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Re: Querying 19million records very slowly

From: Kjell Tore Fossbakk <kjelltore(at)gmail(dot)com>
To: PFC <lists(at)boutiquenumerique(dot)com>
Cc: Paul Ramsey <pramsey(at)refractions(dot)net>,pgsql-performance(at)postgresql(dot)org
Subject: Re: Querying 19million records very slowly
Date: 2005-06-22 07:45:22
Message-ID: e79986c505062200457fe97695@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
database=> set enable_seqscan to on;
SET
Time: 0.34 ms



database=> explain analyze select count(*) from test where p1=53;
                                                      QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=522824.50..522824.50 rows=1 width=0) (actual
time=56380.72..56380.72 rows=1 loops=1)
   ->  Seq Scan on test  (cost=0.00..517383.30 rows=2176479 width=0)
(actual time=9.61..47677.48 rows=2220746 loops=1)
         Filter: (p1 = 53)
 Total runtime: 56380.79 msec
(4 rows)

Time: 56381.40 ms



database=> explain analyze select count(*) from test where p1=53 and
time > now() - interval '24 hours' ;
                                                       QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=661969.01..661969.01 rows=1 width=0) (actual
time=45787.02..45787.02 rows=1 loops=1)
   ->  Seq Scan on test  (cost=0.00..660155.28 rows=725493 width=0)
(actual time=37799.32..45613.58 rows=42424 loops=1)
         Filter: ((p1 = 53) AND ("time" > (now() - '1 day'::interval)))
 Total runtime: 45787.09 msec
(4 rows)

Time: 45787.79 ms



database=> explain analyze select date_trunc('hour', time),count(*) as
total from test where p1=53 and time>now()-interval '24 hours' group
by date_trunc order by date_trunc;
                                                             QUERY
PLAN
------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=755116.97..760558.17 rows=72549 width=8) (actual
time=46040.63..46717.61 rows=23 loops=1)
   ->  Group  (cost=755116.97..758744.44 rows=725493 width=8) (actual
time=46022.06..46548.84 rows=42407 loops=1)
         ->  Sort  (cost=755116.97..756930.70 rows=725493 width=8)
(actual time=46022.04..46198.94 rows=42407 loops=1)
               Sort Key: date_trunc('hour'::text, "time")
               ->  Seq Scan on test  (cost=0.00..660155.28 rows=725493
width=8) (actual time=37784.91..45690.88 rows=42407 loops=1)
                     Filter: ((p1 = 53) AND ("time" > (now() - '1
day'::interval)))
 Total runtime: 46718.43 msec
(7 rows)

Time: 46719.44 ms



database=> create index test_time_p1_idx on test(time,p1) ;
CREATE INDEX
Time: 178926.02 ms

database=> vacuum analyze test ;
VACUUM
Time: 73058.33 ms

database=> \d test
               Table "public.test"
  Column     |           Type           | Modifiers
-------------+--------------------------+-----------
 time        | timestamp with time zone |
 source      | inet                     |
 destination | inet                     |
 p1          | integer                  |
 p2          | integer                  |



database=> \di
 public | test_time_idx        | index | database | test
 public | test_source_idx      | index | database | test
 public | test_destination_idx | index | database | test
 public | test_p1_idx          | index | database | test
 public | test_p2_idx          | index | database | test
 public | test_time_p1_idx     | index | database | test



database=> set enable_seqscan to off ;
SET
Time: 0.28 ms



database=> explain analyze select date_trunc('hour', time),count(*) as
total from test where p1=53 and time>now()-interval '24 hours' group
by date_trunc order by date_trunc;
 Aggregate  (cost=2315252.66..2320767.17 rows=73527 width=8) (actual
time=2081.15..2720.44 rows=23 loops=1)
   ->  Group  (cost=2315252.66..2318929.00 rows=735268 width=8)
(actual time=2079.76..2564.22 rows=41366 loops=1)
         ->  Sort  (cost=2315252.66..2317090.83 rows=735268 width=8)
(actual time=2079.74..2243.32 rows=41366 loops=1)
               Sort Key: date_trunc('hour'::text, "time")
               ->  Index Scan using test_time_p1_idx on test 
(cost=0.00..2218878.46 rows=735268 width=8) (actual
time=29.50..1774.52 rows=41366 loops=1)
                     Index Cond: (("time" > (now() - '1
day'::interval)) AND (p1 = 53))
 Total runtime: 2735.42 msec

