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

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>dets</title>
    <prepared>Claes Wikstr&ouml;m</prepared>
    <responsible>Claes Wikstr&ouml;m</responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2001-06-06</date>
    <rev>B</rev>
    <file>dets.xml</file>
  </header>
  <module>dets</module>
  <modulesummary>A disk-based term storage.</modulesummary>
  <description>
    <p>This module provides a term storage on file. The
      stored terms, in this module called <em>objects</em>, are tuples
      such that one element is defined to be the key. A Dets
      <em>table</em> is a collection of objects with the key at the same
      position stored on a file.</p>

    <p>This module is used by the Mnesia application, and is provided
      "as is" for users who are interested in efficient storage of Erlang
      terms on disk only. Many applications only need to store some
      terms in a file. Mnesia adds transactions, queries, and
      distribution. The size of Dets files cannot exceed 2 GB. If larger
      tables are needed, table fragmentation in Mnesia can be used.</p>

    <p>Three types of Dets tables exist:</p>

    <list type="bulleted">
      <item><p><c>set</c>. A table of this type has at most one object with a
          given key. If an object with a key already present in the
          table is inserted, the existing object is overwritten by the new
          object.</p>
      </item>
      <item><p><c>bag</c>. A table of this type has zero or more different
          objects with a given key.</p>
      </item>
      <item><p><c>duplicate_bag</c>. A table of this type has zero or more
        possibly matching objects with a given key.</p>
      </item>
    </list>

    <p>Dets tables must be opened before they can be updated or read,
      and when finished they must be properly closed. If a table is not
      properly closed, Dets automatically repairs the table.
      This can take a substantial time if the table is large. A Dets
      table is closed when the process which opened the table
      terminates. If many Erlang processes (users) open the same Dets
      table, they share the table. The table is properly closed
      when all users have either terminated or closed the table. Dets
      tables are not properly closed if the Erlang runtime system
      terminates abnormally.</p>

    <note>
      <p>A <c>^C</c> command abnormally terminates an Erlang runtime
        system in a Unix environment with a break-handler.</p>
    </note>

    <p>As all operations performed by Dets are disk operations, it
      is important to realize that a single look-up operation involves a
      series of disk seek and read operations. The Dets functions
      are therefore much slower than the corresponding
      <seealso marker="ets"><c>ets(3)</c></seealso> functions,
      although Dets exports a similar interface.</p>

    <p>Dets organizes data as a linear hash list and the hash list
      grows gracefully as more data is inserted into the table. Space
      management on the file is performed by what is called a buddy
      system. The current implementation keeps the entire buddy system
      in RAM, which implies that if the table gets heavily fragmented,
      quite some memory can be used up. The only way to defragment a
      table is to close it and then open it again with option <c>repair</c>
      set to <c>force</c>.</p>

    <p>Notice that type <c>ordered_set</c> in Ets is not yet
      provided by Dets, neither is the limited support for
      concurrent updates that makes a sequence of <c>first</c> and
      <c>next</c> calls safe to use on fixed ETS tables. Both these
      features may be provided by Dets in a future release of
      Erlang/OTP. Until then, the Mnesia application (or some
      user-implemented method for locking) must be used to implement safe
      concurrency. Currently, no Erlang/OTP library has support for
      ordered disk-based term storage.</p>

    <p>All Dets functions return <c>{error, Reason}</c> if an error
      occurs (<seealso marker="#first/1"><c>first/1</c></seealso> and
      <seealso marker="#next/2"><c>next/2</c></seealso> are exceptions, they
      exit the process with the error tuple). If badly formed arguments are
      specified, all functions exit the process with a <c>badarg</c>
      message.</p>
  </description>

  <datatypes>
    <datatype>
      <name name="access"/>
    </datatype>
    <datatype>
      <name name="auto_save"/>
    </datatype>
    <datatype>
      <name name="bindings_cont"/>
      <desc>
        <p>Opaque continuation used by <seealso marker="#match/1">
          <c>match/1</c></seealso> and <seealso marker="#match/3">
          <c>match/3</c></seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="cont"/>
      <desc>
        <p>Opaque continuation used by <seealso marker="#bchunk/2">
          <c>bchunk/2</c></seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="keypos"/>
    </datatype>
    <datatype>
      <name name="match_spec"/>
      <desc>
        <p>Match specifications, see section
          <seealso marker="erts:match_spec">
          Match Specification in Erlang</seealso> in ERTS User's Guide and the
          <seealso marker="ms_transform"><c>ms_transform(3)</c></seealso>
          module.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="no_slots"/>
    </datatype>
    <datatype>
      <name name="object"/>
    </datatype>
    <datatype>
      <name name="object_cont"/>
      <desc>
        <p>Opaque continuation used by <seealso marker="#match_object/1">
          <c>match_object/1</c></seealso> and
          <seealso marker="#match_object/3"><c>match_object/3</c></seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="pattern"/>
      <desc>
        <p>For a description of patterns, see
          <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="select_cont"/>
      <desc>
        <p>Opaque continuation used by <seealso marker="#select/1">
          <c>select/1</c></seealso> and <seealso marker="#select/3">
          <c>select/3</c></seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="tab_name"/>
    </datatype>
    <datatype>
      <name name="type"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="all" arity="0"/>
      <fsummary>Return a list of the names of all open Dets tables on
        this node.</fsummary>
      <desc>
        <p>Returns a list of the names of all open tables on this node.</p>
      </desc>
    </func>

