From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Dunstan <andrew@dunslane.net>
cc: Andres Freund <andres@anarazel.de>, exclusion@gmail.com,
        pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #17994: Invalidating relcache corrupts tupDesc inside
 ExecEvalFieldStoreDeForm()
In-reply-to: <e6cbff03-32c3-4048-78b6-54d2907ba852@dunslane.net>
References: <17994-5c7100b51b4790e9@postgresql.org>
 <1306569.1687978174@sss.pgh.pa.us>
 <20230628194446.uu47f4vyfyjfl7dd@awork3.anarazel.de>
 <1314841.1687981948@sss.pgh.pa.us>
 <20230628205443.6gu5bmcck6t4pgyv@awork3.anarazel.de>
 <e52f6028-7196-8545-9726-a17a50ea14a3@dunslane.net>
 <e6cbff03-32c3-4048-78b6-54d2907ba852@dunslane.net>
Comments: In-reply-to Andrew Dunstan <andrew@dunslane.net>
	message dated "Thu, 29 Jun 2023 09:56:39 -0400"
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Date: Thu, 29 Jun 2023 10:26:24 -0400
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Andrew Dunstan <andrew@dunslane.net> writes:
> After (not) sleeping on this overnight, and discussing it with a 
> colleague this morning, here's a suggestion. We have a hash table, keyed 
> by (tdtypeid, attnum) where we store a datumCopy'd version of the value. 
> If it's present just return the value instead of getting it from the 
> tupdesc. The hash table is blown away at the end of the transaction. 
> Assuming that's workable I think it would not be a large patch.

That sounds possibly workable.  I'm a bit concerned about added
overhead, and about whether the hashtable needs invalidation support.
It might be better to key it off (relfilenode, attnum).

> My colleague did ask if we had live reproducible case.

I have not attempted to build one, but it'd probably be fairly
easy to do if you turn on debug_discard_caches.

			regards, tom lane