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

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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2013</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
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    </legalnotice>

    <title>dets</title>
    <prepared>Claes Wikstr&ouml;m</prepared>
    <responsible>Claes Wikstr&ouml;m</responsible>
    <docno></docno>
    <approved>nobody</approved>
    <checked>no</checked>
    <date>2001-06-06</date>
    <rev>B</rev>
    <file>dets.sgml</file>
  </header>
  <module>dets</module>
  <modulesummary>A Disk Based Term Storage</modulesummary>
  <description>
    <p>The module <c>dets</c> 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>Dets is used by the Mnesia application, and is provided as is
      for users who are interested in an efficient storage of Erlang
      terms on disk only. Many applications just need to store some
      terms in a file. Mnesia adds transactions, queries, and
      distribution. The size of Dets files cannot exceed 2 GB. If larger
      tables are needed, Mnesia's table fragmentation can be used.</p>
    <p>There are three types of Dets tables: set, bag and
      duplicate_bag. A table of type <em>set</em> has at most one object
      with a given key. If an object with a key already present in the
      table is inserted, the existing object is overwritten by the new
      object. A table of type <em>bag</em> has zero or more different
      objects with a given key. A table of type <em>duplicate_bag</em>
      has zero or more possibly matching objects with a given key.</p>
    <p>Dets tables must be opened before they can be updated or read,
      and when finished they must be properly closed. If a table has not
      been properly closed, Dets will automatically repair the table.
      This can take a substantial time if the table is large. A Dets
      table is closed when the process which opened the table
      terminates. If several Erlang processes (users) open the same Dets
      table, they will share the table. The table is properly closed
      when all users have either terminated or closed the table. Dets
      tables are not properly closed if the Erlang runtime system is
      terminated abnormally.</p>
    <note>
      <p>A ^C command abnormally terminates an Erlang runtime
        system in a Unix environment with a break-handler.</p>
    </note>
    <p>Since all operations performed by Dets are disk operations, it
      is important to realize that a single look-up operation involves a
      series of disk seek and read operations. For this reason, the Dets
      functions are much slower than the corresponding Ets functions,
      although Dets exports a similar interface.</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 the <c>repair</c>
      option set to <c>force</c>.</p>
    <p>It is worth noting that the ordered_set type present in Ets is
      not yet implemented by Dets, neither is the limited support for
      concurrent updates which makes a sequence of <c>first</c> and
      <c>next</c> calls safe to use on fixed Ets tables. Both these
      features will be implemented by Dets in a future release of
      Erlang/OTP. Until then, the Mnesia application (or some user
      implemented method for locking) has to be used to implement safe
      concurrency. Currently, no library of Erlang/OTP has support for
      ordered disk based term storage.</p>
    <p>Two versions of the format used for storing objects on file are
      supported by Dets. The first version, 8, is the format always used
      for tables created by OTP R7 and earlier. The second version, 9,
      is the default version of tables created by OTP R8 (and later OTP
      releases). OTP R8 can create version 8 tables, and convert version
      8 tables to version 9, and vice versa, upon request.
      </p>
    <p>All Dets functions return <c>{error, Reason}</c> if an error
      occurs (<c>first/1</c> and <c>next/2</c> are exceptions, they exit
      the process with the error tuple). If given badly formed
      arguments, 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&nbsp;specifications, 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_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>See <seealso marker="ets#match/2">ets:match/2</seealso> for a
          description of patterns.</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>
    <datatype>
      <name name="version"/>
    </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 giving
          the value <c>bchunk</c> to the <c>format</c> option of the
          <seealso marker="#init_table/3"><c>init_table/3</c></seealso>
          function. 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> may 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>The <c>bchunk/2</c> function 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
          which is
          to be passed on to a subsequent call to <c>bchunk/2</c>. With
          a series of calls to <c>bchunk/2</c> it is possible to extract
          all objects of the table.