    <func>
      <name name="bchunk" arity="2"/>
      <fsummary>Return a chunk of objects stored in a Dets table.
      </fsummary>
      <desc>
        <p>Returns a list of objects stored in a table. The exact
          representation of the returned objects is not public. The
          lists of data can be used for initializing a table by specifying
          value <c>bchunk</c> to option <c>format</c> of function
          <seealso marker="#init_table/3"><c>init_table/3</c></seealso>
          The Mnesia application uses this
          function for copying open tables.</p>
        <p>Unless the table is protected using <c>safe_fixtable/2</c>,
          calls to <c>bchunk/2</c> do possibly not work as expected if
          concurrent updates are made to the table.</p>
        <p>The first time <c>bchunk/2</c> is called, an initial
          continuation, the atom <c>start</c>, must be provided.</p>
        <p><c>bchunk/2</c> returns a tuple
          <c>{<anno>Continuation2</anno>, <anno>Data</anno>}</c>,
          where <c><anno>Data</anno></c> is a list of
          objects. <c><anno>Continuation2</anno></c> is another continuation
          that is to be passed on to a subsequent call to <c>bchunk/2</c>. With
          a series of calls to <c>bchunk/2</c>, all table objects can be
          extracted.</p>
        <p><c>bchunk/2</c> returns <c>'$end_of_table'</c> when all
          objects are returned, or <c>{error, <anno>Reason</anno>}</c>
          if an error occurs.</p>
      </desc>
    </func>

    <func>
      <name name="close" arity="1"/>
      <fsummary>Close a Dets table.</fsummary>
      <desc>
        <p>Closes a table. Only processes that have opened a table are
          allowed to close it.</p>
        <p>All open tables must be closed before the system is
          stopped. If an attempt is made to open a table that is not
          properly closed, Dets automatically tries to repair it.</p>
      </desc>
    </func>

    <func>
      <name name="delete" arity="2"/>
      <fsummary>Delete all objects with a specified key from a Dets
        table.</fsummary>
      <desc>
        <p>Deletes all objects with key <c><anno>Key</anno></c> from
          table <c><anno>Name</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="delete_all_objects" arity="1"/>
      <fsummary>Delete all objects from a Dets table.</fsummary>
      <desc>
        <p>Deletes all objects from a table in almost constant time.
          However, if the table if fixed, <c>delete_all_objects(T)</c>
          is equivalent to <c>match_delete(T, '_')</c>.</p>
      </desc>
    </func>

    <func>
      <name name="delete_object" arity="2"/>
      <fsummary>Delete a specified object from a Dets table.</fsummary>
      <desc>
        <p>Deletes all instances of a specified object from a table. If a
          table is of type <c>bag</c> or <c>duplicate_bag</c>, this
          function can be used to delete only some of
          the objects with a specified key.</p>
      </desc>
    </func>

    <func>
      <name name="first" arity="1"/>
      <fsummary>Return the first key stored in a Dets table.</fsummary>
      <desc>
        <p>Returns the first key stored in table <c><anno>Name</anno></c>
          according to the internal order of the table, or
          <c>'$end_of_table'</c> if the table is empty.</p>
        <p>Unless the table is protected using <c>safe_fixtable/2</c>,
          subsequent calls to <seealso marker="#next/2"><c>next/2</c></seealso>
          do possibly not work as expected if
          concurrent updates are made to the table.</p>
        <p>If an error occurs, the process is exited with an error
          tuple <c>{error, Reason}</c>. The error tuple is not returned,
          as it cannot be distinguished from a key.</p>
        <p>There are two reasons why <c>first/1</c> and <c>next/2</c>
          are not to be used: they are not efficient, and they
          prevent the use of key <c>'$end_of_table'</c>, as this atom
          is used to indicate the end of the table. If possible, use functions
          <seealso marker="#match/1"><c>match</c></seealso>,
          <seealso marker="#match_object/1"><c>match_object</c></seealso>, and
          <seealso marker="#select/1"><c>select</c></seealso>
          for traversing tables.</p>
      </desc>
    </func>

    <func>
      <name name="foldl" arity="3"/>
      <name name="foldr" arity="3"/>
      <fsummary>Fold a function over a Dets table.</fsummary>
      <desc>
        <p>Calls <c><anno>Function</anno></c> on successive elements of
          table <c><anno>Name</anno></c> together with an extra argument
          <c>AccIn</c>. The table elements are traversed in unspecified
          order. <c><anno>Function</anno></c> must return a new
          accumulator that is passed to the next call.
          <c><anno>Acc0</anno></c> is returned if the table is empty.</p>
      </desc>
    </func>

    <func>
      <name name="from_ets" arity="2"/>
      <fsummary>Replace the objects of a Dets table with the objects
        of an ETS table.</fsummary>
      <desc>
        <p>Deletes all objects of table <c><anno>Name</anno></c> and then
          inserts all the objects of the ETS table
          <c><anno>EtsTab</anno></c>. The objects are inserted in unspecified
          order. As <c>ets:safe_fixtable/2</c> is called, the ETS table
          must be public or owned by the calling process.</p>
      </desc>
    </func>

    <func>
      <name name="info" arity="1"/>
      <fsummary>Return information about a Dets table.</fsummary>
      <desc>
        <p>Returns information about table <c><anno>Name</anno></c>
          as a list of tuples:</p>
        <list type="bulleted">
          <item>
            <p><c>{file_size, integer() >= 0}}</c> - The file size, in
            bytes.</p>
          </item>
          <item>
            <p><c>{filename, </c><seealso marker="file#type-name">
            <c>file:name()</c></seealso><c>}</c> - The name of the file
            where objects are stored.</p>
          </item>
          <item>
            <p><c>{keypos, </c><seealso marker="#type-keypos">
              <c>keypos()</c></seealso><c>}</c> - The key position.</p>
          </item>
          <item>
            <p><c>{size, integer() >= 0}</c> - The number of objects
              stored in the table.</p>
          </item>
          <item>
            <p><c>{type, </c><seealso marker="#type-type">
              <c>type()</c></seealso><c>}</c> - The table type.</p>
          </item>
        </list>
      </desc>
    </func>

