Thread
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Tuning queries on large database
Valerie Schneider DSI/DEV <valerie.schneider@meteo.fr> — 2004-08-04T12:44:43Z
Hi, I have some problem of performance on a PG database, and I don't know how to improve. I Have two questions : one about the storage of data, one about tuning queries. If possible ! My job is to compare Oracle and Postgres. All our operational databases have been running under Oracle for about fifteen years. Now I try to replace Oracle by Postgres. I have a test platform under linux (Dell server, 4 Gb RAM, bi-processor, Linux Red Hat 9 (2.4.20-31.9)) with 2 databases, 1 with Oracle (V8i or V9i it's quite the same), 1 with PG (7.4.2). Both databases have the same structure, same content, about 100 Gb each. I developped some benches, representative of our use of databases. My problem is that I have tables (relations) with more than 100 millions rows, and each row has about 160 fields and an average size 256 bytes. For Oracle I have a SGA size of 500 Mb. For PG I have a postgresql.conf as : max_connections = 1500 shared_buffers = 30000 sort_mem = 50000 effective_cache_size = 200000 and default value for other parameters. I have a table named "data" which looks like this : bench=> \d data Table "public.data" Column | Type | Modifiers ------------+-----------------------------+----------- num_poste | numeric(9,0) | not null dat | timestamp without time zone | not null datrecu | timestamp without time zone | not null rr1 | numeric(5,1) | qrr1 | numeric(2,0) | ... ... all numeric fields ... Indexes: "pk_data" primary key, btree (num_poste, dat) "i_data_dat" btree (dat) It contains 1000 different values of "num_poste" and for each one 125000 different values of "dat" (1 row per hour, 15 years). I run a vacuum analyze of the table. bench=> select * from tailledb ; 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. 125 000 000 rows x 256 b = about 32 Gb. This calculation gives an idea not so bad for oracle. What about for PG ? How data is stored ? 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 ! I can't understand these results. The way to execute queries is the same I think. I've read recommended articles on the PG site. I tried with a table containing 30 millions rows, results are similar. What can I do ? Thanks for your help ! ******************************************************************** * Les points de vue exprimes sont strictement personnels et * * n'engagent pas la responsabilite de METEO-FRANCE. * ******************************************************************** * Valerie SCHNEIDER Tel : +33 (0)5 61 07 81 91 * * METEO-FRANCE / DSI/DEV Fax : +33 (0)5 61 07 81 09 * * 42, avenue G. Coriolis Email : Valerie.Schneider@meteo.fr * * 31057 TOULOUSE Cedex - FRANCE http://www.meteo.fr * ******************************************************************** -
Re: [PERFORM] Tuning queries on large database
Christopher Kings-Lynne <chriskl@familyhealth.com.au> — 2004-08-04T13:21:51Z
> 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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Re: Tuning queries on large database
Rod Taylor <rbt@rbt.ca> — 2004-08-04T13:26:42Z
On Wed, 2004-08-04 at 08:44, Valerie Schneider DSI/DEV wrote: > Hi, > > I have some problem of performance on a PG database, and I don't > know how to improve. I Have two questions : one about the storage > of data, one about tuning queries. If possible ! > > My job is to compare Oracle and Postgres. All our operational databases > have been running under Oracle for about fifteen years. Now I try to replace > Oracle by Postgres. You may assume some additional hardware may be required -- this would be purchased out of the Oracle License budget :) > My first remark is that the table takes a lot of place on disk, about > 70 Gb, instead of 35 Gb with oracle. > 125 000 000 rows x 256 b = about 32 Gb. This calculation gives an idea > not so bad for oracle. What about for PG ? How data is stored ? This is due to the datatype you've selected. PostgreSQL does not convert NUMERIC into a more appropriate integer format behind the scenes, nor will it use the faster routines for the math when it is an integer. Currently it makes the assumption that if you've asked for numeric rather than integer or float that you are dealing with either large numbers or require high precision math. Changing most of your columns to integer + Check constraint (where necessary) will give you a large speed boost and reduce disk requirements a little. > The different queries of the bench are "simple" queries (no join, > sub-query, ...) and are using indexes (I "explained" each one to > be sure) : Care to send us the EXPLAIN ANALYZE output for each of the 4 queries after you've improved the datatype selection? -- Rod Taylor <rbt [at] rbt [dot] ca> Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL PGP Key: http://www.rbt.ca/signature.asc -
Re: Tuning queries on large database
Gaetano Mendola <mendola@bigfoot.com> — 2004-08-04T15:34:56Z
Valerie Schneider DSI/DEV wrote: > Hi, > > I have some problem of performance on a PG database, and I don't > know how to improve. I Have two questions : one about the storage > of data, one about tuning queries. If possible ! > > My job is to compare Oracle and Postgres. All our operational databases > have been running under Oracle for about fifteen years. Now I try to replace > Oracle by Postgres. Show us the explain analyze on your queries. Regards Gaetano Mendola
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Re: Tuning queries on large database
PFC <lists@boutiquenumerique.com> — 2004-08-04T15:50:56Z
>> not so bad for oracle. What about for PG ? How data is stored I agree with the datatype issue. Smallint, bigint, integer... add a constraint... Also the way order of the records in the database is very important. As you seem to have a very large static population in your table, you should insert it, ordered by your favourite selection index (looks like it's poste). Also, you have a lot of static data which pollutes your table. Why not create two tables, one for the current year, and one for all the past years. Use a view to present a merged view.