Re: Sorting performance vs. MySQL

On Mon, Feb 22, 2010 at 10:51 PM, Yang Zhang <yanghatespam@gmail.com> wrote:
> nnnnnOn Mon, Feb 22, 2010 at 3:45 PM, Scott Marlowe
> <scott.marlowe@gmail.com> wrote:
>>
>> What do things like vmstat 10 say while the query is running on each
>> db?  First time, second time, things like that.
>
> Awesome -- this actually led me to discover the problem.
>
> vmstat showed no swapping-out for a while, and then suddenly it
> started spilling a lot. Checking psql's memory stats showed that it
> was huge -- apparently, it's trying to store its full result set in
> memory. As soon as I added a LIMIT 10000, everything worked
> beautifully and finished in 4m (I verified that the planner was still
> issuing a Sort).
>
> I'm relieved that Postgresql itself does not, in fact, suck, but
> slightly disappointed in the behavior of psql. I suppose it needs to
> buffer everything in memory to properly format its tabular output,
> among other possible reasons I could imagine.

It's best when working with big sets to do so with a cursor and fetch
a few thousand rows at a time.  It's how we handle really big sets at
work and it works like a charm in keeping the client from bogging down
with a huge memory footprint.