    <func>
      <name name="info" arity="2"/>
      <fsummary>Return the information associated with a specified item for
        a Dets table.</fsummary>
      <desc>
        <p>Returns the information associated with <c><anno>Item</anno></c>
          for table <c><anno>Name</anno></c>.
          In addition to the <c>{<anno>Item</anno>, <anno>Value</anno>}</c>
          pairs defined for <seealso marker="#info/1"><c>info/1</c></seealso>,
          the following items are allowed:</p>
        <list type="bulleted">
          <item>
            <p><c>{access, </c><seealso marker="#type-access">
              <c>access()</c></seealso><c>}</c> - The access mode.</p>
          </item>
          <item>
            <p><c>{auto_save, </c><seealso marker="#type-auto_save">
              <c>auto_save()</c></seealso><c>}</c> - The autosave interval.</p>
          </item>
          <item>
            <p><c>{bchunk_format, binary()}</c> - An opaque binary
              describing the format of the objects returned by
              <c>bchunk/2</c>. The binary can be used as argument to
              <c>is_compatible_chunk_format/2</c>.</p>
          </item>
          <item>
            <p><c>{hash, Hash}</c> - Describes which BIF is
              used to calculate the hash values of the objects stored in the
              Dets table. Possible values of <c>Hash</c>:</p>
            <list>
              <item>
                <p><c>phash</c> - Implies that the <c>erlang:phash/2</c> BIF
                  is used.</p>
              </item>
              <item>
                <p><c>phash2</c> - Implies that the <c>erlang:phash2/1</c> BIF
                  is used.</p>
              </item>
            </list>
          </item>
          <item>
            <p><c>{memory, integer() >= 0}</c> - The file size, in bytes.
              The same value is associated with item <c>file_size</c>.</p>
          </item>
          <item>
            <p><c>{no_keys, integer >= 0()}</c> - The number of different
              keys stored in the table.</p>
          </item>
          <item>
            <p><c>{no_objects, integer >= 0()}</c> - The number of objects
              stored in the table.</p>
          </item>
          <item>
            <p><c>{no_slots, {Min, Used, Max}}</c> - The
              number of slots of the table. <c>Min</c> is the minimum number of
              slots, <c>Used</c> is the number of currently used slots,
              and <c>Max</c> is the maximum number of slots.</p>
          </item>
          <item>
            <p><c>{owner, pid()}</c> - The pid of the process that
              handles requests to the Dets table.</p>
          </item>
          <item>
            <p><c>{ram_file, boolean()}</c> - Whether the table is
              kept in RAM.</p>
          </item>
          <item>
            <p><c>{safe_fixed_monotonic_time, SafeFixed}</c> - If the table
              is fixed, <c>SafeFixed</c> is a tuple
              <c>{FixedAtTime, [{Pid,RefCount}]}</c>.
              <c>FixedAtTime</c> is the time when
              the table was first fixed, and <c>Pid</c> is the pid of
              the process that fixes the table <c>RefCount</c> times.
              There can be any number of processes in the list. If the table
              is not fixed, <c>SafeFixed</c> is the atom <c>false</c>.</p>
            <p><c>FixedAtTime</c> corresponds to the result returned by
              <seealso marker="erts:erlang#monotonic_time/0">
              <c>erlang:monotonic_time/0</c></seealso> at the time of fixation.
              The use of <c>safe_fixed_monotonic_time</c> is
              <seealso marker="erts:time_correction#Time_Warp_Safe_Code">
              time warp safe</seealso>.</p>
          </item>
          <item>
            <p><c>{safe_fixed, SafeFixed}</c> - The same as
              <c>{safe_fixed_monotonic_time, SafeFixed}</c> except
              the format and value of <c>FixedAtTime</c>.</p>
            <p><c>FixedAtTime</c> corresponds to the result returned by
              <seealso marker="erts:erlang#timestamp/0">
              <c>erlang:timestamp/0</c></seealso> at the time of fixation.
              Notice that when the system uses single or multi
              <seealso marker="erts:time_correction#Time_Warp_Modes">time warp
              modes</seealso>, this can produce strange results. This is
              because the use of <c>safe_fixed</c> is not
              <seealso marker="erts:time_correction#Time_Warp_Safe_Code">
              time warp safe</seealso>. Time warp safe code must use
              <c>safe_fixed_monotonic_time</c> instead.</p>
          </item>
        </list>
      </desc>
    </func>

    <func>
      <name name="init_table" arity="2"/>
      <name name="init_table" arity="3"/>
      <fsummary>Replace all objects of a Dets table.</fsummary>
      <desc>
        <p>Replaces the existing objects of table <c><anno>Name</anno></c>
          with objects created by calling the input function
          <c><anno>InitFun</anno></c>,
          see below. The reason for using this function rather than
          calling <c>insert/2</c> is that of efficiency. Notice
          that the input functions are called by the process that
          handles requests to the Dets table, not by the calling
          process.</p>
        <p>When called with argument <c>read</c>, function
          <c><anno>InitFun</anno></c> is assumed to return <c>end_of_input</c>
          when there is no more input, or <c>{Objects, Fun}</c>, where
          <c>Objects</c> is a list of objects and <c>Fun</c> is a new
          input function. Any other value <c>Value</c> is returned as an error
          <c>{error, {init_fun, Value}}</c>. Each input function is
          called exactly once, and if an error occurs, the last
          function is called with argument <c>close</c>, the reply
          of which is ignored.</p>
        <p>If the table type is <c>set</c> and more
          than one object exists with a given key, one of the objects is
          chosen. This is not necessarily the last object with the given
          key in the sequence of objects returned by the input
          functions. Avoid duplicate keys, otherwise the file becomes 
          unnecessarily fragmented. This holds also for duplicated
          objects stored in tables of type <c>bag</c>.</p>
        <p>It is important that the table has a sufficient number of
          slots for the objects. If not, the hash list starts to
          grow when <c>init_table/2</c> returns, which significantly
          slows down access to the table for a period of time. The
          minimum number of slots is set by the <c>open_file/2</c> option
          <c>min_no_slots</c> and returned by the <c>info/2</c>
          item <c>no_slots</c>. See also option <c>min_no_slots</c> below.</p>
        <p>Argument <c><anno>Options</anno></c> is a list of <c>{Key, Val}</c>
          tuples, where the following values are allowed:</p>
        <list type="bulleted">
          <item>
            <p><c>{min_no_slots, no_slots()}</c> - Specifies the
              estimated number of different keys to be stored
              in the table. The <c>open_file/2</c> option with the same
              name is ignored, unless the table is created, in which
              case performance can be enhanced by supplying an
              estimate when initializing the table.</p>
          </item>
          <item>
            <p><c>{format, Format}</c> - Specifies the format of the
              objects returned by function <c><anno>InitFun</anno></c>. If
              <c>Format</c> is <c>term</c> (the default),
              <c><anno>InitFun</anno></c> is assumed to return a list of tuples.
              If <c>Format</c> is <c>bchunk</c>, <c><anno>InitFun</anno></c> is
              assumed to return <c><anno>Data</anno></c> as returned by
              <seealso marker="#bchunk/2"><c>bchunk/2</c></seealso>.
              This option overrides option <c>min_no_slots</c>.</p>
          </item>
        </list>
      </desc>
    </func>

