=?UTF-8?Q?Filip_Rembia=C5=82kowski?= <filip.rembialkowski@gmail.com> writes:
> Among following queries, only THREE runs fast enough for me.
> I can't understand the logic behind this.
I'm not sure why you'd expect real answers when you haven't shown us
what the query is doing, but my first thought is that the discrepancy
comes from additional buffer touches in the first execution of a query
in a given backend; which is not exactly surprising because that backend
has to load up its system catalog caches. IOW, the excess touches
represent accesses to system catalogs not user tables.
In general, if you're annoyed by query execution times measured in
milliseconds, you'd be best advised not to start a fresh connection
for each one. A new connection not only involves a process launch
but a fair amount of loading of local caches, and a large part of
the latter work happens during the first few queries it processes.
regards, tom lane