"Thomas Kellerer" <JUAXQOSZFGQQ@spammotel.com> wrote in message
news:5hg5aiF3kia4nU1@mid.individual.net...
> ciapecki wrote:
>> Hi,
>>
>> Does indexing a very big table (about 5Mio records) on the columnA
>> which can hold only values Y,N,<NULL> make sense?
>>
> Yes, that's what bitmap indexes were made for.
>

Hi Thomas

A Bitmap index is of no use if both Y and N are roughly evenly distributed
and you have no other predicate in the query.

Returning approximately 2.5 millions rows through a bitmap index would be
dramatically slower than a full table scan.

A single bitmap index only would be useless in this scenario, even more so
if the table is subjected to any transactional based DML load.

Cheers

Richard