Re: OFFSET/LIMIT - Disparate Performance w/ Go application

På torsdag 12. juni 2014 kl. 16:58:06, skrev Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us
<mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us>>: Matt Silverlock <matt(at)eatsleeprepeat(dot)net> writes:
> Hi all. This might be tricky in so much as there���s a few moving parts
(when isn���t there?), but I���ve tried to test the postgres side as much as
possible.
> Trying to work out a potential database bottleneck with a HTTP application
(written in Go):
> Pages that render HTML templates but don���t perform DB queries can hit
~36k+ req/s
> Pages that perform a SELECT on a single row net about ~6.6k req/s:
db.Get(l, "SELECT * FROM listings WHERE id = $1 AND expiry_date >
current_date", l.Id)
> Pages that SELECT multiple rows with OFFSET and LIMIT conditions struggle
to top 1.3k req/s

You don't show us exactly what you're doing with OFFSET/LIMIT, but I'm
going to guess that you're using it to paginate large query results.
That's basically always going to suck: Postgres has no way to implement
OFFSET except to generate and then throw away that number of initial rows.
If you do the same query over again N times with different OFFSETs, it's
going to cost you N times as much as the base query would.   Are there any
plans to make PG implement OFFSET more efficiently, so it doesn't have to "read
and throw away"?   I used SQL Server back in 2011 in a project and seem to
remember they implemented offset pretty fast. Paging in a resultset of millions
was much faster than in PG.   -- Andreas Jospeh Krogh CTO / Partner - Visena AS
Mobile: +47 909 56 963 andreas(at)visena(dot)com <mailto:andreas(at)visena(dot)com>
www.visena.com <https://www.visena.com> <https://www.visena.com>