Re: postgres performance - Mailing list pgsql-performance

On 06/12/13 17:36, chidamparam muthusamy wrote:

I rather think Alan is right - you either want a lot more RAM or faster
disks. Have a look at your first query...

> Query:
> EXPLAIN (analyze, buffers) SELECT text(client) as client, text(gateway)
> as gateway,count(*)::bigint as total_calls, (avg(duration)/1000.0)
> ::numeric(10,2) as acd, (avg(pdd)) ::numeric(10,2) as pdd,
> sum(call_duration_recv)/1000.0 as duration_recv,
> sum(call_duration_pay)/1000.0 as duration_pay, sum(call_amount_recv) as
> call_amount_recv, sum(call_amount_pay) as call_amount_
> pay FROM detailed_report WHERE end_time>='2013-05-01 00:00' and
> end_time<'2013-07-01 00:00' and group_id='admin' and client ='CHOICE'
> GROUP by client, gateway ORDER BY call_amount_recv DESC;

> QUERY PLAN
> ------------------------------------------------------
> Sort (cost=3422863.06..3422868.69 rows=2254 width=44) (actual
> time=137852.474..137852.474 rows=5 loops=1)
> Sort Key: (sum(call_amount_recv))
> Sort Method: quicksort Memory: 25kB
> Buffers: shared read=2491664

> -> HashAggregate (cost=3422664.28..3422737.53 rows=2254 width=44)
> (actual time=137852.402..137852.454 rows=5 loops=1)
> Buffers: shared read=2491664

> -> Bitmap Heap Scan on detailed_report (cost=644828.11..3399506.87
> rows=1029218 width=44) (actual time=4499.558..125443.122 rows=5248227
> loops=1)
> Recheck Cond: ((end_time >= '2013-05-01 00:00:00+00'::timestamp with
> time zone) AND (end_time < '2013-07-01 00:00:00+00'::timestamp with time
> zone) AND ((group_id)::text = 'adm
> in'::text) AND ((client)::text = 'CHOICE'::text))
> Buffers: shared read=2491664

> -> Bitmap Index Scan on endtime_groupid_client_tsidx_detail_report
> (cost=0.00..644570.81 rows=1029218 width=0) (actual
> time=3418.754..3418.754 rows=5248227 loops=1)
> Index Cond: ((end_time >= '2013-05-01 00:00:00+00'::timestamp with time
> zone) AND (end_time < '2013-07-01 00:00:00+00'::timestamp with time
> zone) AND ((group_id)::text =
> 'admin'::text) AND ((client)::text = 'CHOICE'::text))
> Buffers: shared read=95055

> Total runtime: *137868.946 ms*
> (13 rows)

The index is being used, but most of your time is going on the "Bitmap
Heap Scan". You're processing 5.2 million rows in about 120 seconds -
that's about 43 rows per millisecond - not too bad. It's not getting any
cache hits though, it's having to read all the blocks. Looking at the
number of blocks, that's ~2.5 million at 8KB each or about 20GB. You
just don't have the RAM to cache that.

If you have lots of similar reporting queries to run, you might get away
with dropping the index and letting them run in parallel. Each
individual query would be slow but they should be smart enough to share
each other's sequential scans - the disks would basically be looping
through you data continuously.

--
   Richard Huxton
   Archonet Ltd