surprising query optimisation

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