Re: Optimizing numeric SUM() aggregate - Mailing list pgsql-hackers

From Heikki Linnakangas
Subject Re: Optimizing numeric SUM() aggregate
Date
Msg-id ac2a41c6-9dba-982f-5e8f-9786ea3555c1@iki.fi
Whole thread Raw
In response to Re: Optimizing numeric SUM() aggregate  (Andrey Borodin <amborodin@acm.org>)
List pgsql-hackers
On 08/03/2016 08:47 PM, Andrey Borodin wrote:
> 1. Currenlty overflow is carried every 9999 addition. I suggest that
> it is possibe to do it only every (INT32_MAX - INT32_MAX / NBASE) /
> (NBASE - 1) addition. Please note that with this overflow count it
> will be neccesary to check whether two finishing carrings are
> required.

I left it at (NBASE -1), but added a comment on that. I couldn't measure 
any performance difference between that and a larger interval, so it 
didn't seem worthwhile. The code wouldn't be much more complicated, but 
the math required to explain what the safe maximum is, would be :-).

> 2. Aggregates and numeric regression tests do not containt
> any test case covering overflows. I recommend adding sum with numer 1
> 000 000 of 99 999 999 values. May be you will come up with more
> clever overflow testing.

Hmm. Added a couple of simple SUM() queries to the regression tests, so 
that we have at least some test coverage of the overflows. But nothing 
very clever.

Looking closer, I don't think this implementation is actually dependent 
on NBASE being 10000. So I removed the comment changes that suggested so.

I did some more minor comment fixes, and committed. Thanks for the review!

- Heikki




pgsql-hackers by date:

Previous
From: Petr Jelinek
Date:
Subject: Re: Logical decoding slots can go backwards when used from SQL, docs are wrong
Next
From: Robert Haas
Date:
Subject: Re: less expensive pg_buffercache on big shmem