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

> On 3 Apr 2024, at 19:02, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> =?utf-8?Q?Micha=C5=82_K=C5=82eczek?= <michal@kleczek.org> writes:
>
>> 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.

This actually sounds scary - looks like there is no way to perform cache clean-up after rescan then?

Do you think it might be useful to introduce a way for per-rescan caching (ie. setting up a dedicated memory context in
gistrescanand passing it to support functions)? 

—
Michal