RE: CBO Predicate selectivity
- From: "Laimutis Nedzinskas" <Laimutis.Nedzinskas@xxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 13 Jul 2006 14:05:21 -0000
It depends.
In my case the num_rows was the most important string to pull.
It was not fts I was fightning, it was the wrong join order.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of amit poddar
Sent: 13. júlí 2006 13:51
To: breitliw@xxxxxxxxxxxxx
Cc: jonathan@xxxxxxxxxxxxxxxxxx; oracle-l@xxxxxxxxxxxxx
Subject: Re: CBO Predicate selectivity
Yes by changing num_rows we can get right cardinality.
But for single table access path oracle would still use a FTS.
Won't we have to adjust clustering factor and/or leaf_blocks of the index on
that table to make the CBO use the index
amit
Wolfgang Breitling wrote:
> Quoting amit poddar <amit.poddar@xxxxxxxx>:
>
>
>> Hi,
>>
>> "You say you can't change the query, so if you want to fake this
>> query, you probably need to use dbms_stats.set_table_stats to tell
>> Oracle that the table has a very small number of rows - but don't
>> change the block count - so that a properly calculated selectivity is
>> fooled into getting the right cardinality."
>>
>> Sorry for being dense, but how does changing num_rows change the
>> selectivity ?
>>
>>
>
> It doesn't. Note that Jonathan said "so that a properly calculated
> selectivity is fooled into getting the right cardinality" i.e. the
> selectivity calculation (5% or 5% of 5%) isn't changed but the
> resulting cardinality estimate (num_rows
> * selectivity) is.
>
>
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