Re: Sort performance

Here you go.

Limit (cost=502843.44..502844.69 rows=501 width=618) (actual
time=561397.940..561429.242 rows=501 loops=1)
-> Sort (cost=502843.44..503923.48 rows=432014 width=618) (actual
time=561397.934..561429.062 rows=501 loops=1)
Sort Key: name
-> Seq Scan on objects (cost=0.00..99157.88 rows=432014
width=618) (actual time=0.172..22267.727 rows=649999 loops=1)
Filter: (((domainid)::text = ANY
(('{111,SmWCGiRp}'::character varying[])::text[])) AND (("type")::text =
'cpe'::text) AND (upper((name)::text) ~~ 'CPE1%'::text) AND
(upper((name)::text) >= 'CPE1'::text) AND (upper((name)::text) <
'CPE2'::text))
Total runtime: 561429.915 ms
(6 rows)

I haven't tried setting that high number. I came up with 500M by
monitoring pgsql_tmp when sort operations were performed. It never went
beyond 450M. Once it reaches 450M it spends some cycles before I see the
output. I guess some sort of merge operation happens to get the first
500 records out.

Thanks,
Stalin

-----Original Message-----
From: Robert Haas [mailto:robertmhaas(at)gmail(dot)com]
Sent: Thursday, January 29, 2009 3:21 PM
To: Subbiah Stalin-XCGF84
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] Sort performance

On Thu, Jan 29, 2009 at 3:15 PM, Subbiah Stalin-XCGF84
<SSubbiah(at)motorola(dot)com> wrote:
> I'm in the process of tuning a query that does a sort on a huge
dataset.
> With work_mem set to 2M, i see the sort operation spilling to disk
> writing upto 430MB and then return the first 500 rows. Our query is of

> the sort
>
> select co1, col2... from table where col1 like 'aa%' order col1 limit
> 500; It took 561Secs to complete. Looking at the execution plan 95% of

> the time is spent on sort vs seq scan on the table.
>
> Now if set the work_mem to 500MB (i did this in a psql session without

> making it global) and ran the same query. One would think the sort
> operations would happen in memory and not spill to disk but i still
> see 430MB written to disk however, the query complete time dropped
> down to 351Secs. So work_mem did have an impact but wondering why its
> still writing to disk when it can all do it memory.
>
> I appreciate if anyone can shed some light on this.

Can you send the EXPLAIN ANALYZE output?

What happens if you set work_mem to something REALLY big, like 5GB?

...Robert