RE: select count(col1) from A where col1=col2. Indexes on

Well YOU might be saying that, but is ORACLE understanding that... Try adding two where clauses( and col1 is not null and col2 is not null ) and see what happens.

>>> ramakrishnav_at_mindtree.com 03/30/01 08:10AM >>> Hi,
  I don't want it to count the rows with col1 as null. When I specify col1=col2, I am implicitly saying that they are both non-null and equal to each other.

regards,
ram

-----Original Message-----
Sent: Friday, March 30, 2001 6:25 PM
To: Multiple recipients of list ORACLE-L

Well, how will it count the rows where col1 is null if it just uses the index? There are no entries in the index for those rows...

>>> ramakrishnav_at_mindtree.com 03/30/01 06:30AM >>>


> 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?

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