Re: [HACKERS] Secondary index access optimizations

From: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
To: Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Antonin Houska <ah(at)cybertec(dot)at>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: [HACKERS] Secondary index access optimizations
Date: 2018-07-09 01:26:40
Message-ID: CAKJS1f_5KYJn8y_42jBYnAre71R50s1Z_8nH_hHkEee-DB7Qvg@mail.gmail.com
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On 22 March 2018 at 22:38, Konstantin Knizhnik
<k(dot)knizhnik(at)postgrespro(dot)ru> wrote:
> Attached please find rebased version of the patch.

Hi,

I started looking over this patch and have a few comments:

I don't think this range type idea is a great one. I don't think it's
going to ever perform very well. I also see you're not checking the
collation of the type anywhere. As of now, no range types have
collation support, but you can't really go and code this with that
assumption. I also don't really like the sequence scan over pg_range.

Probably a better way to do this would be to add a new bt proc, like
what was done in 0a459cec for 2 new functions. Something like
BTISNEXTVAL_PROC and BTISPREVVAL_PROC. You'd then need to define
functions for all the types based on integers, making functions which
take 2 parameters of the type, and an additional collation param. The
functions should return bool. int4_isnextval(2, 3, InvalidOid) would
return true. You'd need to return false on wraparound.

I also think that the patch is worth doing without the additional
predicate_implied_by() smarts. In fact, I think strongly that the two
should be considered as two independent patches. Partial indexes
suffer from the same issue you're trying to address here and that
would be resolved by any patch which makes changes to
predicate_implied_by().

Probably the best place to put the code to skip the redundant quals is
inside set_append_rel_size(). There's already code there that skips
quals that are seen as const TRUE. This applies for UNION ALL targets
with expressions that can be folded to constants once the qual is
passed through adjust_appendrel_attrs()

For example:
explain select * from (select 1 as a from pg_class union all select 2
from pg_class) t where a = 1;

I've attached a patch to show what I had in mind. I had to change how
partition_qual is populated, which I was surprised to see only gets
populated for sub-partitioned tables (the top-level parent won't have
a qual since it's not partitioned) I didn't update the partition
pruning code that assumes this is only populated for sub-partitioned
table. That will need to be done. The patch comes complete with all
the failing regression tests where the redundant quals have been
removed by the new code.

If you want to make this work for CHECK constraints too, then I think
the code for that can be added to the same location as the code I
added in the attached patch. You'd just need to fetch the check
constraint quals and just add some extra code to check if the qual is
redundant.

Some new performance benchmarks would then be useful in order to find
out how much overhead there is. We might learn that we don't want to
enable it by default if it's too expensive.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

Attachment Content-Type Size
skip_redundant_partition_quals_poc.patch application/octet-stream 1.5 KB

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