Re: Release SPI plans for referential integrity with DISCARD ALL

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: yuzuko <yuzukohosoya(at)gmail(dot)com>
Cc: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>, Corey Huinker <corey(dot)huinker(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Release SPI plans for referential integrity with DISCARD ALL
Date: 2021-09-26 19:05:01
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yuzuko <yuzukohosoya(at)gmail(dot)com> writes:
> I improved a patch according to Peter's following comment :

>> but I think the
>> solution of dropping all cached plans as part of DISCARD ALL seems a bit
>> too extreme of a solution. In the context of connection pooling,
>> getting a new session with pre-cached plans seems like a good thing, and
>> this change could potentially have a performance impact without a
>> practical way to opt out.

> In a new patch, I separated discarding SPI Plan caches for RI from DISCARD ALL
> and added a new option "RIPLANS" to DISCARD command to do that.

I'm not really comfortable with exposing this as a user-visible behavior.
In particular ...

> The amount of modification is small and this option can be
> removed instantly when it is no longer needed, so I think we can use
> this patch as a provisional solution.

... this claim seems like nonsense. Once there is user-accessible syntax
for something, we basically can't ever take that out, because it'd break
applications that call that command.

It seems like the real problem with the ri_query_cache cache is that
it will grow without limit. I wonder whether it wouldn't be better
to put some upper bound on it, along with a simple least-recently-used
rule to choose which entry to discard. (Perhaps the same goes for
ri_constraint_cache and/or ri_compare_cache, although their entry sizes
are smaller.)

regards, tom lane

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