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From: Michał Kłeczek <[email protected]>
To: Tom Lane <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Is it safe to cache data by GiST consistent function
Date: Wed, 3 Apr 2024 18:47:03 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

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

> On 3 Apr 2024, at 16:27, Tom Lane <[email protected]> wrote:
> 
> =?utf-8?Q?Micha=C5=82_K=C5=82eczek?= <[email protected]> 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

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