Re: using new bitmap scans to index bit columns?

On Wed, 9 Nov 2005, TJ O'Donnell wrote:

> I like the new bitmap scans and I'm wondering if there is any way
> I can utilize them for my bitmask column, defined as bit(1024).
> I use this column as a sort of fingerprint to quickly scan my tables.
> But it is a scan, not an index.  I have not figured out a way to
> index the bitmask column.  Is there some way it can be used as
> an index now that there are bitmap scans in 8.1?
>
> Currently I do this:
> Select * from mytable where contains(bitmask, fingerprint(user_data))
> and matches(datacolumn, user_data);
>
> user_data is a string, like a regexp but with different semantics for
> chemical data.
> bitmask is precomputed/stored as bit(1024) = fingerprint(datacolumn)
> contains(a,b) returns bool as 'select b=(a&b);'
>
> This works well because matches() is an expensive functions.
> But it would work better if bitmask could be indexed, no?

You can use GiST to do that.

>
> TJ O'Donnell
>
> ---------------------------(end of broadcast)---------------------------
> TIP 5: don't forget to increase your free space map settings
>

     Regards,
         Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83