          </p>
        <p><c>bchunk/2</c> returns <c>'$end_of_table'</c> when all
          objects have been 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 which has not
          been properly closed, Dets automatically tries to repair the
          table.</p>
      </desc>
    </func>
    <func>
      <name name="delete" arity="2"/>
      <fsummary>Delete all objects with a given key from a Dets table.</fsummary>
      <desc>
        <p>Deletes all objects with the key <c><anno>Key</anno></c> from
          the 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 given object from a Dets table.</fsummary>
      <desc>
        <p>Deletes all instances of a given object from a table. If a
          table is of type <c>bag</c> or <c>duplicate_bag</c>, the
          <c>delete/2</c> function cannot be used to delete only some of
          the objects with a given key. This function makes this
          possible.</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 the table <c><anno>Name</anno></c>
          according to the table's internal order, 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>
          may not work as expected if
          concurrent updates are made to the table.</p>
        <p>Should an error occur, the process is exited with an error
          tuple <c>{error, Reason}</c>. The reason for not returning the
          error tuple is that it cannot be distinguished from a key.</p>
        <p>There are two reasons why <c>first/1</c> and <c>next/2</c>
          should not be used: they are not very efficient, and they
          prevent the use of the key <c>'$end_of_table'</c> since this
          atom is used to indicate the end of the table. If possible,
          the <c>match</c>, <c>match_object</c>, and <c>select</c>
          functions should be used 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
          the table <c><anno>Name</anno></c> together with an extra argument
          <c>AccIn</c>. The
          order in which the elements of the table are traversed is
          unspecified. <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
          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 the table <c><anno>Name</anno></c> and then
          inserts all the objects of the Ets table <c><anno>EtsTab</anno></c>.
          The order in which the objects are inserted is not specified.
          Since <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 the table <c><anno>Name</anno></c>
           as a list of tuples:</p>
        <list type="bulleted">
          <item>
            <p><c>{file_size, integer() >= 0}</c>, the size of the file in
              bytes.</p>
          </item>
          <item>
            <p><c>{filename, </c><seealso marker="file#type-name">file:name()</seealso><c>}</c>,
              the name of the file where objects are stored.</p>
          </item>
          <item>
            <p><c>{keypos, </c><seealso marker="#type-keypos">keypos()</seealso>
              <c>}</c>, the position of the key.</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">type()</seealso>
            <c>}</c>, the type of the table.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name name="info" arity="2"/>
      <fsummary>Return the information associated with a given item for a Dets table.</fsummary>
      <desc>
        <p>Returns the information associated with <c><anno>Item</anno></c>
          for the table <c><anno>Name</anno></c>.
          In addition to the <c>{<anno>Item</anno>, <anno>Value</anno>}</c>
          pairs defined for <c>info/1</c>, the following items are
          allowed:</p>
        <list type="bulleted">
          <item>
            <p><c>{access, </c><seealso marker="#type-access">access()</seealso>
            <c>}</c>, the access mode.</p>
          </item>
          <item>
            <p><c>{auto_save, </c><seealso marker="#type-auto_save">
              auto_save()</seealso><c>}</c>, the auto save 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>. Only available for
              version 9 tables.</p>
          </item>
          <item>
            <p><c>{hash,</c> Hash<c>}</c>. Describes which BIF is
              used to calculate the hash values of the objects stored in
              the Dets table. Possible values of Hash are <c>hash</c>,
              which implies that the <c>erlang:hash/2</c> BIF is used,
              <c>phash</c>, which implies that the <c>erlang:phash/2</c>
              BIF is used, and <c>phash2</c>, which implies that the
              <c>erlang:phash2/1</c> BIF is used.</p>
          </item>
          <item>
            <p><c>{memory, integer() >= 0}</c>, the size of the file in
              bytes. The same value is associated with the item
              <c>file_size</c>.</p>
          </item>
          <item>
            <p><c>{no_keys, integer >= 0()}</c>, the number of different
              keys stored in the table. Only available for version 9
              tables.</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, {</c>Min<c>, </c>Used<c>, </c>Max<c>}}</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. Only
              available for version 9 tables.</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,</c> SafeFixed<c>}</c>. If the table
              is fixed, SafeFixed 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 may be any number of processes in the list. If the
              table is not fixed, SafeFixed is the atom <c>false</c>.</p>
          </item>
          <item>
            <p><c>{version, integer()</c>, the version of the format of
              the table.</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 the 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. It should be
          noted that the input functions are called by the process that
          handles requests to the Dets table, not by the calling
          process.</p>
        <p>When called with the argument <c>read</c> the 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 Value is returned as an error
          <c>{error, {init_fun, Value}}</c>. Each input function will be
          called exactly once, and should an error occur, the last
          function is called with the argument <c>close</c>, the reply
          of which is ignored.</p>
        <p>If the type of the table is <c>set</c> and there is more
          than one object with a given key, one of the objects is
          chosen. This is not necessarily the last object with the given
          key in the sequence of objects returned by the input
          functions. Duplicate keys should be avoided, or the file 
	  will be unnecessarily fragmented. This holds also for duplicated
          objects stored in tables of type <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 will start to
          grow when <c>init_table/2</c> returns which will significantly
          slow 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 the <c>min_no_slots</c> option
          below.
