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33a82e7fd8
undocumented items in TD. Should doc patches alse be sent to pgsql-patches, or do I have to subscribe to pgsql-docs? The archive link for pgsql-patches is broken, and I don't see any patches in spot checking the archive for pgsql-docs. -Brad McLean.
167 lines
6.1 KiB
Plaintext
167 lines
6.1 KiB
Plaintext
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/plpython.sgml,v 1.3 2001/09/12 03:58:15 momjian Exp $ -->
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<chapter id="plpython">
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<title>PL/Python - Python Procedural Language</title>
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<note>
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<para>
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This chapter is not fully developed yet.
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</para>
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</note>
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<sect1 id="plpython-install">
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<title>Installation</title>
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<para>
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... needs to be worked out.
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</para>
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</sect1>
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<sect1 id="plpython-using">
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<title>Using</title>
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<para>
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There are sample functions in
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<filename>plpython_function.sql</filename>. The Python code you
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write gets transformed into a function. E.g.,
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<programlisting>
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CREATE FUNCTION myfunc(text) RETURNS text AS
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'return args[0]'
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LANGUAGE 'plpython';
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</programlisting>
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gets transformed into
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<programlisting>
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def __plpython_procedure_myfunc_23456():
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return args[0]
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</programlisting>
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where 23456 is the Oid of the function.
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</para>
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<para>
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If you do not provide a return value, Python returns the default
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<symbol>None</symbol> which may or may not be what you want. The
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language module translates Python's None into SQL NULL.
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</para>
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<para>
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PostgreSQL function variables are available in the global
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<varname>args</varname> list. In the <function>myfunc</function>
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example, <varname>args[0]</> contains whatever was passed in as the text
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argument. For <literal>myfunc2(text, integer)</literal>, <varname>args[0]</>
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would contain the <type>text</type> variable and <varname>args[1]</varname> the <type>integer</type> variable.
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</para>
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<para>
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The global dictionary SD is available to store data between
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function calls. This variable is private static data. The global
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dictionary GD is public data, available to all python functions
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within a backend. Use with care.
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</para>
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<para>
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Each function gets its own restricted execution object in the
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Python interpreter, so that global data and function arguments from
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<function>myfunc</function> are not available to
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<function>myfunc2</function>. The exception is the data in the GD
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dictionary, as mentioned above.
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</para>
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</para>
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When a function is used in a trigger, the dictionary TD contains
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transaction related values. The trigger tuples are in TD["new"]
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and/or TD["old"] depending on the trigger event. TD["event"]
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contains the event as a string ("INSERT", "UPDATE", "DELETE", or
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"UNKNOWN"). TD["when"] contains one of ("BEFORE", "AFTER", or
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"UNKNOWN"). TD["level"] contains one of ("ROW", "STATEMENT", or
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"UNKNOWN"). TD["name"] contains the trigger name, and TD["relid"]
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contains the relation id of the table on which the trigger occurred.
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If the trigger was called with arguments they are available
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in TD["args"][0] to TD["args"][(n -1)]
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</para>
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<para>
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If the trigger 'when' is "BEFORE", you may Return 'None' or "OK"
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from the python function to indicate the tuple is unmodified,
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"SKIP" to abort the event, or "MODIFIED" to indicate you've
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modified the tuple.
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</para>
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<para>
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The PL/Python language module automatically imports a Python module
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called <literal>plpy</literal>. The functions and constants in
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this module are available to you in the Python code as
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<literal>plpy.<replaceable>foo</replaceable></literal>. At present
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<literal>plpy</literal> implements the functions
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<literal>plpy.error("msg")</literal>,
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<literal>plpy.fatal("msg")</literal>,
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<literal>plpy.debug("msg")</literal>, and
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<literal>plpy.notice("msg")</literal>. They are mostly equivalent
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to calling <literal>elog(<replaceable>LEVEL</>, "msg")</literal>,
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where <replaceable>LEVEL</> is DEBUG, ERROR, FATAL or NOTICE.
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<function>plpy.error</function> and <function>plpy.fatal</function>
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actually raise a Python exception which, if uncaught, causes the
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PL/Python module to call <literal>elog(ERROR, msg)</literal> when
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the function handler returns from the Python interpreter. Long
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jumping out of the Python interpreter is probably not good.
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<literal>raise plpy.ERROR("msg")</literal> and <literal>raise
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plpy.FATAL("msg")</literal> are equivalent to calling
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<function>plpy.error</function> or <function>plpy.fatal</function>.
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</para>
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<para>
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Additionally, the <literal>plpy</literal> module provides two functions called
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<function>execute</function> and <function>prepare</function>.
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Calling <function>plpy.execute</function> with a query string, and
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an optional limit argument, causes that query to be run, and the
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result returned in a result object. The result object emulates a
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list or dictionary object. The result object can be accessed by
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row number, and field name. It has these additional methods:
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<function>nrows()</function> which returns the number of rows
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returned by the query, and <function>status</function> which is the
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<function>SPI_exec</function> return variable. The result object
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can be modified.
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<programlisting>
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rv = plpy.execute("SELECT * FROM my_table", 5)
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</programlisting>
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returns up to 5 rows from my_table. Ff my_table has a column
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my_field it would be accessed as
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<programlisting>
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foo = rv[i]["my_field"]
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</programlisting>
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The second function <function>plpy.prepare</function> is called
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with a query string, and a list of argument types if you have bind
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variables in the query.
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<programlisting>
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plan = plpy.prepare("SELECT last_name FROM my_users WHERE first_name = $1", [ "text" ])
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</programlisting>
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text is the type of the variable you will be passing as $1. After
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preparing you use the function <function>plpy.execute</function> to
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run it.
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<programlisting>
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rv = plpy.execute(plan, [ "name" ], 5)
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</programlisting>
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The limit argument is optional in the call to
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<function>plpy.execute</function>.
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</para>
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<para>
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When you prepare a plan using the PL/Python module it is
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automatically saved. Read the SPI documentation (<xref
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linkend="spi">) for a description of what this means. The take
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home message is if you do
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<programlisting>
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plan = plpy.prepare("SOME QUERY")
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plan = plpy.prepare("SOME OTHER QUERY")
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</programlisting>
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you are leaking memory, as I know of no way to free a saved plan.
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The alternative of using unsaved plans it even more painful (for
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me).
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</para>
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</sect1>
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</chapter>
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