Time: 2736.48 ms



database=> explain analyze select date_trunc('hour', time),count(*) as
total from test where p1=80 and time>now()-interval '24 hours' group
by date_trunc order by date_trunc;
 Aggregate  (cost=1071732.15..1074305.59 rows=34313 width=8) (actual
time=6353.93..7321.99 rows=22 loops=1)
   ->  Group  (cost=1071732.15..1073447.77 rows=343125 width=8)
(actual time=6323.76..7078.10 rows=64267 loops=1)
         ->  Sort  (cost=1071732.15..1072589.96 rows=343125 width=8)
(actual time=6323.75..6579.42 rows=64267 loops=1)
               Sort Key: date_trunc('hour'::text, "time")
               ->  Index Scan using test_time_p1_idx on test 
(cost=0.00..1035479.58 rows=343125 width=8) (actual time=0.20..5858.67
rows=64267 loops=1)
                     Index Cond: (("time" > (now() - '1
day'::interval)) AND (p1 = 80))
 Total runtime: 7322.82 msec

Time: 7323.90 ms



database=> explain analyze select date_trunc('hour', time),count(*) as
total from test where p1=139 and time>now()-interval '24 hours' group
by date_trunc order by date_trunc;
 Aggregate  (cost=701562.34..703250.12 rows=22504 width=8) (actual
time=2448.41..3033.80 rows=22 loops=1)
   ->  Group  (cost=701562.34..702687.53 rows=225037 width=8) (actual
time=2417.39..2884.25 rows=36637 loops=1)
         ->  Sort  (cost=701562.34..702124.94 rows=225037 width=8)
(actual time=2417.38..2574.19 rows=36637 loops=1)
               Sort Key: date_trunc('hour'::text, "time")
               ->  Index Scan using test_time_p1_idx on test 
(cost=0.00..679115.34 rows=225037 width=8) (actual time=8.47..2156.18
rows=36637 loops=1)
                     Index Cond: (("time" > (now() - '1
day'::interval)) AND (p1 = 139))
 Total runtime: 3034.57 msec

Time: 3035.70 ms



Now, this query gives me all the hours in a day, with the count of all
p1=53 for each hour. Pg uses 46.7 seconds to run with seqscan, while
2.7 seconds indexing on (time,p1). I think I turned "set
enable_seqscan to on;" again, and then the planner used seqscan, and
not index.
- Why does Pg not see the benefits of using index?
- and how can i tune the optimisation fields in postgresql.conf to help him?

So now my PG uses a reasonable amout of time on these queries (with
enable_seqscan turned off)

The next place which seems to slow my queries, is probably my
connection to PHP. I got a bash script running in cron on my server
(freebsd 4.11), which runs php on a php file. To force PG to not use
seqscans, I have modifies the postgresql.conf:

..
enable_seqscan = false
enable_indexscan = true
..
effective_cache_size = 10000
random_page_cost = 2
..

I save the file, type 'pg_crl reload' then enter 'psql database'.

database=> show enable_seqscan ;
 enable_seqscan
----------------
 on
(1 row)


argus=> show effective_cache_size ;
 effective_cache_size
----------------------
 1000
(1 row)

I have used the manual pages on postgresql, postmaster, and so on, but
I cant find anywhere to specify which config file Pg is to use. I'm
not entirely sure if he uses the one im editing
(/usr/local/etc/postgresql.conf).

Any hints, tips or help is most appreciated!

Kjell Tore.





On 6/21/05, PFC <lists(at)boutiquenumerique(dot)com> wrote:
> 
> use CURRENT_TIME which is a constant instead of now() which is not  
> considered constant... (I think)
>

In response to

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Next:From: Tobias BroxDate: 2005-06-22 08:03:57
Subject: Re: Querying 19million records very slowly
Previous:From: Radu-Adrian PopescuDate: 2005-06-22 07:24:15
Subject: Re: Prepared statements vs. Stored Procedures

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