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

Thanks for taking your time to answer. Not sure if I understand though.

> On 3 Apr 2024, at 16:27, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> =?utf-8?Q?Micha=C5=82_K=C5=82eczek?= <michal(at)kleczek(dot)org> writes:
>> When implementing a GiST consistent function I found the need to cache pre-processed query across invocations.
>> I am not sure if it is safe to do (or I need to perform some steps to make sure cached info is not leaked between rescans).
>
> 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.

>
>> The comment in gistrescan says:
>
>> /*
>> * If this isn't the first time through, preserve the fn_extra
>> * pointers, so that if the consistentFns are using them to cache
>> * data, that data is not leaked across a rescan.
>> */
>
>> which seems to me self-contradictory as fn_extra is preserved between rescans (so leaks are indeed possible).
>
> I think you're reading it wrong. If we cleared fn_extra during
> rescan, access to the old extra value would be lost so a new one
> would have to be created, leaking the old value for the rest of
> the query.

I understand that but not sure what “that data is not leaked across a rescan” means.

—
Michal