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Bitmap Index

August 25th, 2011 by igogo

A few days ago I got an email about a problem with a system that would use a bitmap index to satisfy a query but wouldn’t use the equivalent B-tree index – and the DBA wanted to make the switch he wanted to downgrade to Standard Edition from Enterprise Edition.

In outline there was a table with a mapping column defined as varchar2(N) holding a string of 1′s and 0′s representing a bit mask. Typically each map value had about 800 rows associated with it and the users’ queries were all about matching bits using the utl_raw.bitand() and utl_raw.bit_or() functions.

My response was that the only surprise was that Oracle was using the bitmap index, not that it wasn’t using the B-tree index as it seemed that the only way the index could be used was with an index fast full scan. I was curious, so I said I’d take a look at the query, the object definitions the plan, and the 10053 trace file if the DBA cared to send them to me.

It turned out that I was correct – the index fast full scan was the plan used by the bitmap index because the queries were of the form:

select  count(*)
from    tableX t0
        utl_raw.bit_and(t0.mapping, '0000.....001') = '0000.....001'

But, as the DBA pointed out, Oracle didn’t even consider this plan when he changed the bitmap to a B-tree. Why not ? For the same old reason that Oracle often surprises people by ignoring indexes – the column was not declared as NOT NULL, which means there could be rows in the table that are not in the B-tree index, so the index was not a valid target for comparison. (In this case the human eye can see that this is irrelevant, but the optimizer is blindly following a heurisitc – or rule – at this point.)

Key point: Oracle’s standard B-tree indexes do not hold index entries that are completely null. Bitmap indexes (and cluster indexes) do have entries for the nulls.

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