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

On Mon, Jan 11, 2010 at 2:45 AM, Thilo Schneider
<Thilo(dot)Schneider(at)math(dot)uni-giessen(dot)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