    <func>
      <name name="insert" arity="2"/>
      <fsummary>Insert one or more objects into a Dets table.</fsummary>
      <desc>
        <p>Inserts one or more objects into the table <c><anno>Name</anno></c>.
          If there already exists an object with a key matching the key of
          some of the given objects and the table type is <c>set</c>,
          the old object will be replaced.</p>
      </desc>
    </func>

    <func>
      <name name="insert_new" arity="2"/>
      <fsummary>Insert one or more objects into a Dets table.</fsummary>
      <desc>
        <p>Inserts one or more objects into table <c><anno>Name</anno></c>.
          If there already exists some object with a key matching the key
          of any of the specified objects, the table is not updated and
          <c>false</c> is returned. Otherwise the objects are inserted
          and <c>true</c> returned.</p>
      </desc>
    </func>

    <func>
      <name name="is_compatible_bchunk_format" arity="2"/>
      <fsummary>Test compatibility of chunk data of a table.</fsummary>
      <desc>
        <p>Returns <c>true</c> if it would be possible to initialize
          table <c><anno>Name</anno></c>, using
          <seealso marker="#init_table/3"><c>init_table/3</c></seealso> with
          option <c>{format,&nbsp;bchunk}</c>,  with objects read with
          <seealso marker="#bchunk/2"><c>bchunk/2</c></seealso> from some
          table <c>T</c>, such that calling
          <c>info(T,&nbsp;bchunk_format)</c> returns
          <c>BchunkFormat</c>.</p>
      </desc>
    </func>

    <func>
      <name name="is_dets_file" arity="1"/>
      <fsummary>Test for a Dets table.</fsummary>
      <desc>
        <p>Returns <c>true</c> if file <c><anno>Filename</anno></c>
          is a Dets table, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="lookup" arity="2"/>
      <fsummary>Return all objects with a specified key stored in a
        Dets table.</fsummary>
      <desc>
        <p>Returns a list of all objects with key <c><anno>Key</anno></c>
          stored in table <c><anno>Name</anno></c>, for example:</p>
        <pre>
2> <input>dets:open_file(abc, [{type, bag}]).</input>
{ok,abc}
3> <input>dets:insert(abc, {1,2,3}).</input>
ok
4> <input>dets:insert(abc, {1,3,4}).</input>
ok
5> <input>dets:lookup(abc, 1).</input>
[{1,2,3},{1,3,4}]</pre>
        <p>If the table type is <c>set</c>, the function returns
          either the empty list or a list with one object, as there
          cannot be more than one object with a given key. If the table
          type is <c>bag</c> or <c>duplicate_bag</c>, the function
          returns a list of arbitrary length.</p>
        <p>Notice that the order of objects returned is unspecified. In
          particular, the order in which objects were inserted is not
          reflected.</p>
      </desc>
    </func>

    <func>
      <name name="match" arity="1"/>
      <fsummary>Match a chunk of objects stored in a Dets table and
        return a list of variable bindings.</fsummary>
      <desc>
        <p>Matches some objects stored in a table and returns a
          non-empty list of the bindings matching a specified pattern in
          some unspecified order. The table, the pattern, and the number
          of objects that are matched are all defined by
          <c><anno>Continuation</anno></c>, which has been returned by a
          previous call to <c>match/1</c> or <c>match/3</c>.</p>
        <p>When all table objects are matched,
          <c>'$end_of_table'</c> is returned.</p>
      </desc>
    </func>

    <func>
      <name name="match" arity="2"/>
      <fsummary>Match the objects stored in a Dets table and return a
        list of variable bindings.</fsummary>
      <desc>
        <p>Returns for each object of table <c><anno>Name</anno></c> that
          matches <c><anno>Pattern</anno></c> a list of bindings in some
          unspecified order. For a description of patterns, see
          <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.
          If the keypos'th element of
          <c><anno>Pattern</anno></c> is unbound, all table objects are
          matched. If the keypos'th element is bound, only the
          objects with the correct key are matched.</p>
      </desc>
    </func>