          </p>
        <p>The <c><anno>Options</anno></c> argument 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 that will be stored
              in the table. The <c>open_file</c> option with the same
              name is ignored unless the table is created, and in that
              case performance can be enhanced by supplying an
              estimate when initializing the table.</p>
          </item>
          <item>
            <p><c>{format, Format}</c>. Specifies the format of the
              objects returned by the 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 the
              <c>min_no_slots</c> option.</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 the table <c><anno>Name</anno></c>.
          If there already exists some object with a key matching the key
          of any of the given 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 a table's chunk data.</fsummary>
      <desc>
        <p>Returns <c>true</c> if it would be possible to initialize
          the table <c><anno>Name</anno></c>, using
          <seealso marker="#init_table/3"><c>init_table/3</c></seealso>
          with the
          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 the file <c><anno>Filename</anno></c>
          is a Dets table, <c>false</c> otherwise.</p>
      </desc>
    </func>
    <func>
      <name name="lookup" arity="2"/>
      <fsummary>Return all objects with a given key stored in a Dets table.</fsummary>
      <desc>
        <p>Returns a list of all objects with the key <c><anno>Key</anno></c>
          stored in the 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 is of type <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
          is of type <c>bag</c> or <c>duplicate_bag</c>, the function
          returns a list of arbitrary length.</p>
        <p>Note 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 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 prior
          call to <c>match/1</c> or <c>match/3</c>.</p>
        <p>When all objects of the table have been 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 the table <c><anno>Name</anno></c> that
          matches <c><anno>Pattern</anno></c> a list of bindings in some unspecified
          order. See <seealso marker="ets#match/2">ets:match/2</seealso> for a
          description of patterns. If the keypos'th element of
          <c><anno>Pattern</anno></c> is unbound, all objects of the table are
          matched. If the keypos'th element is bound, only the
          objects with the right key are matched.</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 the 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.
          See <seealso marker="ets#match/2">ets:match/2</seealso> for a
          description of patterns.</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 objects of the table are matched. If the keypos'th element is
          unbound, all objects of the table are matched, <c><anno>N</anno></c>
          objects at a time, until at least one object matches or the
          end of the table has been 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. If
          <c><anno>Name</anno></c> is a version 9 table, all objects with the same
          key are always matched at the same time which implies that
          more than <anno>N</anno> objects may sometimes be matched.
          </p>
        <p>The table should always be protected using
          <c>safe_fixtable/2</c> before calling <c>match/3</c>, or
          errors may 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 the
          table <c><anno>Name</anno></c>.
          See <seealso marker="ets#match/2">ets:match/2</seealso> for a
          description of patterns.</p>
        <p>If the keypos'th element of <c>Pattern</c> is bound,
          only the objects with the right 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 prior call to <c>match_object/1</c> or
          <c>match_object/3</c>.</p>
        <p>When all objects of the table have been 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 the table <c><anno>Name</anno></c> that
          match <c><anno>Pattern</anno></c> in some unspecified order.
