RE: query uses function based index in DEV1 db but does NOT use it in DEV2 db
- From: "Taylor, Chris David" <ChrisDavid.Taylor@xxxxxxxxxxxxxxx>
- To: 'Li Li' <litanli@xxxxxxxxx>
- Date: Wed, 25 Apr 2012 12:34:40 -0500
Oracle will often favor a full tablescan when the data you want is spread out
across multiple Oracle blocks - so say the table only has 50 Oracle blocks, and
your data occupies 45 of them then it's 'cheaper' to do a FTS since it has to
read most of the Oracle blocks anyway.
That's the point I was trying to find floating around in my head...
Chris
-----Original Message-----
From: Taylor, Chris David
Sent: Wednesday, April 25, 2012 12:09 PM
To: 'Li Li'
Cc: 'oracle-l@xxxxxxxxxxxxx'
Subject: RE: query uses function based index in DEV1 db but does NOT use it in
DEV2 db
<snip>
There are times where if you've asked for some percentage of a table's rows,
but because that percentage is spread out across the whole table, that Oracle
will favor a FTS even when asked for a small percentage of the table's rows.
(I was trying to find an example of this but can't put my fingers on it at the
moment...)
</snip>
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