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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Heikki Linnakangas <hlinnaka@iki.fi>
cc: Joel Jacobson <joel@compiler.org>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async
 queue
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Comments: In-reply-to Heikki Linnakangas <hlinnaka@iki.fi>
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Heikki Linnakangas <hlinnaka@iki.fi> writes:
> Fortunately that's easy to fix: We can move the IsListeningOn() check 
> after releasing the lock. See attached.
> The elephant in the room of course is that a lookup in a linked list is 
> O(n) and it would be very straightforward to replace it with e.g. a hash 
> table. We should do that irrespective of this bug fix. But I'm inclined 
> to do it as a separate followup patch.

There is a different patch series that's concerned with improving
NOTIFY throughput [1].  I think this one should just focus on
fixing the XID problem.

			regards, tom lane

[1] https://commitfest.postgresql.org/patch/6078/