From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
Cc: regrog <andrea.vencato@gmail.com>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: view reading information_schema is slow in PostgreSQL 12
Date: Sat, 13 Jun 2020 13:06:46 -0400
Message-ID: <845804.1592068006@sss.pgh.pa.us> (raw)
In-Reply-To: <CAApHDvpeZ1q79YZwn7qnCvaRFPMkTixPnuaBgn3JF=1+skd9Yg@mail.gmail.com>
References: <1591975278775-0.post@n3.nabble.com>
<798686.1592017869@sss.pgh.pa.us>
<CAApHDvrnop0h+du73FZKEzxd0CGP3RUoWPNha2ekz_F0YMNaYA@mail.gmail.com>
<800752.1592021253@sss.pgh.pa.us>
<CAApHDvrj9b3OrDt_Yc-21LUF23_hd=Ni5o856xXGidaQtQ__GQ@mail.gmail.com>
<CAApHDvpeZ1q79YZwn7qnCvaRFPMkTixPnuaBgn3JF=1+skd9Yg@mail.gmail.com>
David Rowley <dgrowleyml@gmail.com> writes:
> On Sat, 13 Jun 2020 at 19:52, David Rowley <dgrowleyml@gmail.com> wrote:
>> On Sat, 13 Jun 2020 at 16:07, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Hm. Maybe we could reasonably assume that the equality operators used
>>> for such constructs are error-and-side-effect-free, thus dodging the
>>> semantic problem I mentioned in the other thread?
>> I'm only really talking about selectivity estimation only for now.
>> I'm not really sure why we'd need to ensure that the equality operator
>> is error and side effect free. We'd surely only be executing the case
>> statement's operator's oprrest function? We'd need to ensure we don't
>> invoke any casts that could error out.
> Hmm, after a bit of thought I now see what you mean.
No, you were right the first time: we're considering different things.
I was wondering about how to constant-fold a "CASE = constant" construct
as was being requested in the other thread. Obviously, if that succeeds
then it'll simplify selectivity estimation too --- but it's reasonable
to also think about what to do for "CASE = constant" in selectivity
estimation, because with or without such a constant-folding rule,
there would be lots of cases that the rule fails to simplify. Further
we should be thinking about how to get some estimate for cases that
the folding rule would fail at, so I'm not sure that we ought to restrict
our thoughts to constant comparisons.
In the cases I've seen so far, even a rule as dumb as "if the CASE has
N arms then estimate selectivity as 1/N" would be a lot better than
what we get now.
regards, tom lane
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, dgrowleyml@gmail.com, andrea.vencato@gmail.com
Subject: Re: view reading information_schema is slow in PostgreSQL 12
In-Reply-To: <845804.1592068006@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox