Re: Queries with timestamp II

Dnia 2004-01-26 15:12, Użytkownik Arnau napisał:
> Hi all,
>
> First of all thanks to Josh and Richard for their replies. What I have
> done to test
> their indications is the following. I have created a new table identical to
> STATISTICS,
> and an index over the TIMESTAMP_IN field.
>
> CREATE TABLE STATISTICS2
> (
> STATISTIC_ID NUMERIC(10) NOT NULL DEFAULT
> NEXTVAL('STATISTIC_ID_SEQ')
> CONSTRAINT pk_st_statistic2_id PRIMARY KEY,
> TIMESTAMP_IN TIMESTAMP,
> VALUE NUMERIC(10)
> );

Do you really have to use numeric as primary key? Integer datatypes
(int4/int8) are much faster than numeric.

>
> CREATE INDEX i_stats2_tin ON STATISTICS2(TIMESTAMP_IN);
>
> After that I inserted the data from STATISTICS and vacuumed the DB:
>
> INSERT INTO STATISTICS2 ( SELECT * FROM STATISTICS );
> vacuumdb -f -z -d test
>
> once the vacuum has finished I do the following query
>
> explain analyze select * from statistics2 where timestamp_in <
> to_timestamp( '20031201', 'YYYYMMDD' );
> NOTICE: QUERY PLAN:
>
> Seq Scan on statistics2 (cost=0.00..638.00 rows=9289 width=35) (actual
> time=0.41..688.34 rows=27867 loops=1)
> Total runtime: 730.82 msec
>
> That query is not using the index. Anybody knows what I'm doing wrong?

Over 25000 rows match your condition:
timestamp_in < to_timestamp( '20031201', 'YYYYMMDD' );

How many rows do you have in your table? It's possible, that seq scan is
just faster than using index when getting so many rows output.

Regards,
Tomasz Myrta