Re: Bitmap Index

That's a pretty good question. In fact, I've never seen a bitmap index as large as a btree index
but I don't create them on nearly unique columns, either. The highest cardinality field that we
use them on has about 1210 different values. The key is the relationship to the number of rows
in the table in the first place. If there were only 1210 rows in the table, a btree
would be more effective, but with 24 million, the bitmap works pretty well.

Sean

Marcel Romahn <rom_at_messer.de> wrote in article <34EAAB1C.28E5B8C9_at_messer.de>...
> Conditions for using bitmap indexes essentialy are the low cardinality
> of the indexed columns with low selectivity, which means huge amout of
> retrieved rows.
>
> What is "low cardinality" in this context. Yes, a GENDER or YES/NO
> column is easy to understand. But what about attributes with 20 or 30
> different values. Where is the break even point in comparison with
> b-tree indexes.
>
> Thanks in advance
>
> Marcel Romahn
>
>
  Received on Sun Mar 01 1998 - 00:00:00 CST