Set Returning C-Function with cache over multiple calls (with different arguments)

Dear list,

Currently I am working on a user C-Function which should create a cache object on the first call and afterwards return a set of computed values for each argument combination it is called with.

My Problem is how to get the cache object saved over multiple calls. Without the SRF I could use fcinfo->flinfo->fn_extra for my pointer to the data. This is now used by the FuncCallContext structure. This structure is destroyed every time SRF_RETURN_DONE is called, thus user_fctx also is not the way to go.

As a minimal example look at the function provided below.

--------------------------------------------------- snip ---------------------------------------------------
PG_FUNCTION_INFO_V1(test);
Datum test(PG_FUNCTION_ARGS)
{
MemoryContext old_context;
FuncCallContext *funcctx;


if (SRF_IS_FIRSTCALL()) {
funcctx = SRF_FIRSTCALL_INIT();

// This is the structure potentially generated in previous calls
str = funcctx->user_fctx;

// If the structure does not exist or the geometry array has changed, it has to be created.
if ( ! str) {
elog(NOTICE, "create new");

old_context = MemoryContextSwitchTo(fcinfo->flinfo->fn_mcxt);

// Fill str with data around here ...

MemoryContextSwitchTo(old_context);

funcctx->user_fctx = str;
}

}
funcctx = SRF_PERCALL_SETUP();
SRF_RETURN_DONE(funcctx);
}
--------------------------------------------------- snip ---------------------------------------------------

To make the problem perfectly clear the SQL-Code this should work with:

--------------------------------------------------- snip ---------------------------------------------------
CREATE OR REPLACE FUNCTION test(int, int[] )
RETURNS SETOF int AS 'myfunc', 'test'
LANGUAGE 'c' IMMUTABLE STRICT
COST 1;

SELECT test(number, array(SELECT integers FROM another_table)) FROM numbers;
--------------------------------------------------- snip ---------------------------------------------------

As creating the cache object is by far the most expensive part in the desired function, it should be possible to create the cache only once over the whole query - using only the arguments in the array, which do not change over multiple calls.

Is there a way around this problem? Another pointer I could use and do not know of yet?

Thanks in advance,
Thilo Schneider