Re: same sql_id with different plan_hash_value

From: Sreejith S Nair <sreejithsna_at_gmail.com>
Date: Sun, 25 Mar 2012 17:01:15 +0530
Message-Id: <A6FE7A92-E5FC-4ABC-9D25-07C9153F5556_at_gmail.com>

Please see few comments for a similar question posted sometime back

There are many reasons why a child cursor is created. It could be that the optimizer environment differs or because the bind variablen had a different length, ... . But, this does not have to mean that a different sql plan will be used.

You can check the reasons why a certain child exists for a cursor in the v$sql_shared_cursor view.

Apart from a new 11.2 scalability option, things like adaptive cursor sharing and cardinality feedback, and anomalies caused by bugs, a new child cursor is generated when a session tries to use a statement that is in the library cache and finds that every child cursors for that statement already in the cache has a different optimizer environment from the session's optimizer environment. It is possible that different optimizer environments will, however, still produce the same execution plan.

A common example of changing the optimizer environment is to enable SQL_trace; other options include local changes to workarea_size_policy, sort_area_size, db_file_multiblock_read_count, and so on. If you check v$sql.optimizer_env_hash_value for the statements you may find that they differ.

Regards,
Sreejith
-- Sent from my iPhone

On 24-Mar-2012, at 5:08 AM, GovindanK <gkatteri_at_gmail.com> wrote:

> A child number will be generated if AUTH_CHECK_MISMATCH occurs too for the
> same sql_id / query.
> HTH
> GovindanK
> 
> On Fri, Mar 23, 2012 at 12:49 PM, Dba DBA <oracledbaquestions_at_gmail.com>wrote:
> 

>> When I query v$sql_plan, I see some SQL_IDs with different plan_hash_value.
>> I thought if the plan changed while the query was cached, it would generate
>> a child_number.
>> When would a query generate a new child number, vs. a new plan_hash_value?
>> I have one query with
>>
>> 2 plan_hash_value
>>
>> first plan_hash_value has 3 child_numbers
>> second one has 2.
>>
>>
>> --
>> http://www.freelists.org/webpage/oracle-l
>>
>>
>>

> 
> 
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Received on Sun Mar 25 2012 - 06:31:15 CDT