doc/src/sgml/tableam.sgml

   1 <!-- doc/src/sgml/tableam.sgml -->
   2
   3 <chapter id="tableam">
   4  <title>Table Access Method Interface Definition</title>
   5
   6  <indexterm>
   7   <primary>Table Access Method</primary>
   8  </indexterm>
   9  <indexterm>
  10   <primary>tableam</primary>
  11   <secondary>Table Access Method</secondary>
  12  </indexterm>
  13
  14  <para>
  15   This chapter explains the interface between the core
  16   <productname>PostgreSQL</productname> system and <firstterm>table access
  17   methods</firstterm>, which manage the storage for tables. The core system
  18   knows little about these access methods beyond what is specified here, so
  19   it is possible to develop entirely new access method types by writing
  20   add-on code.
  21  </para>
  22
  23  <para>
  24   Each table access method is described by a row in the <link
  25   linkend="catalog-pg-am"><structname>pg_am</structname></link> system
  26   catalog. The <structname>pg_am</structname> entry specifies a name and a
  27   <firstterm>handler function</firstterm> for the table access method.  These
  28   entries can be created and deleted using the <xref
  29   linkend="sql-create-access-method"/> and <xref
  30   linkend="sql-drop-access-method"/> SQL commands.
  31  </para>
  32
  33  <para>
  34   A table access method handler function must be declared to accept a single
  35   argument of type <type>internal</type> and to return the pseudo-type
  36   <type>table_am_handler</type>.  The argument is a dummy value that simply
  37   serves to prevent handler functions from being called directly from SQL commands.
  38
  39   The result of the function must be a pointer to a struct of type
  40   <structname>TableAmRoutine</structname>, which contains everything that the
  41   core code needs to know to make use of the table access method. The return
  42   value needs to be of server lifetime, which is typically achieved by
  43   defining it as a <literal>static const</literal> variable in global
  44   scope. The <structname>TableAmRoutine</structname> struct, also called the
  45   access method's <firstterm>API struct</firstterm>, defines the behavior of
  46   the access method using callbacks. These callbacks are pointers to plain C
  47   functions and are not visible or callable at the SQL level. All the
  48   callbacks and their behavior is defined in the
  49   <structname>TableAmRoutine</structname> structure (with comments inside the
  50   struct defining the requirements for callbacks). Most callbacks have
  51   wrapper functions, which are documented from the point of view of a user
  52   (rather than an implementor) of the table access method.  For details,
  53   please refer to the <ulink url="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/access/tableam.h;hb=HEAD">
  54   <filename>src/include/access/tableam.h</filename></ulink> file.
  55  </para>
  56
  57  <para>
  58   To implement an access method, an implementor will typically need to
  59   implement an <acronym>AM</acronym>-specific type of tuple table slot (see
  60   <ulink url="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/executor/tuptable.h;hb=HEAD">
  61    <filename>src/include/executor/tuptable.h</filename></ulink>), which allows
  62    code outside the access method to hold references to tuples of the AM, and
  63    to access the columns of the tuple.
  64  </para>
  65
  66  <para>
  67   Currently, the way an AM actually stores data is fairly unconstrained. For
  68   example, it's possible, but not required, to use postgres' shared buffer
  69   cache.  In case it is used, it likely makes sense to use
  70   <productname>PostgreSQL</productname>'s standard page layout as described in
  71   <xref linkend="storage-page-layout"/>.
  72  </para>
  73
  74  <para>
  75   One fairly large constraint of the table access method API is that,
  76   currently, if the AM wants to support modifications and/or indexes, it is
  77   necessary for each tuple to have a tuple identifier (<acronym>TID</acronym>)
  78   consisting of a block number and an item number (see also <xref
  79   linkend="storage-page-layout"/>).  It is not strictly necessary that the
  80   sub-parts of <acronym>TIDs</acronym> have the same meaning they e.g., have
  81   for <literal>heap</literal>, but if bitmap scan support is desired (it is
  82   optional), the block number needs to provide locality.
  83  </para>
  84
  85  <para>
  86   For crash safety, an AM can use postgres' <link
  87   linkend="wal"><acronym>WAL</acronym></link>, or a custom implementation.
  88   If <acronym>WAL</acronym> is chosen, either <link
  89   linkend="generic-wal">Generic WAL Records</link> can be used,
  90   or a <link linkend="custom-rmgr">Custom WAL Resource Manager</link> can be
  91   implemented.
  92  </para>
  93
  94  <para>
  95   To implement transactional support in a manner that allows different table
  96   access methods be accessed within a single transaction, it likely is
  97   necessary to closely integrate with the machinery in
  98   <filename>src/backend/access/transam/xlog.c</filename>.
  99  </para>
 100
 101  <para>
 102   Any developer of a new <literal>table access method</literal> can refer to
 103   the existing <literal>heap</literal> implementation present in
 104   <filename>src/backend/access/heap/heapam_handler.c</filename> for details of
 105   its implementation.
 106  </para>
 107
 108 </chapter>