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

On Thu, Jun 12, 2014 at 2:48 PM, Andreas Joseph Krogh
<andreas(at)visena(dot)com> wrote:
>
> På torsdag 12. juni 2014 kl. 16:58:06, skrev Tom Lane <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.

I doubt it. Offset is widely regarded as being pretty dubious. SQL
has formally defined the way to do this (cursors) and optimizing
offset would be complex for such a little benefit. Speaking
generally SQL server also has some trick optimizations of other
constucts like fast count(*) but in my experience underperforms pg in
many areas.

merlin