Re: Is it safe to cache data by GiST consistent function

=?utf-8?Q?Micha=C5=82_K=C5=82eczek?= <michal(at)kleczek(dot)org> writes:
> On 3 Apr 2024, at 16:27, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> AFAIK it works. I don't see any of the in-core ones doing so,
>> but at least range_gist_consistent and multirange_gist_consistent
>> are missing a bet by repeating their cache search every time.

> pg_trgm consistent caches tigrams but it has some logic to make sure cached values are recalculated:

> cache = (gtrgm_consistent_cache *) fcinfo->flinfo->fn_extra;
> if (cache == NULL ||
> cache->strategy != strategy ||
> VARSIZE(cache->query) != querysize ||
> memcmp((char *) cache->query, (char *) query, querysize) != 0)

> What I don’t understand is if it is necessary or it is enough to check fn_extra==NULL.

Ah, I didn't think to search contrib. Yes, you need to validate the
cache entry. In this example, a rescan could insert a new query
value. In general, an opclass support function could get called using
a pretty long-lived FunctionCallInfo (e.g. one in the index's relcache
entry), so it's unwise to assume that cached data is relevant to the
current call without checking.

regards, tom lane