          See <seealso marker="ets#match/2">ets:match/2</seealso> for a
          description of patterns.
          </p>
        <p>If the keypos'th element of <c><anno>Pattern</anno></c> is
          unbound, all objects of the table are matched. If the
          keypos'th element of <c><anno>Pattern</anno></c> is bound, only the
          objects with the right key are matched.</p>
        <p>Using the <c>match_object</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="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 the 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.
          See <seealso marker="ets#match/2">ets:match/2</seealso> for a
          description of patterns.</p>
        <p>A list of objects and a continuation is returned, unless
          the table is empty, in which case <c>'$end_of_table'</c>
          is returned. The continuation is to be used when matching
          further objects by calling <c>match_object/1</c>.</p>
        <p>If the keypos'th element of <c><anno>Pattern</anno></c> is bound, all
          objects of the table are matched. If the keypos'th element is
          unbound, all objects of the table are matched, <c><anno>N</anno></c>
          objects at a time, until at least one object matches or the
          end of the table has been reached. The default, indicated by
          giving <c><anno>N</anno></c> the value <c>default</c>, is to let the number
          of objects vary depending on the sizes of the objects. If
          <c><anno>Name</anno></c> is a version 9 table, all matching objects with
          the same key are always returned in the same reply which
          implies that more than <anno>N</anno> objects may sometimes be returned.
          </p>
        <p>The table should always be protected using
          <c>safe_fixtable/2</c> before calling <c>match_object/3</c>,
          or errors may occur when calling <c>match_object/1</c>.</p>
      </desc>
    </func>
    <func>
      <name name="member" arity="2"/>
      <fsummary>Test for occurrence of a key in a Dets table.</fsummary>
      <desc>
        <p>Works like <c>lookup/2</c>, but does not return the
          objects. The function returns <c>true</c> if one or more
          elements of the table has the key <c><anno>Key</anno></c>, <c>false</c>
          otherwise.</p>
      </desc>
    </func>
    <func>
      <name name="next" arity="2"/>
      <fsummary>Return the next key in a Dets table.</fsummary>
      <desc>
        <p>Returns the key following <c><anno>Key1</anno></c> in the table
          <c><anno>Name</anno></c> according to the table's internal order, or
          <c>'$end_of_table'</c> if there is no next key.</p>
        <p>Should an error occur, the process is exited with an error
          tuple <c>{error, Reason}</c>.</p>
        <p>Use <seealso marker="#first/1"><c>first/1</c></seealso> to find
          the first key in the table.</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 has not been properly
          closed, it will be repaired. The returned reference is to be
          used as the name of the table. This function is most useful
          for debugging purposes.</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 name of the table. The table
          name must be provided in all subsequent operations on the
          table. The name can be used by other processes as well, and
          several process can share one table.
          </p>
        <p>If two processes open the same table by giving the same
          name and arguments, then the table will have two users. If one
          user closes the table, it still remains open until the second
          user closes the table.</p>
        <p>The <c><anno>Args</anno></c> argument 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">
              access()</seealso><c>}</c>. It is possible to open
              existing tables in read-only mode. A table which is opened
              in read-only mode is not subjected to the automatic file
              reparation algorithm if it is later opened after a crash.