    <func>
      <name name="match" arity="3"/>
      <fsummary>Match the first chunk of objects stored in a Dets table
        and return a list of variable bindings.</fsummary>
      <desc>
        <p>Matches some or all objects of table <c><anno>Name</anno></c> and
          returns a non-empty list of the bindings that match
          <c><anno>Pattern</anno></c> in some unspecified order.
          For a description of patterns, see
          <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p>
        <p>A tuple of the bindings and a continuation is returned,
          unless the table is empty, in which case
          <c>'$end_of_table'</c> is returned. The continuation is to be
          used when matching further objects by calling
          <seealso marker="#match/1"><c>match/1</c></seealso>.</p>
        <p>If the keypos'th element of <c><anno>Pattern</anno></c> is bound,
          all table objects are matched. If the keypos'th element is
          unbound, all table objects are matched, <c><anno>N</anno></c>
          objects at a time, until at least one object matches or the
          end of the table is reached. The default, indicated by
          giving <c><anno>N</anno></c> the value <c>default</c>, is to let
          the number of objects vary depending on the sizes of the objects.
	  All objects with the
          same key are always matched at the same time, which implies that
          more than <anno>N</anno> objects can sometimes be matched.</p>
        <p>The table is always to be protected using
          <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso>
          before calling <c>match/3</c>, otherwise
          errors can occur when calling <c>match/1</c>.</p>
      </desc>
    </func>

    <func>
      <name name="match_delete" arity="2"/>
      <fsummary>Delete all objects that match a given pattern from a
        Dets table.</fsummary>
      <desc>
        <p>Deletes all objects that match <c><anno>Pattern</anno></c> from
          table <c><anno>Name</anno></c>. For a description of patterns,
          see <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p>
        <p>If the keypos'th element of <c>Pattern</c> is bound,
          only the objects with the correct key are matched.</p>
      </desc>
    </func>

    <func>
      <name name="match_object" arity="1"/>
      <fsummary>Match a chunk of objects stored in a Dets table and
        return a list of objects.</fsummary>
      <desc>
        <p>Returns a non-empty list of some objects stored in a table
          that match a given pattern in some unspecified order. The
          table, the pattern, and the number of objects that are matched
          are all defined by <c><anno>Continuation</anno></c>, which has been
          returned by a previous call to <c>match_object/1</c> or
          <c>match_object/3</c>.</p>
        <p>When all table objects are matched,
          <c>'$end_of_table'</c> is returned.</p>
      </desc>
    </func>

    <func>
      <name name="match_object" arity="2"/>
      <fsummary>Match the objects stored in a Dets table and return
        a list of objects.</fsummary>
      <desc>
        <p>Returns a list of all objects of table <c><anno>Name</anno></c> that
          match <c><anno>Pattern</anno></c> in some unspecified order.
          For a description of patterns, see
          <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p>
        <p>If the keypos'th element of <c><anno>Pattern</anno></c> is
          unbound, all table objects are matched. If the
          keypos'th element of <c><anno>Pattern</anno></c> is bound, only the
          objects with the correct key are matched.</p>
        <p>Using the <c>match_object</c> functions for traversing all
          table objects is more efficient than calling
          <c>first/1</c> and <c>next/2</c> or <c>slot/2</c>.</p>
      </desc>
    </func>

    <func>
      <name name="match_object" arity="3"/>
      <fsummary>Match the first chunk of objects stored in a Dets table
        and return a list of objects.</fsummary>
      <desc>
        <p>Matches some or all objects stored in table <c><anno>Name</anno></c>
          and returns a non-empty list of the objects that match
          <c><anno>Pattern</anno></c> in some unspecified order.
          For a description of patterns, see
          <seealso marker="ets#match/2"><c>ets:match/2</c></seealso>.</p>
        <p>A list of objects and a continuation is returned, unless
          the table is empty, in which case <c>'$end_of_table'</c>
          is returned. The continuation is to be used when matching
          further objects by calling
          <seealso marker="#match_object/1"><c>match_object/1</c></seealso>.</p>
        <p>If the keypos'th element of <c><anno>Pattern</anno></c> is bound,
          all table objects are matched. If the keypos'th element is
          unbound, all table objects are matched, <c><anno>N</anno></c>
          objects at a time, until at least one object matches or the
          end of the table is reached. The default, indicated by
          giving <c><anno>N</anno></c> the value <c>default</c>,
          is to let the number
          of objects vary depending on the sizes of the objects. All
	  matching objects with the same key are always returned
	  in the same reply, which implies
          that more than <anno>N</anno> objects can sometimes be returned.</p>
        <p>The table is always to be protected using
          <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso>
          before calling <c>match_object/3</c>, otherwise
          errors can occur when calling <c>match_object/1</c>.</p>
      </desc>
    </func>

    <func>
      <name name="member" arity="2"/>
      <fsummary>Test for occurrence of a key in a Dets table.</fsummary>
      <desc>
        <p>Works like <seealso marker="#lookup/2"><c>lookup/2</c></seealso>,
          but does not return the objects. Returns <c>true</c> if one or more
          table elements has key <c><anno>Key</anno></c>, otherwise
          <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="next" arity="2"/>
      <fsummary>Return the next key in a Dets table.</fsummary>
      <desc>
        <p>Returns either the key following <c><anno>Key1</anno></c> in table
          <c><anno>Name</anno></c> according to the internal order of the
            table, or <c>'$end_of_table'</c> if there is no next key.</p>
        <p>If an error occurs, the process is exited with an error
          tuple <c>{error, Reason}</c>.</p>
        <p>To find the first key in the table, use
          <seealso marker="#first/1"><c>first/1</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="open_file" arity="1"/>
      <fsummary>Open an existing Dets table.</fsummary>
      <desc>
        <p>Opens an existing table. If the table is not properly closed,
          it is repaired. The returned reference is to be used as the table
          name. This function is most useful for debugging purposes.</p>
      </desc>
    </func>

