Re: Cached plans and statement generalization

From: Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Cached plans and statement generalization
Date: 2017-04-26 10:30:25
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On 26.04.2017 10:49, Konstantin Knizhnik wrote:
> On 26.04.2017 04:00, Tsunakawa, Takayuki wrote: Are you considering
> some upper limit on the number of prepared statements?
> In this case we need some kind of LRU for maintaining cache of
> autoprepared statements.
> I think that it is good idea to have such limited cached - it can
> avoid memory overflow problem.
> I will try to implement it.

I attach new patch which allows to limit the number of autoprepared
statements (autoprepare_limit GUC variable).
Also I did more measurements, now with several concurrent connections
and read-only statements.
Results of pgbench with 10 connections, scale 10 and read-only
statements are below:


As you can see, autoprepare provides more than 2 times speed improvement.

Also I tried to measure overhead of parsing (to be able to substitute
all literals, not only string literals).
I just added extra call of pg_parse_query. Speed is reduced to 181k.
So overhead is noticeable, but still making such optimization useful.
This is why I want to ask question: is it better to implement slower
but safer and more universal solution?

Konstantin Knizhnik
Postgres Professional:
The Russian Postgres Company

Attachment Content-Type Size
autoprepare.patch text/x-patch 19.4 KB

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