doc/src/sgml/plhandler.sgml

   1 <!-- $PostgreSQL$ -->
   2
   3  <chapter id="plhandler">
   4    <title>Writing A Procedural Language Handler</title>
   5
   6    <indexterm zone="plhandler">
   7     <primary>procedural language</primary>
   8     <secondary>handler for</secondary>
   9    </indexterm>
  10
  11    <para>
  12     All calls to functions that are written in a language other than
  13     the current <quote>version 1</quote> interface for compiled
  14     languages (this includes functions in user-defined procedural languages,
  15     functions written in SQL, and functions using the version 0 compiled
  16     language interface), go through a <firstterm>call handler</firstterm>
  17     function for the specific language.  It is the responsibility of
  18     the call handler to execute the function in a meaningful way, such
  19     as by interpreting the supplied source text.  This chapter outlines
  20     how a new procedural language's call handler can be written.
  21    </para>
  22
  23    <para>
  24     The call handler for a procedural language is a
  25     <quote>normal</quote> function that must be written in a compiled
  26     language such as C, using the version-1 interface, and registered
  27     with <productname>PostgreSQL</productname> as taking no arguments
  28     and returning the type <type>language_handler</type>.  This
  29     special pseudotype identifies the function as a call handler and
  30     prevents it from being called directly in SQL commands.
  31    </para>
  32
  33    <para>
  34     The call handler is called in the same way as any other function:
  35     It receives a pointer to a
  36     <structname>FunctionCallInfoData</structname> <type>struct</> containing
  37     argument values and information about the called function, and it
  38     is expected to return a <type>Datum</type> result (and possibly
  39     set the <structfield>isnull</structfield> field of the
  40     <structname>FunctionCallInfoData</structname> structure, if it wishes
  41     to return an SQL null result).  The difference between a call
  42     handler and an ordinary callee function is that the
  43     <structfield>flinfo-&gt;fn_oid</structfield> field of the
  44     <structname>FunctionCallInfoData</structname> structure will contain
  45     the OID of the actual function to be called, not of the call
  46     handler itself.  The call handler must use this field to determine
  47     which function to execute.  Also, the passed argument list has
  48     been set up according to the declaration of the target function,
  49     not of the call handler.
  50    </para>
  51
  52    <para>
  53     It's up to the call handler to fetch the entry of the function from the
  54     system table
  55     <classname>pg_proc</classname> and to analyze the argument
  56     and return types of the called function. The <literal>AS</> clause from the
  57     <command>CREATE FUNCTION</command> command for the function will be found
  58     in the <literal>prosrc</literal> column of the
  59     <classname>pg_proc</classname> row. This is commonly source
  60     text in the procedural language, but in theory it could be something else,
  61     such as a path name to a file, or anything else that tells the call handler
  62     what to do in detail.
  63    </para>
  64
  65    <para>
  66     Often, the same function is called many times per SQL statement.
  67     A call handler can avoid repeated lookups of information about the
  68     called function by using the
  69     <structfield>flinfo-&gt;fn_extra</structfield> field.  This will
  70     initially be <symbol>NULL</>, but can be set by the call handler to point at
  71     information about the called function.  On subsequent calls, if
  72     <structfield>flinfo-&gt;fn_extra</structfield> is already non-<symbol>NULL</>
  73     then it can be used and the information lookup step skipped.  The
  74     call handler must make sure that
  75     <structfield>flinfo-&gt;fn_extra</structfield> is made to point at
  76     memory that will live at least until the end of the current query,
  77     since an <structname>FmgrInfo</structname> data structure could be
  78     kept that long.  One way to do this is to allocate the extra data
  79     in the memory context specified by
  80     <structfield>flinfo-&gt;fn_mcxt</structfield>; such data will
  81     normally have the same lifespan as the
  82     <structname>FmgrInfo</structname> itself.  But the handler could
  83     also choose to use a longer-lived memory context so that it can cache
  84     function definition information across queries.
  85    </para>
  86
  87    <para>
  88     When a procedural-language function is invoked as a trigger, no arguments
  89     are passed in the usual way, but the
  90     <structname>FunctionCallInfoData</structname>'s
  91     <structfield>context</structfield> field points at a
  92     <structname>TriggerData</structname> structure, rather than being <symbol>NULL</>
  93     as it is in a plain function call.  A language handler should
  94     provide mechanisms for procedural-language functions to get at the trigger
  95     information.
  96    </para>
  97
  98    <para>
  99     This is a template for a procedural-language handler written in C:
 100 <programlisting>
 101 #include "postgres.h"
 102 #include "executor/spi.h"
 103 #include "commands/trigger.h"
 104 #include "fmgr.h"
 105 #include "access/heapam.h"
 106 #include "utils/syscache.h"
 107 #include "catalog/pg_proc.h"
 108 #include "catalog/pg_type.h"
 109
 110 PG_FUNCTION_INFO_V1(plsample_call_handler);
 111
 112 Datum
 113 plsample_call_handler(PG_FUNCTION_ARGS)
 114 {
 115     Datum          retval;
 116
 117     if (CALLED_AS_TRIGGER(fcinfo))
 118     {
 119         /*
 120          * Called as a trigger procedure
 121          */
 122         TriggerData    *trigdata = (TriggerData *) fcinfo-&gt;context;
 123
 124         retval = ...
 125     }
 126     else
 127     {
 128         /*
 129          * Called as a function
 130          */
 131
 132         retval = ...
 133     }
 134
 135     return retval;
 136 }
 137 </programlisting>
 138     Only a few thousand lines of code have to be added instead of the
 139     dots to complete the call handler.
 140    </para>
 141
 142    <para>
 143     After having compiled the handler function into a loadable module
 144     (see <xref linkend="dfunc">), the following commands then
 145     register the sample procedural language:
 146 <programlisting>
 147 CREATE FUNCTION plsample_call_handler() RETURNS language_handler
 148     AS '<replaceable>filename</replaceable>'
 149     LANGUAGE C;
 150 CREATE LANGUAGE plsample
 151     HANDLER plsample_call_handler;
 152 </programlisting>
 153    </para>
 154
 155    <para>
 156     The procedural languages included in the standard distribution
 157     are good references when trying to write your own call handler.
 158     Look into the <filename>src/pl</> subdirectory of the source tree.
 159    </para>
 160
 161  </chapter>