    <func>
      <name name="open_file" arity="2"/>
      <fsummary>Open a Dets table.</fsummary>
      <desc>
        <p>Opens a table. An empty Dets table is created if no file
          exists.</p>
        <p>The atom <c><anno>Name</anno></c> is the table name. The table
          name must be provided in all subsequent operations on the
          table. The name can be used by other processes as well, and
          many processes can share one table.</p>
        <p>If two processes open the same table by giving the same
          name and arguments, the table has two users. If one
          user closes the table, it remains open until the second
          user closes it.</p>
        <p>Argument <c><anno>Args</anno></c> is a list of <c>{Key, Val}</c>
          tuples, where the following values are allowed:</p>
        <list type="bulleted">
          <item>
            <p><c>{access, </c><seealso marker="#type-access">
              <c>access()</c></seealso><c>}</c> - Existing tables can be
              opened in read-only mode. A table that is opened
              in read-only mode is not subjected to the automatic file
              reparation algorithm if it is later opened after a crash.
              Defaults to <c>read_write</c>.</p>
          </item>
          <item>
            <p><c>{auto_save, </c><seealso marker="#type-auto_save">
              <c>auto_save()</c></seealso><c>}</c> - The autosave
              interval. If the interval is an integer <c>Time</c>, the
              table is flushed to disk whenever it is not accessed for
              <c>Time</c> milliseconds. A table that has been flushed
              requires no reparation when reopened after an
              uncontrolled emulator halt. If the interval is the atom
              <c>infinity</c>, autosave is disabled. Defaults to
              180000 (3 minutes).</p>
          </item>
          <item>
            <p><c>{estimated_no_objects, </c><seealso marker="#type-no_slots">
              <c>no_slots()</c></seealso><c>}</c> - Equivalent to option
              <c>min_no_slots</c>.</p>
          </item>
          <item>
            <p><c>{file, </c><seealso marker="file#type-name">
              <c>file:name()</c></seealso><c>}</c> - The name of the file to be
              opened. Defaults to the table name.</p>
          </item>
          <item>
            <p><c>{max_no_slots, </c><seealso marker="#type-no_slots">
              <c>no_slots()</c></seealso><c>}</c> - The maximum number
              of slots to be used. Defaults to 32 M, which is the
              maximal value. Notice that a higher value can
              increase the table fragmentation, and
              a smaller value can decrease the fragmentation, at
              the expense of execution time.</p>
          </item>
          <item>
            <p><c>{min_no_slots, </c><seealso marker="#type-no_slots">
              <c>no_slots()</c></seealso><c>}</c> - Application
              performance can be enhanced with this flag by specifying,
              when the table is created, the estimated number of
              different keys to be stored in the table. Defaults to 256,
              which is the minimum value.</p>
          </item>
          <item>
            <p><c>{keypos, </c><seealso marker="#type-keypos">
              <c>keypos()</c></seealso><c>}</c> - The position of the
              element of each object to be used as key. Defaults to 1.
              The ability to explicitly state the key
              position is most convenient when we want to store Erlang
              records in which the first position of the record is the
              name of the record type.</p>
          </item>
          <item>
            <p><c>{ram_file, boolean()}</c> - Whether the table is to
              be kept in RAM. Keeping the table in RAM can sound like an
              anomaly, but can enhance the performance of applications
              that open a table, insert a set of objects, and then
              close the table. When the table is closed, its contents
              are written to the disk file. Defaults to <c>false</c>.</p>
          </item>
          <item>
            <p><c>{repair, Value}</c> - <c>Value</c> can be either
              a <c>boolean()</c> or the atom <c>force</c>. The flag
              specifies if the Dets server is to invoke the
              automatic file reparation algorithm. Defaults to
              <c>true</c>. If <c>false</c> is specified, no attempt is
              made to repair the file, and <c>{error, {needs_repair,
              FileName}}</c> is returned if the table must be repaired.</p>
            <p>Value <c>force</c> means that a reparation
              is made even if the table is properly closed.
	      This is a seldom needed option.</p>
            <p>Option <c>repair</c> is ignored if the table is already open.</p>
          </item>
          <item>
            <p><c>{type, </c><seealso marker="#type-type">
              <c>type()</c></seealso><c>}</c> - The table type. Defaults to
              <c>set</c>.</p>
          </item>
        </list>
      </desc>
    </func>

    <func>
      <name name="pid2name" arity="1"/>
      <fsummary>Return the name of the Dets table handled by a pid.</fsummary>
      <desc>
        <p>Returns the table name given the pid of a process
          that handles requests to a table, or <c>undefined</c> if
          there is no such table.</p>
        <p>This function is meant to be used for debugging only.</p>
      </desc>
    </func>

    <func>
      <name name="repair_continuation" arity="2"/>
      <fsummary>Repair a continuation from <c>select/1</c> or <c>select/3</c>.
      </fsummary>
      <desc>
        <p>This function can be used to restore an opaque continuation
          returned by
          <seealso marker="#select/3"><c>select/3</c></seealso> or
          <seealso marker="#select/1"><c>select/1</c></seealso> if the
          continuation has passed through external term format (been
          sent between nodes or stored on disk).</p>
        <p>The reason for this function is that continuation terms
          contain compiled match specifications and therefore are
          invalidated if converted to external term format. Given that
          the original match specification is kept intact, the
          continuation can be restored, meaning it can once again be
          used in subsequent <c>select/1</c> calls even though it has
          been stored on disk or on another node.</p>
        <p>For more information and examples, see the
          <seealso marker="ets"><c>ets(3)</c></seealso> module.</p>
        <note>
          <p>This function is rarely needed in application code. It is used by
            application Mnesia to provide distributed <c>select/3</c>
            and <c>select/1</c> sequences. A normal application would
            either use Mnesia or keep the continuation from being
            converted to external format.</p>
          <p>The reason for not having an external representation of
            compiled match specifications is performance. It can be
            subject to change in future releases, while this interface
            remains for backward compatibility.</p>
        </note>
      </desc>
    </func>

