</c> option can be used to cache
          the answers to a query list comprehension. The answers are
          stored in one ETS table for each cached query list
          comprehension. When a cached query list comprehension is
          evaluated again, answers are fetched from the table without
          any further computations. As a consequence, when all answers
          to a cached query list comprehension have been found, the
          ETS tables used for caching answers to the query list
          comprehension's qualifiers can be emptied. The option
          <c>cache</c> is equivalent to <c>{cache,&nbsp;ets}</c>.</p>

        <p>The <c>{cache,&nbsp;list}</c> option can be used to cache
          the answers to a query list comprehension just like
          <c>{cache,&nbsp;ets}</c>. 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. The option
          <c>max_list_size</c> sets the limit in bytes and the temporary
          file is put on the directory set by the <c>tmpdir</c> option.</p>

        <p>The <c>cache</c> option has no effect if it is known that
          the query list comprehension will be evaluated at most once.
          This is always true for the top-most query list
          comprehension and also for the list expression of the first
          generator in a list of qualifiers. Note that in the presence
          of side effects in filters or callback functions the answers
          to query list comprehensions can be affected by the
          <c>cache</c> option.</p>

        <p>The <c>{unique,&nbsp;true}</c> option can be used to remove
          duplicate answers to a query list comprehension. The unique
          answers are stored in one ETS table for each query list
          comprehension. The table is emptied every time it is known
          that there are no more answers to the query list
          comprehension. The option <c>unique</c> is equivalent to
          <c>{unique,&nbsp;true}</c>. If the <c>unique</c> option is
          combined with the <c>{cache,&nbsp;ets}</c> option, two ETS
          tables are used, but the full answers are stored in one
          table only. If the <c>unique</c> option is combined with the
          <c>{cache,&nbsp;list}</c> option the answers are sorted
          twice using <c>keysort/3</c>; once to remove duplicates, and
          once to restore the order.</p>

        <p>The <c>cache</c> and <c>unique</c> options apply not only
          to the query list comprehension itself but also to the
          results of looking up constants, running match
          specifications, and joining handles. </p>

        <pre>
1> <input>Q = qlc:q([{A,X,Z,W} ||</input>
<input>A &lt;- [a,b,c],</input>
<input>{X,Z} &lt;- [{a,1},{b,4},{c,6}],</input>
<input>{W,Y} &lt;- [{2,a},{3,b},{4,c}],</input>
<input>X =:= Y],</input>
<input>{cache, list}),</input>
<input>io:format("~s~n", [qlc:info(Q)]).</input>
begin
    V1 =
        qlc:q([
               P0 ||
                   P0 = {X,Z} &lt;-
                       qlc:keysort(1, [{a,1},{b,4},{c,6}], [])
              ]),
    V2 =
        qlc:q([
               P0 ||
                   P0 = {W,Y} &lt;-
                       qlc:keysort(2, [{2,a},{3,b},{4,c}], [])
              ]),
    V3 =
        qlc:q([
               [G1|G2] ||
                   G1 &lt;- V1,
                   G2 &lt;- V2,
                   element(1, G1) == element(2, G2)
              ],
              [{join,merge},{cache,list}]),
    qlc:q([
           {A,X,Z,W} ||
               A &lt;- [a,b,c],
               [{X,Z}|{W,Y}] &lt;- V3,
               X =:= Y
          ])
end</pre>

        <p>In this example the cached results of the merge join are
          traversed for each value of <c>A</c>. Note that without the
          <c>cache</c> option the join would have been carried out
          three times, once for each value of <c>A</c></p>

        <p><c>sort/1,2</c> and <c>keysort/2,3</c> 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.</p>

        <p>Sometimes (see <seealso
          marker="#lookup_fun">qlc:table/2</seealso> below) traversal
          of tables can be done by looking up key values, which is
          assumed to be fast. Under certain (rare) circumstances it
          could happen that there are too many key values to look up.
          <marker id="max_lookup"></marker> The
          <c>{max_lookup,&nbsp;MaxLookup}</c> option can then be used
          to limit the number of lookups: if more than
          <c>MaxLookup</c> lookups would be required no lookups are
          done but the table traversed instead. The default value is
          <c>infinity</c> which means that there is no limit on the
          number of keys to look up.</p>
        <pre>
1> <input>T = gb_trees:empty(),</input>
<input>QH = qlc:q([X || {{X,Y},_} &lt;- gb_table:table(T),</input>
<input>((X == 1) or (X == 2)) andalso</input>
<input>((Y == a) or (Y == b) or (Y == c))]),</input>
<input>io:format("~s~n", [qlc:info(QH)]).</input>
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']}]))</pre>

