JL Computer Consultancy
Bitmap indexes in Oracle 7.3.2 |
January 1999 |
This attached Word document is in Word 7 / Windows 95 format. The size is 47KB.
The content is an article published in Relate (the magazine of the UK Oracle User Group) in Autumn 1997. The article describes bitmap indexes, how they work, and includes some interesting test results when they are applied to a fairly large (750 MB) table.
Since this article was written very little has changed, although there have been bugs in various versions of Oracle which show up as massive memory consumption when using bitmap indexes, particularly when parsing queries against partition views with bitmap indexes.
It is also a little surprising what can happen if you enable 'b tree to bitmap conversion' plans. Almost invariably the main problem is that a bitmap index is physically VERY small, so the cost-based optimiser has a strong bias towards using it if at all possible since the cost of reading it is low.