    <func>
      <name name="safe_fixtable" arity="2"/>
      <fsummary>Fix a Dets table for safe traversal.</fsummary>
      <desc>
        <p>If <c><anno>Fix</anno></c> is <c>true</c>, table
          <c><anno>Name</anno></c> is
          fixed (once more) by the calling process, otherwise the table
          is released. The table is also released when a fixing process
          terminates.</p>
        <p>If many processes fix a table, the table remains
          fixed until all processes have released it or terminated. A
          reference counter is kept on a per process basis, and N
          consecutive fixes require N releases to release the table.</p>
        <p>It is not guaranteed that calls to <c>first/1</c>,
          <c>next/2</c>, or select and match functions work as expected
          even if the table is fixed; the limited support for
          concurrency provided by the
          <seealso marker="ets"><c>ets(3)</c></seealso> module is not yet
          provided by Dets.
          Fixing a table currently only disables resizing of
          the hash list of the table.</p>
        <p>If objects have been added while the table was fixed, the
          hash list starts to grow when the table is released, which
          significantly slows down access to the table for a period
          of time.</p>
      </desc>
    </func>

    <func>
      <name name="select" arity="1"/>
      <fsummary>Apply a match specification to some objects stored in a
        Dets table.</fsummary>
      <desc>
        <p>Applies a match specification to some objects stored in a
          table and returns a non-empty list of the results. The
          table, the match specification, and the number of objects
          that are matched are all defined by <c><anno>Continuation</anno></c>,
          which is returned by a previous call to
          <seealso marker="#select/1"><c>select/1</c></seealso> or
          <seealso marker="#select/3"><c>select/3</c></seealso>.</p>
        <p>When all objects of the table have been matched,
          <c>'$end_of_table'</c> is returned.</p>
      </desc>
    </func>

    <func>
      <name name="select" arity="2"/>
      <fsummary>Apply a match specification to all objects stored in a
        Dets table.</fsummary>
      <desc>
        <p>Returns the results of applying match specification
          <c><anno>MatchSpec</anno></c> to all or some objects stored in table
          <c><anno>Name</anno></c>. The order of the objects is not specified.
          For a description of match specifications, see the
          <seealso marker="erts:match_spec">ERTS User's Guide</seealso>.</p>
        <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is
          unbound, the match specification is applied to all objects of
          the table. If the keypos'th element is bound, the match
          specification is applied to the objects with the correct key(s)
          only.</p>
        <p>Using the <c>select</c> functions for traversing all
          objects of a table is more efficient than calling
          <c>first/1</c> and <c>next/2</c> or <c>slot/2</c>.
          </p>
      </desc>
    </func>

    <func>
      <name name="select" arity="3"/>
      <fsummary>Apply a match specification to the first chunk of objects
        stored in a Dets table.</fsummary>
      <desc>
        <p>Returns the results of applying match specification
          <c><anno>MatchSpec</anno></c> to some or all objects stored in table
          <c><anno>Name</anno></c>. The order of the objects is not specified.
          For a description of match specifications, see the
          <seealso marker="erts:match_spec">ERTS User's Guide</seealso>.</p>
        <p>A tuple of the results of applying the match specification
          and a continuation is returned, unless the table is empty,
          in which case <c>'$end_of_table'</c> is returned. The
          continuation is to be used when matching more objects by calling
          <seealso marker="#select/1"><c>select/1</c></seealso>.</p>
        <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is bound,
          the  match specification is applied to all objects of the table
          with the correct key(s). If the keypos'th element of
          <c><anno>MatchSpec</anno></c> is unbound, the match specification is
          applied to all objects of the table, <c><anno>N</anno></c> objects at
          a time, until at least one object matches or the end of the
          table is reached. The default, indicated by giving
          <c><anno>N</anno></c> the value <c>default</c>, is to let the number
          of objects vary depending on the sizes of the objects. All
	  objects with the
          same key are always handled at the same time, which implies that the
          match specification can be applied to more than <anno>N</anno>
          objects.</p>
        <p>The table is always to be protected using
          <seealso marker="#safe_fixtable/2"><c>safe_fixtable/2</c></seealso>
          before calling <c>select/3</c>, otherwise
          errors can occur when calling <c>select/1</c>.</p>
      </desc>
    </func>

    <func>
      <name name="select_delete" arity="2"/>
      <fsummary>Delete all objects that match a given pattern from a
        Dets table.</fsummary>
      <desc>
        <p>Deletes each object from table <c><anno>Name</anno></c> such that
          applying match specification <c><anno>MatchSpec</anno></c> to the
          object returns value <c>true</c>.
          For a description of match specifications, see the
          <seealso marker="erts:match_spec">ERTS User's Guide</seealso>.
          Returns the number of deleted objects.</p>
        <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is
          bound, the match specification is applied to the objects
          with the correct key(s) only.</p>
      </desc>
    </func>

    <func>
      <name name="slot" arity="2"/>
      <fsummary>Return the list of objects associated with a slot of a
        Dets table.</fsummary>
      <desc>
        <p>The objects of a table are distributed among slots,
          starting with slot <c>0</c> and ending with slot <c>n</c>.
          Returns the list of objects associated with slot
          <c><anno>I</anno></c>. If <c><anno>I</anno></c> &gt; <c>n</c>,
          <c>'$end_of_table'</c> is returned.</p>
      </desc>
    </func>

    <func>
      <name name="sync" arity="1"/>
      <fsummary>Ensure that all updates made to a Dets table are written
        to disk.</fsummary>
      <desc>
        <p>Ensures that all updates made to table <c><anno>Name</anno></c> are
          written to disk. This also applies to tables that have been
          opened with flag <c>ram_file</c> set to <c>true</c>. In
          this case, the contents of the RAM file are flushed to disk.</p>
        <p>Notice that the space management data structures kept in RAM,
          the buddy system, is also written to the disk. This can take
          some time if the table is fragmented.</p>
      </desc>
    </func>

