Re: surprising query optimisation

On 29/11/2018 11:26, Chris Withers wrote:
> Hi All,
>
> We have an app that deals with a lot of queries, and we've been slowly
> seeing performance issues emerge. We take a lot of free form queries
> from users and stumbled upon a very surprising optimisation.
>
> So, we have a 'state' column which is a 3 character string column with
> an index on it. Despite being a string, this column is only used to
> store one of three values: 'NEW', 'ACK', or 'RSV'.
>
> One of our most common queries clauses is "state!='RSV'" and we've
> found that by substituting this clause with "state='ACK' or
> state='NEW'" wherever it was used, we've dropped the postgres server's
> load average from 20 down to 4 and the CPU usage from 60% in user
> space down to <5%.
>
> This seems counter-intuitive to me, so thought I'd ask here. Why would
> this be likely to make such a difference? We're currently on 9.4, is
> this something that's likely to be different (better? worse?) if we
> got all the way up to 10 or 11?
>
> cheers,
>
> Chris
>
>
At a guess...

    "state!='RSV'"  ==> pg only has to check one value

and

    "state='ACK' or state='NEW'"   ==> pg has to check two values

so I would expect the '!=' to be faster.

Cheers,
Gavin