From: | Pavel Stehule <stehule(at)kix(dot)fsv(dot)cvut(dot)cz> |
---|---|
To: | Ryszard Lach <siaco(at)autograf(dot)pl> |
Cc: | pgsql-performance(at)postgresql(dot)org, Ryszard Lach <rla(at)debian(dot)org> |
Subject: | Re: "select max/count(id)" not using index |
Date: | 2003-12-22 11:03:05 |
Message-ID: | Pine.LNX.4.44.0312221201260.27697-100000@kix.fsv.cvut.cz |
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Thread: | |
Lists: | pgsql-performance |
Hello
It is normal behavior PostgreSQL. Use
SELECT id FROM tabulka ORDER BY id DESC LIMIT 1;
regards
Pavel
On Mon, 22 Dec 2003, Ryszard Lach wrote:
> Hi.
>
> I have a table with 24k records and btree index on column 'id'. Is this
> normal, that 'select max(id)' or 'select count(id)' causes a sequential
> scan? It takes over 24 seconds (on a pretty fast machine):
>
> => explain ANALYZE select max(id) from ogloszenia;
> QUERY PLAN
> ----------------------------------------------------------------------
> Aggregate (cost=3511.05..3511.05 rows=1 width=4) (actual
> time=24834.629..24834.629 rows=1 loops=1)
> -> Seq Scan on ogloszenia (cost=0.00..3473.04 rows=15204 width=4)
> (actual time=0.013..24808.377 rows=16873 loops=1)
> Total runtime: 24897.897 ms
>
> Maybe it's caused by a number of varchar fields in this table? However,
> 'id' column is 'integer' and is primary key.
>
> Clustering table on index created on 'id' makes such a queries
> many faster, but they still use a sequential scan.
>
> Richard.
>
>
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