Wired behavor with LIMIT - Mailing list pgsql-general
From | Thomas Munz |
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Subject | Wired behavor with LIMIT |
Date | |
Msg-id | 4656BD3E.90506@ecommerce.com Whole thread Raw |
In response to | Re: phpPgAdmin - prior version available? (Robert Treat <xzilla@users.sourceforge.net>) |
Responses |
Re: Wired behavor with LIMIT
|
List | pgsql-general |
Hello List! I tried today to optmize in our companies internal Application the querys. I come to a point where I tried, if querys with LIMIT are slower then querys without limit I tried following query in 8.2.4. Keep in mind that the table hs_company only contains 10 rows. thomas@localhost:~$ psql testdb testsuer Welcome to psql 8.2.4, the PostgreSQL interactive terminal. Type: \copyright for distribution terms \h for help with SQL commands \? for help with psql commands \g or terminate with semicolon to execute query \q to quit ghcp=# explain analyze select * from hs_company; explain analyze select * from hs_company limit 10; QUERY PLAN -------------------------------------------------------------------------------------------------------- Seq Scan on hs_company (cost=0.00..1.10 rows=10 width=186) (actual time=0.012..0.034 rows=10 loops=1) Total runtime: 0.102 ms (2 rows) QUERY PLAN -------------------------------------------------------------------------------------------------------------- Limit (cost=0.00..1.10 rows=10 width=186) (actual time=0.012..0.063 rows=10 loops=1) -> Seq Scan on hs_company (cost=0.00..1.10 rows=10 width=186) (actual time=0.007..0.025 rows=10 loops=1) Total runtime: 0.138 ms (3 rows) I runned this query about 100 times and always resulted, that this query without limit is about 40 ms faster Now I putted the same query in the file 'sql.sql' and runned it 100 times with: psql test testuser -f sql.sql with following results thomas@localhost:~$ psql testdb testuser -f sql.sql QUERY PLAN -------------------------------------------------------------------------------------------------------- Seq Scan on hs_company (cost=0.00..1.10 rows=10 width=186) (actual time=0.013..0.034 rows=10 loops=1) Total runtime: 0.200 ms (2 rows) QUERY PLAN -------------------------------------------------------------------------------------------------------------- Limit (cost=0.00..1.10 rows=10 width=186) (actual time=0.016..0.069 rows=10 loops=1) -> Seq Scan on hs_company (cost=0.00..1.10 rows=10 width=186) (actual time=0.008..0.025 rows=10 loops=1) Total runtime: 0.153 ms (3 rows) The querys are equal but has different speeds. Can me someone explain why that is? Thomas
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