Re: Can a bitmap index be used for index-only access?

Steve

  Bitmap index 'index only access' is supported.   Query 'select b from t where a='a1' ' can be satisfied by index range scan on that bitmap index, with out any need for table block access.   Query 'select a from t where b='a1' ' also can be satisfied without any table block access. But in this case, full index must be scanned and CBO choices include Fast full scan/Index Full Scan. [ I don't think, skip scan is possible with bitmap index, at least, I have not seen it ].  

  Same is true for B-tree index also, just index access is enough for both cases of these statements. Access path choice is an optimizer's decision and so inn your b-tree case, it is possible that another access method is cheaper..

Cheers
Riyaj
blog: http://orainternals.wordpress.com

Tolkin, Steve wrote:
> Can a bitmap index be used for index-only access?
>
> Suppose there is a single bitmap index defined on two columns A and B in
> table T.
> Can that bitmap index alone be used for certain queries, without needing
> to read any blocks from the base table?
> For example these two queries:
> Select B from T where A ='a1'
> Select A from T where B ='b2'
>
> (An ordinary B-tree index on columns A and B, in that order, supports
> index only access on the first query above, but not the second.)
>
> Where, if anywhere, is this described in the Oracle documentation?
>
> Thanks,
> Steve
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