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From: David Rowley <dgrowleyml@gmail.com>
Date: Wed, 8 May 2024 10:47:32 +1200
Message-ID: <CAApHDvrvcJd2DCAV0vDKmRxp5tvXq3=+RwhRns4OjoHQ1UbDEg@mail.gmail.com>
Subject: Re: Revert: Remove useless self-joins *and* -DREALLOCATE_BITMAPSETS
 make server crash, regress test fail.
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andrew Dunstan <andrew@dunslane.net>, Richard Guo <guofenglinux@gmail.com>, 
	jian he <jian.universality@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Wed, 8 May 2024 at 10:40, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> David Rowley <dgrowleyml@gmail.com> writes:
> > We could make the policy copy-on-modify.  If you put bms_copy around
> > the bms_del_member() calls in remove_rel_from_query(), does it pass
> > then?
>
> Didn't test, but that route seems awfully invasive and fragile: how
> will we find all the places to modify, or ensure that the policy
> is followed by future patches?

REALLOCATE_BITMAPSETS was invented for this and IMO, it found exactly
the problem it was invented to find.

Copy-on-modify is our policy for node mutation. Why is it ok there but
awfully fragile here?

David