Re: Tuning queries on large database

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>Date: Wed, 4 Aug 2004 21:21:51 +0800 (WST)
>From: Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au>
>To: Valerie Schneider DSI/DEV <Valerie(dot)Schneider(at)meteo(dot)fr>
>Cc: pgsql-performance(at)postgresql(dot)org, <pgsql-general(at)postgresql(dot)org>
>Subject: Re: [PERFORM] Tuning queries on large database
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>> sort_mem = 50000
>
>That is way, way too large. Try more like 5000 or lower.
>
>> num_poste | numeric(9,0) | not null
>
>For starters numerics are really, really slow compared to integers. Why
>aren't you using an integer for this field since youhave '0' decimal
>places.
>
>> schema | relfilenode | table | index | reltuples | size
>>
--------+-------------+------------------+------------+-------------+----------
>> public | 125615917 | data | | 1.25113e+08 |
72312040
>> public | 251139049 | data | i_data_dat | 1.25113e+08 |
2744400
>> public | 250870177 | data | pk_data | 1.25113e+08 |
4395480
>>
>> My first remark is that the table takes a lot of place on disk, about
>> 70 Gb, instead of 35 Gb with oracle.
>
>Integers will take a lot less space than numerics.
>
>> The different queries of the bench are "simple" queries (no join,
>> sub-query, ...) and are using indexes (I "explained" each one to
>> be sure) :
>> Q1 select_court : access to about 700 rows : 1 "num_poste" and 1 month
>> (using PK : num_poste=p1 and dat between p2 and p3)
>> Q2 select_moy : access to about 7000 rows : 10 "num_poste" and 1 month
>> (using PK : num_poste between p1 and p1+10 and dat between p2 and p3)
>> Q3 select_long : about 250 000 rows : 2 "num_poste"
>> (using PK : num_poste in (p1,p1+2))
>> Q4 select_tres_long : about 3 millions rows : 25 "num_poste"
>> (using PK : num_poste between p1 and p1 + 25)
>>
>> The result is that for "short queries" (Q1 and Q2) it runs in a few
>> seconds on both Oracle and PG. The difference becomes important with
>> Q3 : 8 seconds with oracle
>> 80 sec with PG
>> and too much with Q4 : 28s with oracle
>> 17m20s with PG !
>>
>> Of course when I run 100 or 1000 parallel queries such as Q3 or Q4,
>> it becomes a disaster !
>
>Please reply with the EXPLAIN ANALYZE output of these queries so we can
>have some idea of how to help you.
>
>Chris
>
>
>
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Q1 :
bench=> explain analyze select 'Q1',min(td),max(u) from data where
num_poste=1000 and dat between
(date_trunc('month',to_timestamp('31012004','ddmmyyyy')-interval '2000
days'))::timestamp and
(date_trunc('month',to_timestamp('31012004','ddmmyyyy')-interval '2000 days') +
interval '1 month' - interval '1 hour')::timestamp;


QUERY PLAN
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
----------------------------------------------------------------------
Aggregate (cost=2501.90..2501.90 rows=1 width=21) (actual
time=581.460..581.461 rows=1 loops=1)
-> Index Scan using pk_data on data (cost=0.00..2498.80 rows=619 width=21)
(actual time=92.986..579.089 rows=744 loops=1)
Index Cond: ((num_poste = 1000::numeric) AND (dat >=
(date_trunc('month'::text, (to_timestamp('31012004'::text, 'ddmmyyyy'::text) -
'2000 days'::interval)))::timestamp without time zone) AND (dat <=
(((date_trunc('month'::text, (to_timestamp('31012004'::text, 'ddmmyyyy'::text) -
'2000 days'::interval)) + '1 mon'::interval) - '01:00:00'::interval))::timestamp
without time zone))
Total runtime: 609.149 ms
(4 rows)

Q2 :
bench=> explain analyze select 'Q2',count(*) from data where num_poste between
100 and 100+10 and dat between
(date_trunc('month',to_timestamp('31012004','ddmmyyyy')-interval '3000
days'))::timestamp and
(date_trunc('month',to_timestamp('31012004','ddmmyyyy')-interval '3000 days') +
interval '1 month' - interval '1 hour')::timestamp;


QUERY PLAN
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
----------------------
Aggregate (cost=23232.05..23232.05 rows=1 width=0) (actual
time=5678.849..5678.850 rows=1 loops=1)
-> Index Scan using pk_data on data (cost=0.00..23217.68 rows=5747 width=0)
(actual time=44.408..5669.387 rows=7920 loops=1)
Index Cond: ((num_poste >= 100::numeric) AND (num_poste <=
110::numeric) AND (dat >= (date_trunc('month'::text,
(to_timestamp('31012004'::text, 'ddmmyyyy'::text) - '3000
days'::interval)))::timestamp without time zone) AND (dat <=
(((date_trunc('month'::text, (to_timestamp('31012004'::text, 'ddmmyyyy'::text) -
'3000 days'::interval)) + '1 mon'::interval) - '01:00:00'::interval))::timestamp
without time zone))
Total runtime: 5679.059 ms
(4 rows)

Q3 :
bench=> explain analyze select 'Q3',sum(rr1),count(ff) from data where num_poste
in (50,50+2);
QUERY PLAN
--------------------------------------------------------------------------------
------------------------------------------------------------------
Aggregate (cost=986770.56..986770.56 rows=1 width=17) (actual
time=75401.030..75401.031 rows=1 loops=1)
-> Index Scan using pk_data, pk_data on data (cost=0.00..985534.43
rows=247225 width=17) (actual time=35.823..74885.689 rows=250226 loops=1)
Index Cond: ((num_poste = 50::numeric) OR (num_poste = 52::numeric))
Total runtime: 75405.666 ms
(4 rows)

Q4 :
bench=> explain analyze select 'Q4',count(*) from data where num_poste between
600 and 625;
QUERY PLAN
--------------------------------------------------------------------------------
--------------------------------------------------------------
Aggregate (cost=12166763.62..12166763.62 rows=1 width=0) (actual
time=1162090.302..1162090.303 rows=1 loops=1)
-> Index Scan using pk_data on data (cost=0.00..12159021.19 rows=3096971
width=0) (actual time=94.679..1158266.561 rows=3252938 loops=1)
Index Cond: ((num_poste >= 600::numeric) AND (num_poste <=
625::numeric))
Total runtime: 1162102.217 ms
(4 rows)

Now I'm going to recreate my table with integer and real datatype,
and to decrease sort_mem to 5000.
Then I'll try these queries again.
Thanks.

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