Bit count

I'm trying to convert this method using bitmaps in redis for real time
metrics to postgresql:
http://blog.getspool.com/2011/11/29/fast-easy-realtime-metrics-using-redis-bitmaps/

I'll use a bit varying field with unlimited length.
To record unique visitors per day, I'll flip the bit corresponding to a
user's id.
Each event type will have a separate record.
It is possible to get useful information using union(&) and intersection(|)
of several fields.
A field for 1 M users will required 1M bits = 125KB.

Is it reasonable to assume that this will result with a better performance
and lower space and memory than using a table with a unique index?

Is there a built-in function to count bits similar to BIT_COUNT in mysql
and BITCOUNT in redis?

I've found several plpgsql functions in the mailing list.
What is the best function to use for large bitstrings (>100KB)?

Thanks

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;

create or replace function bitsetlen(bit) returns int as $$
declare i int;
c int;
begin
c:=0;
for i in 1..length($1) loop
if substring($1,i,1)=B'1' then
c:=c+1;
end if;
end loop;
return c;
end;
$$ language plpgsql;

SELECT (myBit & 1 + myBit >> 1 & 1 + myBit >> 2 & 1) AS bitCount FROM
myBitTable;

SELECT LENGTH( REPLACE( CAST( B'101000000000000000000010' AS TEXT ), '0',
''));