        <p>In this example using the <c>gb_table</c> module from the
          <seealso marker="#implementing_a_qlc_table">Implementing a
          QLC table</seealso> section there are six keys to look up:
          <c>{1,a}</c>, <c>{1,b}</c>, <c>{1,c}</c>, <c>{2,a}</c>,
          <c>{2,b}</c>, and <c>{2,c}</c>. The reason is that the two
          elements of the key {X,&nbsp;Y} are compared separately.</p>

        <p>The <c>{lookup,&nbsp;true}</c> option can be used to ensure
          that the <c>qlc</c> module will look up constants in some
          QLC table. If there
          are more than one QLC table among the generators' list
          expressions, constants have to 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 in
          situations when it would be unacceptable to traverse all
          objects in some table. Setting the <c>lookup</c> option to
          <c>false</c> ensures that no constants will be looked up
          (<c>{max_lookup,&nbsp;0}</c> has the same effect). The
          default value is <c>any</c> which means that constants will
          be looked up whenever possible.</p>

        <p>The <c>{join,&nbsp;Join}</c> option can be used to ensure
          that a certain join method will be used:
          <c>{join,&nbsp;lookup}</c> invokes the lookup join method;
          <c>{join,&nbsp;merge}</c> invokes the merge join method; and
          <c>{join,&nbsp;nested_loop}</c> invokes the method of
          matching every pair of objects from two handles. The last
          method is mostly very slow. The evaluation of the query
          fails if the <c>qlc</c> module cannot carry out the chosen
          join method. The
          default value is <c>any</c> which means that some fast join
          method will be used if possible.</p>
      </desc>
    </func>

    <func>
      <name name="sort" arity="1"/>
      <name name="sort" arity="2"/>
      <fsummary>Return a query handle.</fsummary>
      <desc>
        <p>Returns a query handle. When evaluating the query handle
          <c><anno>QH2</anno></c> the answers to the query handle
          <c><anno>QH1</anno></c> are sorted by <seealso
          marker="file_sorter">file_sorter:sort/3</seealso> according
          to the options.</p>

        <p>The sorter will use temporary files only if
          <c><anno>QH1</anno></c> does not evaluate to a list and the
          size of the binary representation of the answers exceeds
          <c>Size</c> bytes, where <c>Size</c> is the value of the
          <c>size</c> option.</p>

        <p><c>sort(<anno>QH1</anno>)</c> is equivalent to
          <c>sort(<anno>QH1</anno>, [])</c>.</p>

      </desc>
    </func>

    <func>
      <name name="string_to_handle" arity="1"/>
      <name name="string_to_handle" arity="2"/>
      <name name="string_to_handle" arity="3"/>
      <fsummary>Return a handle for a query list comprehension.</fsummary>
      <desc>
        <p>A string version of <c>qlc:q/1,2</c>. When the query handle
          is evaluated the fun created by the parse transform is
          interpreted by <c>erl_eval(3)</c>. The query string is to be
          one single query list comprehension terminated by a
          period.</p>

        <pre>
1> <input>L = [1,2,3],</input>
<input>Bs = erl_eval:add_binding('L', L, erl_eval:new_bindings()),</input>
<input>QH = qlc:string_to_handle("[X+1 || X &lt;- L].", [], Bs),</input>
<input>qlc:eval(QH).</input>
[2,3,4]</pre>

        <p><c>string_to_handle(<anno>QueryString</anno>)</c>
          is equivalent to
          <c>string_to_handle(<anno>QueryString</anno>, [])</c>.</p>

        <p><c>string_to_handle(<anno>QueryString</anno>,
          <anno>Options</anno>)</c>
          is equivalent to
          <c>string_to_handle(<anno>QueryString</anno>,
          <anno>Options</anno>, erl_eval:new_bindings())</c>.</p>

