PostgreSQL 7.2.1 Documentation
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# 12.7. Procedural Language Handlers

All calls to functions that are written in a language other than the current "version 1" interface for compiled languages (this includes functions in user-defined procedural languages, functions written in SQL, and functions using the version 0 compiled language interface), go through a call handler function for the specific language. It is the responsibility of the call handler to execute the function in a meaningful way, such as by interpreting the supplied source text. This section describes how a language call handler can be written. This is not a common task, in fact, it has only been done a handful of times in the history of PostgreSQL, but the topic naturally belongs in this chapter, and the material might give some insight into the extensible nature of the PostgreSQL system.

The call handler for a procedural language is a "normal" function, which must be written in a compiled language such as C and registered with PostgreSQL as taking no arguments and returning the `opaque` type, a placeholder for unspecified or undefined types. This prevents the call handler from being called directly as a function from queries. (However, arguments may be supplied in the actual call to the handler when a function in the language offered by the handler is to be executed.)

> Note: In PostgreSQL 7.1 and later, call handlers must adhere to the "version 1" function manager interface, not the old-style interface.

The call handler is called in the same way as any other function: It receives a pointer to a `FunctionCallInfoData` struct containing argument values and information about the called function, and it is expected to return a `Datum` result (and possibly set the `isnull` field of the `FunctionCallInfoData` struct, if it wishes to return an SQL NULL result). The difference between a call handler and an ordinary callee function is that the `flinfo->fn_oid` field of the `FunctionCallInfoData` struct will contain the OID of the actual function to be called, not of the call handler itself. The call handler must use this field to determine which function to execute. Also, the passed argument list has been set up according to the declaration of the target function, not of the call handler.

It's up to the call handler to fetch the `pg_proc` entry and to analyze the argument and return types of the called procedure. The AS clause from the CREATE FUNCTION of the procedure will be found in the `prosrc` attribute of the `pg_proc` table entry. This may be the source text in the procedural language itself (like for PL/Tcl), a path name to a file, or anything else that tells the call handler what to do in detail.

Often, the same function is called many times per SQL statement. A call handler can avoid repeated lookups of information about the called function by using the `flinfo->fn_extra` field. This will initially be NULL, but can be set by the call handler to point at information about the PL function. On subsequent calls, if `flinfo->fn_extra` is already non-NULL then it can be used and the information lookup step skipped. The call handler must be careful that `flinfo->fn_extra` is made to point at memory that will live at least until the end of the current query, since an `FmgrInfo` data structure could be kept that long. One way to do this is to allocate the extra data in the memory context specified by `flinfo->fn_mcxt`; such data will normally have the same lifespan as the `FmgrInfo` itself. But the handler could also choose to use a longer-lived context so that it can cache function definition information across queries.

When a PL function is invoked as a trigger, no explicit arguments are passed, but the `FunctionCallInfoData`'s `context` field points at a `TriggerData` node, rather than being NULL as it is in a plain function call. A language handler should provide mechanisms for PL functions to get at the trigger information.

This is a template for a PL handler written in C:

    #include "postgres.h"
    #include "executor/spi.h"
    #include "commands/trigger.h"
    #include "utils/elog.h"
    #include "fmgr.h"
    #include "access/heapam.h"
    #include "utils/syscache.h"
    #include "catalog/pg_proc.h"
    #include "catalog/pg_type.h"

    PG_FUNCTION_INFO_V1(plsample_call_handler);

    Datum
    plsample_call_handler(PG_FUNCTION_ARGS)
    {
        Datum          retval;

        if (CALLED_AS_TRIGGER(fcinfo))
        {
            /*
             * Called as a trigger procedure
             */
            TriggerData    *trigdata = (TriggerData *) fcinfo->context;

            retval = ...
        }
        else {
            /*
             * Called as a function
             */

            retval = ...
        }

        return retval;
    }

Only a few thousand lines of code have to be added instead of the dots to complete the call handler. See Section 12.5 for information on how to compile it into a loadable module.

The following commands then register the sample procedural language:

    CREATE FUNCTION plsample_call_handler () RETURNS opaque
        AS '/usr/local/pgsql/lib/plsample'
        LANGUAGE C;
    CREATE LANGUAGE plsample
        HANDLER plsample_call_handler;

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