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

From: Tim Sawmiller <sawmillert_at_state.mi.us>
Date: Fri, 30 Mar 2001 06:43:41 -0800
Message-ID: <F001.002DD00B.20010330064544@fatcity.com>

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?
>
> Thanks in advance,
> Ram
>
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--
Author: Ramakrishna V
  INET: ramakrishnav_at_mindtree.com 

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Author: Tim Sawmiller
  INET: sawmillert_at_state.mi.us 

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Author: Ramakrishna V
  INET: ramakrishnav_at_mindtree.com 

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--
Author: Tim Sawmiller
  INET: sawmillert_at_state.mi.us

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
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Received on Fri Mar 30 2001 - 08:43:41 CST

Original text of this message

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