        <p>This function is probably useful mostly when called from
          outside of Erlang, for instance from a driver written in C.</p>
      </desc>
    </func>

    <func>
      <name name="table" arity="2"/>
      <fsummary>Return a query handle for a table.</fsummary>
      <desc>
        <p><marker id="table"></marker>Returns a query handle for a
          QLC table. In Erlang/OTP there is support for ETS, Dets and
          Mnesia tables, but it is also possible to turn many other
          data structures into QLC tables. The way to accomplish this
          is to let function(s) in the module implementing the data
          structure create a query handle by calling
          <c>qlc:table/2</c>. The different ways to traverse the table
          as well as properties of the table are handled by callback
          functions provided as options to <c>qlc:table/2</c>.</p>

        <p>The callback function <c><anno>TraverseFun</anno></c> is
          used for traversing the table. It is to return a list of
          objects terminated by either <c>[]</c> 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
          <c><anno>TraverseFun</anno></c>s 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 <c>qlc:table/2</c> and filters using
          variables introduced in the pattern. If the parse transform
          cannot find a match specification equivalent to the pattern
          and filters, <c><anno>TraverseFun</anno></c> will be called
          with a match specification returning every object. Modules
          that can utilize match specifications for optimized
          traversal of tables should call <c>qlc:table/2</c> with a
          unary
          <c><anno>TraverseFun</anno></c> while other modules can
          provide a nullary
          <c><anno>TraverseFun</anno></c>. <c>ets:table/2</c> is an
          example of the former; <c>gb_table:table/1</c> in the
          <seealso marker="#implementing_a_qlc_table">Implementing a
          QLC table</seealso> section is an example of the latter.</p>

        <p><c><anno>PreFun</anno></c> is a unary callback function
          that is called once before the table is read for the first
          time. If the call fails, the query evaluation fails.
          Similarly, the nullary callback function
          <c><anno>PostFun</anno></c> is called once after the table
          was last read. The return value, which is caught, is
          ignored. If <c><anno>PreFun</anno></c> has been called for a
          table,
          <c><anno>PostFun</anno></c> is guaranteed to be called for
          that table, even if the evaluation of the query fails for
          some reason. The order in which pre (post) functions for
          different tables are evaluated is not specified. Other table
          access than reading, such as calling
          <c><anno>InfoFun</anno></c>, is assumed to be OK at any
          time. The argument <c><anno>PreArgs</anno></c> is a list of
          tagged values. Currently there are two tags,
          <c>parent_value</c> and <c>stop_fun</c>, used by Mnesia for
          managing transactions. The value of <c>parent_value</c> is
          the value returned by <c><anno>ParentFun</anno></c>, or
          <c>undefined</c> if there is no <c>ParentFun</c>.
          <c><anno>ParentFun</anno></c> is called once just before the
          call of
          <c><anno>PreFun</anno></c> in the context of the process
          calling
          <c>eval</c>, <c>fold</c>, or
          <c>cursor</c>. The value of <c>stop_fun</c> is a nullary fun
          that deletes the cursor if called from the parent, or
          <c>undefined</c> if there is no cursor.</p>

        <p><marker id="lookup_fun"></marker>The binary callback
          function <c><anno>LookupFun</anno></c> is used for looking
          up objects in the table. The first argument
          <c><anno>Position</anno></c> is the key position or an
          indexed position and the second argument
          <c><anno>Keys</anno></c> is a sorted list of unique values.
          The return value is to be a list of all objects (tuples)
          such that the element at <c>Position</c> is a member of
          <c><anno>Keys</anno></c>. Any other return value is
          immediately returned as value of the query evaluation.
          <c><anno>LookupFun</anno></c> is called instead of
          traversing the table if the parse transform at compile time
          can find out that the filters match and compare the element
          at <c><anno>Position</anno></c> in such a way that only
          <c><anno>Keys</anno></c> need to be looked up in order to
          find all potential answers. The key position is obtained by
          calling
          <c><anno>InfoFun</anno>(keypos)</c> and the indexed
          positions by calling
          <c><anno>InfoFun</anno>(indices)</c>. 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
          <c><anno>InfoFun</anno></c> is chosen. Positions requiring
          more than <seealso marker="#max_lookup">max_lookup</seealso>
          lookups are ignored.</p>

