Bitmap Sizing

The Tuning book says bitmap indexes are typically 25 percent the size of a regular B-tree index. In another place it says bitmap indexes are typically only a fraction of the size of regular indexes. I'm dealing with tables with millions of rows, and I'd like to create initial extents with a little more accuracy than "probably around 25 percent" of their current size or "likely even smaller." Does anyone have any ideas on a possible formula for estimating the size of bitmap indexes?

Thanks.

--brad

PS. One initial test with a bitmapped index reduced the explain plan cost of a query from 6,297 down to 208. The time of the query went from over 2 minutes to about 20 seconds. This was on a table with 1.1 million records.  The indexed column has only 474 unique values. The Rows Per Key is 2,417.  

• Bradley W. Skiles bwskiles_at_adpc.purdue.edu =
• Purdue University 317-494-0481 (o) =
• Freehafer Hall 317-428-3957 (p) =
• West Lafayette, IN 47907-1061 =
• Kellogg's Meteoric Theory of Data: "Data doesn't move; =
• it lands and sticks. =

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