              The default value is <c>read_write</c>.</p>
          </item>
          <item>
            <p><c>{auto_save, </c><seealso marker="#type-auto_save">
              auto_save()</seealso><c>}</c>, the auto save
              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
              will require no reparation when reopened after an
              uncontrolled emulator halt. If the interval is the atom
              <c>infinity</c>, auto save is disabled. The default value
              is 180000 (3 minutes).</p>
          </item>
          <item>
            <p><c>{estimated_no_objects, </c><seealso marker="#type-no_slots">
              no_slots()</seealso><c>}</c>. Equivalent to the
              <c>min_no_slots</c> option.</p>
          </item>
          <item>
            <p><c>{file, </c><seealso marker="file#type-name">
              file:name()</seealso><c>}</c>, the name of the file to be
              opened. The default value is the name of the table.</p>
          </item>
          <item>
            <p><c>{max_no_slots, </c><seealso marker="#type-no_slots">
              no_slots()</seealso><c>}</c>, the maximum number
              of slots that will be used. The default value as well as
              the maximal value is 32 M. Note that a higher value may
              increase the fragmentation of the table, and conversely,
              that a smaller value may decrease the fragmentation, at
              the expense of execution time. Only available for version
              9 tables.</p>
          </item>
          <item>
            <p><c>{min_no_slots, </c><seealso marker="#type-no_slots">
              no_slots()</seealso><c>}</c>. Application
              performance can be enhanced with this flag by specifying,
              when the table is created, the estimated number of
              different keys that will be stored in the table. The
              default value as well as the minimum value is 256.</p>
          </item>
          <item>
            <p><c>{keypos, </c><seealso marker="#type-keypos">
              keypos()</seealso><c>}</c>, the position of the
              element of each object to be used as key. The default
              value is 1. The ability to explicitly state the key
              position is most convenient when we want to store Erlang
              records in which the first position of the record is the
              name of the record type.</p>
          </item>
          <item>
            <p><c>{ram_file, boolean()}</c>, whether the table is to
              be kept in RAM. Keeping the table in RAM may sound like an
              anomaly, but can enhance the performance of applications
              which open a table, insert a set of objects, and then
              close the table. When the table is closed, its contents
              are written to the disk file. The default value is
              <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 whether the Dets server should invoke the
              automatic file reparation algorithm. The default is
              <c>true</c>. If <c>false</c> is specified, there is no
              attempt to repair the file and <c>{error, {needs_repair,
              FileName}}</c> is returned if the table needs to be
              repaired.</p>
            <p>The value <c>force</c> means that a reparation will
              take place even if the table has been properly closed.
              This is how to convert tables created by older versions of
              STDLIB. An example is tables hashed with the deprecated
              <c>erlang:hash/2</c> BIF. Tables created with Dets from a
              STDLIB version of 1.8.2 and later use the
              <c>erlang:phash/2</c> function or the
              <c>erlang:phash2/1</c> function, which is preferred.</p>
            <p>The <c>repair</c> option is ignored if the table is
              already open.</p>
          </item>
          <item>
            <p><c>{type, </c><seealso marker="#type-type">type()</seealso><c>}</c>,
              the type of the table. The default value is <c>set</c>.</p>
          </item>
          <item>
            <p><c>{version, </c><seealso marker="#type-version">
              version()</seealso><c>}</c>, the version of the format
              used for the table. The default value is <c>9</c>. Tables
              on the format used before OTP R8 can be created by giving
              the value <c>8</c>. A version 8 table can be converted to
              a version 9 table by giving the options <c>{version,9}</c>
              and <c>{repair,force}</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 name of the table 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 select/1 or select/3.</fsummary>
      <desc>
        <p>This function can be used to restore an opaque continuation
          returned by <c>select/3</c> or <c>select/1</c> 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 will be
          invalidated if converted to external term format. Given that
          the original match specification is kept intact, the
          continuation can be restored, meaning it can once again be
          used in subsequent <c>select/1</c> calls even though it has
          been stored on disk or on another node.</p>
        <p>See also <c>ets(3)</c> for further explanations and
          examples.
