Re: Bit count

On Tue, Nov 24, 2009 at 07:18:00PM +0100, Rikard Bosnjakovic wrote:
> On Tue, Nov 24, 2009 at 16:47, Nathaniel Trellice <naptrel(at)yahoo(dot)co(dot)uk> wrote:
>
> [...]
> > If not, can anyone recommend the most efficient way within postgres to implement the kind of bit-counting tricks found at:
>
> Perhaps something like this:
>
> CREATE OR REPLACE FUNCTION bitcount(i integer) RETURNS integer AS $$
> DECLARE n integer;
> DECLARE amount integer;
> BEGIN
> amount := 0;
> FOR n IN 1..16 LOOP
> amount := amount + ((i >> (n-1)) & 1);
> END LOOP;
> RETURN amount;
> END
> $$ LANGUAGE plpgsql;
>
>
> bos=# select bitcount(6);
> bitcount
> ----------
> 2
> (1 row)
>
> bos=# select bitcount(7);
> bitcount
> ----------
> 3
>
> bos=# select bitcount(4711);
> bitcount
> ----------
> 7
> (1 row)
>
> bos=# select bitcount(1024);
> bitcount
> ----------
> 1
> (1 row)
>
>
> --
> - Rikard
>

You could also setup a lookup table and just select the
bit count from the table. There was also a thread on how
to efficiently count the set bits that concluded with the
posting of a C function that could be used. It all depends
on your need for speed.

Regards,
Ken