Обсуждение: I don't understand this
Hi! i create some table with primary key and one index key Then i insert data and try some explain on my select. it tells me that index was used. then i do some other queries and vacuum and when i try again SAME explain it prints that it was used Seq Scan It this normal or what. Or is some kind of a bug. -- tia, Uros mailto:uros.gruber@sir-mag.com
On Mon, Apr 29, 2002 at 07:09:47PM +0200, Uros Gruber wrote: > then i do some other queries and vacuum and when i try again > SAME explain it prints that it was used Seq Scan > > It this normal or what. Or is some kind of a bug. PostgreSQL attempt to adjust it's plan depending on what data is in the table. If by adding new data the distribution has changed, it's decided it's now better to do this other plan. Is this other plan significantly slower? -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > Canada, Mexico, and Australia form the Axis of Nations That > Are Actually Quite Nice But Secretly Have Nasty Thoughts About America
Uros Gruber wrote:
> Hi!
>
> i create some table with primary key and one index key
> Then i insert data and try some explain on my select.
>
> it tells me that index was used.
>
>
> then i do some other queries and vacuum and when i try again
> SAME explain it prints that it was used Seq Scan
>
> It this normal or what. Or is some kind of a bug.
Yes, this is normal.
The statistics defaults for an un-vacuumed table usually
cause the optimizer to use an index scan. When you analyze
the table later, the statistics reflect the real status and
for very small tables, a sequential scan is normally better
than an index scan.
Jan
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