Re: Regarding Bitmap Scan

Thanks a lot for your help.

Thanks,
Soni

On 10/17/06, Dawid Kuroczko <qnex42(at)gmail(dot)com> wrote:
>
> On 10/17/06, soni de <soni(dot)de(at)gmail(dot)com> wrote:
> >
> > I didn't understand the "Bitmap Scan" and the sentence "indexes will be
> > dynamically converted to bitmaps in memory". What does mean by "Bitmap Scan"
> > in database?
> >
> >
> >
> > Can anybody help us regarding above query?
> >
>
> Assume you have a table:
> CREATE TABLE foo (
> some_key int,
> some_time timestamp with time zone,
> some_data text
> );
> And two indexes:
> CREATE INDEX foo_key ON foo (some_key);
> CREATE INDEX foo_time ON foo (some_time);
>
> Now, you make a query:
> SELECT * from foo WHERE some_key > 10 AND some_time >
> '2006-10-01'::timestamptz;
>
> ...originally planner would choose only one index to use -- and would use
> the
> one which it think its best.
>
> The 8.1 version does differently: It will scan foo_key index -- make a
> bitmap out of it,
> scan foo_time index -- make another bitmap out of it, binary AND these
> bitmaps,
> and will read the data from the table using such combined bitmap. It
> could as well
> use "OR" if you used OR in your query.
>
> Hence -- it can be faster, especially for large tables and selective
> queries.
>
> Regards,
> DAwid
>
>
>