select count(col1) from A where col1=col2. Indexes on col1,col2 NOT USED, WHY?

> Hi,
> I have a table A with two identical, integer, nullable columns col1 and
> col2, both of which are indexed.
> The table A has 100,000 rows and takes up 11,000 blocks.
> The indexes on the two columns take about 225 leaf blocks each with
> BLevel=1.
> The optimizer goal is set to CHOOSE and the table analyzed with compute
> stats.
> Oracle version 8.1.7 on Win' 2000.
>
> Right, when I now run the query:
>
> SELECT COUNT(col1)
> FROM A
> WHERE col1=col2;
>
> the CBO is doing a full table scan on A (with 11000 physical reads)
> rather than just joining
> the two indexes to get the result.
>
> I have two questions:
> 1. Why is the CBO not choosing to join the indexes by default?
> 2. When I gave an INDEX_JOIN hint
> /*+ INDEX_JOIN (A COL1IDX COL2IDX) */,
> it did do a hash join of the two indexes which was faster but still
> took about 6500 physical
> reads while the two indexes put together take only about 452
> blocks. Any idea what the
> extra disk reads are doing?
>
> Thanks in advance,
> Ram
>

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Author: Ramakrishna V
  INET: ramakrishnav_at_mindtree.com

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