On Mon, Jan 11, 2010 at 2:45 AM, Thilo Schneider
<thilo.schnei...@math.uni-giessen.de> wrote:
> 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?
have you ruled out simply keeping a static pointer around and using malloc()?
merlin
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