Re: weird EXPLAIN
От | Tom Jenkins |
---|---|
Тема | Re: weird EXPLAIN |
Дата | |
Msg-id | 1024346202.15174.205.camel@asimov обсуждение исходный текст |
Ответ на | weird EXPLAIN (Varun Kacholia <varunk@cse.iitb.ac.in>) |
Список | pgsql-general |
On Mon, 2002-06-17 at 16:04, Varun Kacholia wrote: > hi , > I have just migrated to PostgreSQL...and found the following wierd : > > suryadb=# explain select * from dbwin where id in (select id from wdwin > where word='bacd'); > NOTICE: QUERY PLAN: > Seq Scan on dbwin (cost=0.00..8158.20 rows=1000 width=76) > ^^^^^^^^^^^^^^^^^^ > SubPlan > -> Materialize (cost=8.14..8.14 rows=10 width=4) > -> Index Scan using wdkwin on wdwin (cost=0.00..8.14 rows=10 width=4) > > > i wanted to know why a sequential scan is done on the field id > while it is a primary key in the table dbwin.this can be clearly > confirmed from the query below : > > suryadb=# explain select * from dbwin where id=1234; > NOTICE: QUERY PLAN: > Index Scan using dbwin_pkey on dbwin (cost=0.00..8.14 rows=10 width=76) > > why does it do a sequential scan for the query above?? If the table is small, the planner figures (usually correctly) that it is quicker to load it all in memory and do an in-memory scan, rather than loading the index into memory, getting the row from the index, then loading the row from disk (2 disc reads to 1) -- Tom Jenkins Development InfoStructure http://www.devis.com
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