    <func>
      <name name="table" arity="1"/>
      <name name="table" arity="2"/>
      <fsummary>Return a QLC query handle.</fsummary>
      <desc>
        <p>Returns a Query List
          Comprehension (QLC) query handle. The
          <seealso marker="qlc"><c>qlc(3)</c></seealso> module
          provides a query language aimed mainly for Mnesia, but
          ETS tables, Dets tables, and lists are also recognized
          by <c>qlc</c> as sources of data. Calling
          <seealso marker="dets#table/1"><c>dets:table/1,2</c></seealso> is the
          means to make Dets table <c><anno>Name</anno></c> usable to
          <c>qlc</c>.</p>
        <p>When there are only simple restrictions on the key position,
          <c>qlc</c> uses
          <seealso marker="dets#lookup/2"><c>dets:lookup/2</c></seealso>
          to look up the keys. When
          that is not possible, the whole table is traversed.
          Option <c>traverse</c> determines how this is done:</p>
        <list type="bulleted">
          <item>
            <p><c>first_next</c> - The table is traversed one key at
              a time by calling <c>dets:first/1</c> and <c>dets:next/2</c>.</p>
          </item>
          <item>
            <p><c>select</c> - The table is traversed by calling
              <seealso marker="dets:select/3"><c>dets:select/3</c></seealso> and
              <seealso marker="dets:select/1"><c>dets:select/1</c></seealso>.
              Option <c>n_objects</c> determines the number of objects
              returned (the third argument of <c>select/3</c>). The
              match specification (the second argument of
              <c>select/3</c>) is assembled by <c>qlc</c>:</p>
            <list type="bulleted">
              <item>
                <p>Simple filters are translated into equivalent match
                  specifications.</p>
              </item>
              <item>
                <p>More complicated filters must be applied to all
                  objects returned by <c>select/3</c> given a match
                  specification that matches all objects.</p>
              </item>
            </list>
          </item>
          <item>
            <p><c>{select, </c><seealso marker="#type-match_spec">
              match_spec()</seealso><c>}</c> - As for <c>select</c>,
              the table is traversed by calling <c>dets:select/3</c>
              and <c>dets:select/1</c>. The difference is that the
              match specification is specified explicitly. This is how to
              state match specifications that cannot easily be
              expressed within the syntax provided by <c>qlc</c>.</p>
          </item>
        </list>
        <p>The following example uses an explicit match specification
          to traverse the table:</p>
        <pre>
1> <input>dets:open_file(t, []),</input>
<input>ok = dets:insert(t, [{1,a},{2,b},{3,c},{4,d}]),</input>
<input>MS = ets:fun2ms(fun({X,Y}) when (X > 1) or (X &lt; 5) -> {Y} end),</input>
<input>QH1 = dets:table(t, [{traverse, {select, MS}}]).</input></pre>
        <p>An example with implicit match specification:</p>
        <pre>
2> <input>QH2 = qlc:q([{Y} || {X,Y} &lt;- dets:table(t), (X > 1) or (X &lt; 5)]).</input></pre>
        <p>The latter example is equivalent to the former, which 
          can be verified using function <c>qlc:info/1</c>:</p>
        <pre>
3> <input>qlc:info(QH1) =:= qlc:info(QH2).</input>
true</pre>
        <p><c>qlc:info/1</c> returns information about a query handle.
          In this case identical information is returned for the
          two query handles.</p>
      </desc>
    </func>

    <func>
      <name name="to_ets" arity="2"/>
      <fsummary>Insert all objects of a Dets table into an ETS
        table.</fsummary>
      <desc>
        <p>Inserts the objects of the Dets table <c><anno>Name</anno></c>
          into the ETS table <c><anno>EtsTab</anno></c>. The order in
          which the objects are
          inserted is not specified. The existing objects of the ETS
          table are kept unless overwritten.</p>
      </desc>
    </func>

    <func>
      <name name="traverse" arity="2"/>
      <fsummary>Apply a function to all or some objects stored in a Dets
        table.</fsummary>
      <desc>
        <p>Applies <c><anno>Fun</anno></c> to each object stored in table
          <c><anno>Name</anno></c> in some unspecified order. Different
          actions are
          taken depending on the return value of <c><anno>Fun</anno></c>. The
          following <c><anno>Fun</anno></c> return values are allowed:</p>
        <taglist>
          <tag><c>continue</c></tag>
          <item>
            <p>Continue to perform the traversal. For example, the
              following function can be used to print the contents
              of a table:</p>
            <pre>
fun(X) -> io:format("~p~n", [X]), continue end.</pre>
          </item>
          <tag><c>{continue, Val}</c></tag>
          <item>
            <p>Continue the traversal and accumulate <c><anno>Val</anno></c>.
              The following function is supplied to collect all
              objects of a table in a list:</p>
            <pre>
fun(X) -> {continue, X} end.</pre>
          </item>
          <tag><c>{done, <anno>Value</anno>}</c></tag>
          <item>
            <p>Terminate the traversal and return
              <c>[<anno>Value</anno> | Acc]</c>.</p>
          </item>
        </taglist>
        <p>Any other value <c><anno>OtherValue</anno></c> returned by
          <c><anno>Fun</anno></c> terminates the
          traversal and is returned immediately.</p>
      </desc>
    </func>

    <func>
      <name name="update_counter" arity="3"/>
      <fsummary>Update a counter object stored in a Dets table.
      </fsummary>
      <desc>
        <p>Updates the object with key <c><anno>Key</anno></c> stored in
          table <c><anno>Name</anno></c> of type <c>set</c> by adding
          <c><anno>Incr</anno></c> to the
          element at the <c><anno>Pos</anno></c>:th position.
          The new counter value
          is returned. If no position is specified, the element directly
          following the key is updated.</p>
        <p>This functions provides a way of updating a counter,
          without having to look up an object, update the object by
          incrementing an element, and insert the resulting object into
          the table again.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seealso marker="ets"><c>ets(3)</c></seealso>,
      <seealso marker="mnesia:mnesia"><c>mnesia(3)</c></seealso>,
      <seealso marker="qlc"><c>qlc(3)</c></seealso></p>
  </section>
</erlref>