From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
cc: balazs@obiserver.hu, pgsql-bugs@postgresql.org,
        pgsql-hackers@postgresql.org
Subject: Re: [BUGS] BUG #14825: enum type: unsafe use?
In-reply-to: <4942e2c0-3302-f496-d4c1-a967f77d0143@2ndQuadrant.com>
References: <20170922185904.1448.16585@wrigleys.postgresql.org>
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 <3cece400-af56-7145-2e65-f044de1b0b68@2ndQuadrant.com>
 <28968.1506136792@sss.pgh.pa.us>
 <f29e2d7e-7d71-b3b7-2fed-d52bddd8ca61@2ndQuadrant.com>
 <24380.1506179805@sss.pgh.pa.us> <16272.1506189656@sss.pgh.pa.us>
 <928f5974-151f-0273-0221-fac213e7624f@2ndQuadrant.com>
 <19137.1506196325@sss.pgh.pa.us>
 <4942e2c0-3302-f496-d4c1-a967f77d0143@2ndQuadrant.com>
Comments: In-reply-to Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
	message dated "Sat, 23 Sep 2017 17:45:46 -0400"
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Date: Sat, 23 Sep 2017 18:06:21 -0400
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Andrew Dunstan <andrew.dunstan@2ndquadrant.com> writes:
> OK, I think I'm convinced. Here's is the WIP code I put together for the
> blacklist. I'm was looking for a place to put the init call, but since
> it's possibly not going anywhere I stopped :-) . My initial thought
> about substransactions was that we should ignore them for this purpose
> (That's why I used TopTransactionContext for the table).

For the blacklist, I agree we could just ignore subtransactions: all
subtransaction levels are equally uncommitted for this purpose, and
leaving entries from failed subtransactions in place seems like a
non-issue, since they'd never be referenced again.  (Well, barring OID
wraparound and an enum-value-OID collision while the transaction runs,
but I think we can ignore that as having probability epsilon.)

But you need to actually put the table in TopTransactionContext, not
CurTransactionContext ;-).  Also, I don't think you need an init call
so much as an end-of-transaction cleanup call.  Maybe call it
AtEOXactEnum(), for consistency with other functions called in the
same area.

> w.r.t. table size - how large? I confess I haven't seen any systems with
> more than a few hundred enum types. But even a million or two shouldn't
> consume a huge amount of memory, should it?

Dynahash tables are self-expanding, so I don't see a need to stress about
that too much.  Anything in 10-100 seems reasonable for initial size.

			regards, tom lane


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