RE: Strategies for dealing with (NOT EQUAL) conditions and indexes

Argh. I sent that carefully formatted to not wrap and be proportional, but
apparently the mail chain stripped all that off and wrapped the heck out of
it.

This:

select --+ gather_plan_statistics a,d from junk13 where (a < '1' or a >'1')
and d != 'not aA'

was the key point.

-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Mark W. Farnham
Sent: Thursday, November 17, 2011 8:54 AM
To: ChrisDavid.Taylor@xxxxxxxxxxxxxxx; 'Stephens, Chris'
Cc: oracle-l@xxxxxxxxxxxxx
Subject: RE: Strategies for dealing with (NOT EQUAL) conditions and indexes

Okay, I did not realize the not equals was on the indexed column rather than
the non-indexed column.
 

In the following, there is an index on columns (a,b,c), and none on d. But a
is highly selective (one row in fact, since I didn't want to fool around and
find the break point).

 

The value on D is then just filtered out from the one row the index returns.
So if the costs are right, you can turn a single not equals into the
concatenation of a pair of index range scans with a table probe

instead of a fast full index scan or a full table scan.
<snip>


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