        <p>The unary callback function <c><anno>InfoFun</anno></c> is
          to return information about the table. <c>undefined</c>
          should be returned if the value of some tag is unknown:</p>

        <list type="bulleted">
          <item><c>indices</c>. Returns a list of indexed
           positions, a list of positive integers.
          </item>
          <item><c>is_unique_objects</c>. Returns <c>true</c> if
           the objects returned by <c>TraverseFun</c> are unique.
          </item>
          <item><c>keypos</c>. Returns the position of the table's
           key, a positive integer.
          </item>
          <item><c>is_sorted_key</c>. Returns <c>true</c> if
           the objects returned by <c>TraverseFun</c> are sorted
           on the key.
          </item>
          <item><c>num_of_objects</c>. Returns the number of
           objects in the table, a non-negative integer.
          </item>
        </list>

        <p>The unary callback function <c><anno>FormatFun</anno></c>
          is used by <seealso marker="#info">qlc:info/1,2</seealso>
          for displaying the call that created the table's query
          handle. The default value, <c>undefined</c>, means that
          <c>info/1,2</c> displays a call to <c>'$MOD':'$FUN'/0</c>.
          It is up to <c><anno>FormatFun</anno></c> 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 will be added by <c>qlc:info</c> though).
          <c><anno>FormatFun</anno></c> 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
          <c>qlc:table/2</c> occurs. The possible values of the
          argument are:</p>

        <list type="bulleted">
          <item><c>{lookup, Position, Keys, NElements, DepthFun}</c>.
          <c>LookupFun</c> is used for looking up objects in the
           table.
          </item>
          <item><c>{match_spec, MatchExpression}</c>. 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
          <c>TraverseFun(MatchExpression)</c>.
          </item>
          <item><c>{all, NElements, DepthFun}</c>. No optimization was
           found. A match specification matching all objects will be
           used if <c>TraverseFun</c> is unary.
          </item>
        </list>

        <p><c>NElements</c> is the value of the <c>info/1,2</c> option
          <c>n_elements</c>, and <c>DepthFun</c> is a function that
          can be used for limiting the size of terms; calling
          <c>DepthFun(Term)</c> substitutes <c>'...'</c> for parts of
          <c>Term</c> below the depth specified by the <c>info/1,2</c>
          option <c>depth</c>. If calling
          <c><anno>FormatFun</anno></c> with an argument including
          <c>NElements</c> and <c>DepthFun</c> fails,
          <c><anno>FormatFun</anno></c> is called once again with an
          argument excluding
          <c>NElements</c> and <c>DepthFun</c>
          (<c>{lookup,&nbsp;Position,&nbsp;Keys}</c> or
          <c>all</c>).</p>

        <p><marker id="key_equality"></marker>The value of
          <c>key_equality</c> is to be <c>'=:='</c> if the table
          considers two keys equal if they match, and to be
          <c>'=='</c> if two keys are equal if they compare equal. The
          default is <c>'=:='</c>.</p>

        <p>See <seealso marker="ets#qlc_table">ets(3)</seealso>,
          <seealso marker="dets#qlc_table">dets(3)</seealso> and
          <seealso marker="mnesia:mnesia#qlc_table">mnesia(3)</seealso>
          for the various options recognized by <c>table/1,2</c> in
          respective module.</p>
      </desc>
    </func>

  </funcs>

  <section>
    <title>See Also</title>
    <p><seealso marker="dets">dets(3)</seealso>,
      <seealso marker="doc/reference_manual:users_guide">
           Erlang Reference Manual</seealso>,
      <seealso marker="erl_eval">erl_eval(3)</seealso>,
      <seealso marker="erts:erlang">erlang(3)</seealso>,
      <seealso marker="ets">ets(3)</seealso>,
      <seealso marker="kernel:file">file(3)</seealso>,
      <seealso marker="error_logger:file">error_logger(3)</seealso>,
      <seealso marker="file_sorter">file_sorter(3)</seealso>,
      <seealso marker="mnesia:mnesia">mnesia(3)</seealso>,
      <seealso marker="doc/programming_examples:users_guide">
           Programming Examples</seealso>,
      <seealso marker="shell">shell(3)</seealso></p>
  </section>
</erlref>