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
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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
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