Re: table scan vs idx scan

Prasad:

Have you run an explain plan on the the query on tab2 to see what Oracle is doing?

Jon Walthour
----- Original Message -----
To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Saturday, July 21, 2001 12:15 PM

> Hi dbas
>
> we have a situation here. can anybody tell me
> why this is happening
> :
>
> we have 2 tables with identical rows(almost) with same
> columns even indexed in same manner.
>
> tab1 51000 rows
> tab2 51500 rows
>
> tab1 indexed with idx1 on col1,col2,col3
> tab2 indexed with idx2 on col1,col2,col3
>
> TAB1:
> -----
> just i am selecting as
> select col1,col2,col3 from tab1
>
> this is using index scan and taking
> 90 seconds to get the data
> (Explain plan )
>
> TAB2:
> -----
> i am select as above
> select col1,col2,col3 from tab2
>
> this is using fulltable scan and taking
> 18 minutes to get data.
> (Explain plan)
>
> we tried in all possible ways for tab2 ,like
>
> analyzed objects (both idx,table (compute statistics))
> changed optimizer modes and tried,
> giving hints to use index ,
> rebuilt of index,
> after all we dropped the idx2 and recreated it.
>
> but the case is same. still it is taking >18 minutes.
>
> even when i provided hints it is not searching for
> that
> index, doing the same old full table scan.
>
> why oracle doesn't follow hints in some cases.
>
> and what should i do to reduce the time to fetch the
> data.
>
>
> thnx in adv
>
> prasad
>
>
>
>
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Jon Walthour
  INET: jonw_at_fuse.net

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
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