Oracle-L: Re: FIRST

From: <Rajesh.Rao_at_jpmchase.com>
Date: Wed, 08 Oct 2003 11:29:29 -0800
Message-ID: <F001.005D26B4.20031008112929@fatcity.com>

Very Much expected behaviour. Your increase in the db_block_size from 8 to 16, is making the optimizer choose full table scans, where applicable, since you have to scan through only half the number of blocks now.

A nested loop will starting throwing the initial results faster. A sort merge join might take more time to start throwing out the results, but the entire job will finish faster. So, the first rows hint is forcing the optimizer to choose nested loops.

Regards
Raj

                                                                                                                     
                    Sai Selvaganesan                                                                                 
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                    Sent by:               Subject:     FIRST_ROWS hints                                             
                    ml-errors_at_fatcit                                                                                 
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                    10/08/2003 02:54                                                                                 
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                    to ORACLE-L

i had a migration from 9.2.0.3 to 9.2.0.4 of a database and here are a couple of observations. please help me in understanding this.

i changed the db block size from 8k to 16k and all sql queires which were using nested loops earlier moved to sort merge joins. i ran 10053 and form whatever i could understand the 9204 db has fewer number of blocks compared to the existing 9203 (db size changed to 16k) and sort merge join turned out to be less costlier than nested loops (i couldnt understand the sort statistics). no parameter other than db block size was changed.

after breaking my head i changed the optimizer mode from choose to first rows and the query is back to the old explain plans.

please clarify
1. whether this is a expected behaviour
2. what is first_rows hint and whether it is good move to go to first_rows to fix this problem.

thanks
sai

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INET: Rajesh.Rao_at_jpmchase.com

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Re: FIRST_ROWS hints