| Subject | Re: [firebird-support] Why "SELECT FIRST 1 ..." and "EXISTS ( SELECT * FROM ... )" are so slow in a big database? |
|---|---|
| Author | Dmitry Yemanov |
| Post date | 2006-07-15T09:03:57Z |
"Antti Nivala" <antti.nivala@...> wrote:
scan. I.e. no record can be fetched until the index scan completes for all
existing matches. For a non-selective index, the scan may take quite a long
time.
Dmitry
>Records are fetched from a table based on the bitmap filled by the index
> I don't understand why poor selectivity would make
> the "FIRST 1" type of a query slow
scan. I.e. no record can be fetched until the index scan completes for all
existing matches. For a non-selective index, the scan may take quite a long
time.
Dmitry