          </p>
        <note>
          <p>This function is very rarely needed in application code. It
            is used by Mnesia to implement 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 may be
            subject to change in future releases, while this interface
            will remain 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>, the 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 several processes fix a table, the table will remain
          fixed until all processes have released it or terminated. A
          reference counter is kept on a per process basis, and N
          consecutive fixes require N releases to release the table.</p>
        <p>It is not guaranteed that calls to <c>first/1</c>,
          <c>next/2</c>, select and match functions work as expected
          even if the table has been fixed; the limited support for
          concurrency implemented in Ets has not yet been implemented
          in Dets. Fixing a table currently only disables resizing of
          the hash list of the table.</p>
        <p>If objects have been added while the table was fixed, the
          hash list will start to grow when the table is released which
          will significantly slow down access to the table for a period
          of time.</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 has been returned by a prior call to <c>select/1</c>
          or <c>select/3</c>.</p>
        <p>When all objects of the table have been matched,
          <c>'$end_of_table'</c> is returned.</p>
      </desc>
    </func>
    <func>
      <name name="select" arity="2"/>
      <fsummary>Apply a match specification to all objects stored in a Dets table.</fsummary>
      <desc>
        <p>Returns the results of applying the match specification
          <c><anno>MatchSpec</anno></c> to all or some objects stored in the table
          <c><anno>Name</anno></c>. The order of the objects is not specified. See
          the ERTS User's Guide for a description of match
          specifications.</p>
        <p>If the keypos'th element of <c><anno>MatchSpec</anno></c> is
          unbound, the match specification is applied to all objects of
          the table. If the keypos'th element is bound, the match
          specification is applied to the objects with the right key(s)
          only.</p>
        <p>Using the <c>select</c> functions for traversing all
          objects of a table is more efficient than calling
          <c>first/1</c> and <c>next/2</c> or <c>slot/2</c>.
          </p>
      </desc>
    </func>
    <func>
      <name name="select" arity="3"/>
      <fsummary>Apply a match specification to the first chunk of objects stored in a Dets table.</fsummary>
      <desc>
        <p>Returns the results of applying the match specification
          <c><anno>MatchSpec</anno></c> to some or all objects stored in the table
          <c><anno>Name</anno></c>. The order of the objects is not specified. See
          the ERTS User's Guide for a description of match
          specifications.</p>
        <p>A tuple of the results of applying the match specification
          and a continuation is returned, unless the table is empty,
          in which case <c>'$end_of_table'</c> is returned. The
          continuation is to be used when matching further objects by
          calling <c>select/1</c>.</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 right 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 has been 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. If
          <c><anno>Name</anno></c> is a version 9 table, all objects with the same
          key are always handled at the same time which implies that the
          match specification may be applied to more than <anno>N</anno> objects.
          </p>
        <p>The table should always be protected using
          <c>safe_fixtable/2</c> before calling <c>select/3</c>, or
          errors may 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 the table <c><anno>Name</anno></c> such that
          applying the match specification <c><anno>MatchSpec</anno></c> to the
          object returns the value <c>true</c>. See the ERTS
          User's Guide for a description of match
          specifications. 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 right 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 n. This
          function returns the list of objects associated with slot
          <c><anno>I</anno></c>. If <c><anno>I</anno></c> is greater than n
          <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 the table <c><anno>Name</anno></c> are
          written to disk. This also applies to tables which have been
          opened with the <c>ram_file</c> flag set to <c>true</c>. In
          this case, the contents of the RAM file are flushed to
          disk.</p>
        <p>Note that the space management data structures kept in RAM,
          the buddy system, is also written to the disk. This may take
          some time if the table is fragmented.</p>
      </desc>
    </func>
    <func>
      <name name="table" arity="1"/>
      <name name="table" arity="2"/>
      <fsummary>Return a QLC query handle.</fsummary>
      <desc>
        <p><marker id="qlc_table"></marker>Returns a QLC (Query List
          Comprehension) query handle. The module <c>qlc</c>
          implements a query language aimed mainly at Mnesia but Ets
          tables, Dets tables, and lists are also recognized by <c>qlc</c>
          as sources of data. Calling <c>dets:table/1,2</c> is the
          means to make the 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 <c>dets:lookup/2</c> to look up the keys, but when
          that is not possible the whole table is traversed. The
          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
              <c>dets:select/3</c> and <c>dets:select/1</c>. The 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>: simple filters are
              translated into equivalent match specifications while
              more complicated filters have to be applied to all
              objects returned by <c>select/3</c> given a match
              specification that matches all objects.</p>
          </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 explicitly given. 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 in fact equivalent to the former which 
          can be verified using the 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,
          and 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 the 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 out 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 in order 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 immediately returned.
          </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 the
          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">ets(3)</seealso>, 
      mnesia(3), 
      <seealso marker="qlc">qlc(3)</seealso></p>
  </section>
</erlref>