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 chapter outlines how a new procedural language's call handler can be written.
The call handler for a procedural language is a
“normal” function that must be written in a compiled
language such as C, using the version-1 interface, and registered
with PostgreSQL as taking no arguments
and returning the type language_handler
. This
special pseudo-type identifies the function as a call handler and
prevents it from being called directly in SQL commands.
For more details on C language calling conventions and dynamic loading,
see Section 37.9.
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
structure, 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
structure 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 entry of the function from the
pg_proc
system catalog and to analyze the argument
and return types of the called function. The AS
clause from the
CREATE FUNCTION
command for the function will be found
in the prosrc
column of the
pg_proc
row. This is commonly source
text in the procedural language, but in theory it could be something else,
such as 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 called 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 make sure 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 memory context so that it can cache
function definition information across queries.
When a procedural-language function is invoked as a trigger, no arguments
are passed in the usual way, but the
FunctionCallInfoData
's
context
field points at a
TriggerData
structure, rather than being NULL
as it is in a plain function call. A language handler should
provide mechanisms for procedural-language functions to get at the trigger
information.
This is a template for a procedural-language handler written in C:
#include "postgres.h" #include "executor/spi.h" #include "commands/trigger.h" #include "fmgr.h" #include "access/heapam.h" #include "utils/syscache.h" #include "catalog/pg_proc.h" #include "catalog/pg_type.h" #ifdef PG_MODULE_MAGIC PG_MODULE_MAGIC; #endif 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.
After having compiled the handler function into a loadable module (see Section 37.9.5), the following commands then register the sample procedural language:
CREATE FUNCTION plsample_call_handler() RETURNS language_handler
AS 'filename
'
LANGUAGE C;
CREATE LANGUAGE plsample
HANDLER plsample_call_handler;
Although providing a call handler is sufficient to create a minimal procedural language, there are two other functions that can optionally be provided to make the language more convenient to use. These are a validator and an inline handler. A validator can be provided to allow language-specific checking to be done during CREATE FUNCTION. An inline handler can be provided to allow the language to support anonymous code blocks executed via the DO command.
If a validator is provided by a procedural language, it
must be declared as a function taking a single parameter of type
oid
. The validator's result is ignored, so it is customarily
declared to return void
. The validator will be called at
the end of a CREATE FUNCTION
command that has created
or updated a function written in the procedural language.
The passed-in OID is the OID of the function's pg_proc
row. The validator must fetch this row in the usual way, and do
whatever checking is appropriate.
First, call CheckFunctionValidatorAccess()
to diagnose
explicit calls to the validator that the user could not achieve through
CREATE FUNCTION
. Typical checks then include verifying
that the function's argument and result types are supported by the
language, and that the function's body is syntactically correct
in the language. If the validator finds the function to be okay,
it should just return. If it finds an error, it should report that
via the normal ereport()
error reporting mechanism.
Throwing an error will force a transaction rollback and thus prevent
the incorrect function definition from being committed.
Validator functions should typically honor the check_function_bodies parameter: if it is turned off then
any expensive or context-sensitive checking should be skipped. If the
language provides for code execution at compilation time, the validator
must suppress checks that would induce such execution. In particular,
this parameter is turned off by pg_dump so that it can
load procedural language functions without worrying about side effects or
dependencies of the function bodies on other database objects.
(Because of this requirement, the call handler should avoid
assuming that the validator has fully checked the function. The point
of having a validator is not to let the call handler omit checks, but
to notify the user immediately if there are obvious errors in a
CREATE FUNCTION
command.)
While the choice of exactly what to check is mostly left to the
discretion of the validator function, note that the core
CREATE FUNCTION
code only executes SET
clauses
attached to a function when check_function_bodies
is on.
Therefore, checks whose results might be affected by GUC parameters
definitely should be skipped when check_function_bodies
is
off, to avoid false failures when reloading a dump.
If an inline handler is provided by a procedural language, it
must be declared as a function taking a single parameter of type
internal
. The inline handler's result is ignored, so it is
customarily declared to return void
. The inline handler
will be called when a DO
statement is executed specifying
the procedural language. The parameter actually passed is a pointer
to an InlineCodeBlock
struct, which contains information
about the DO
statement's parameters, in particular the
text of the anonymous code block to be executed. The inline handler
should execute this code and return.
It's recommended that you wrap all these function declarations,
as well as the CREATE LANGUAGE
command itself, into
an extension so that a simple CREATE EXTENSION
command is sufficient to install the language. See
Section 37.15 for information about writing
extensions.
The procedural languages included in the standard distribution
are good references when trying to write your own language handler.
Look into the src/pl
subdirectory of the source tree.
The CREATE LANGUAGE
reference page also has some useful details.