Re: Child Cursor and Plan

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

Jonathan Lewis
http://jonathanlewis.wordpress.com
Oracle Core (Apress 2011)
http://www.apress.com/9781430239543


----- Original Message ----- 
From: "Sreejith S Nair" <Sreejith.Sreekantan@xxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Sent: Thursday, February 02, 2012 1:36 PM
Subject: Child Cursor and Plan


Hi Friends,
Oracle 11.2.0.2 on Solaris 10.

From my understanding a child cursor is generated when a sql statement
will have multiple execution plans. Here is what I see in one of our
database

SYS> select sql_id,child_number,hash_value,PLAN_HASH_VALUE,CHILD_ADDRESS
from v$sql where sql_id='c8gnrhxma4tas';

SQL_ID        CHILD_NUMBER HASH_VALUE PLAN_HASH_VALUE CHILD_ADDRESS
------------- ------------ ---------- --------------- ----------------
c8gnrhxma4tas            0 1721918808      4024720576 00000004481E01A8
c8gnrhxma4tas            1 1721918808      4024720576 0000000456DEF070
c8gnrhxma4tas            2 1721918808      4024720576 0000000448F3C210
c8gnrhxma4tas            3 1721918808      4024720576 000000047CA5BD30
c8gnrhxma4tas            4 1721918808      4024720576 0000000448476B88
c8gnrhxma4tas            5 1721918808      4024720576 000000047C217998
c8gnrhxma4tas            6 1721918808      4024720576 00000004483B0B78
c8gnrhxma4tas            7 1721918808      4024720576 000000047C3F9668

Here I see that CHILD_NUMBER and different CHILD_ADDRESS are generated for
same PLAN_HASH_VALUE. Can someone please give some light on what exactly
is a child cursor ?
This is what I got from Google. Could not find a proper definition in
documentation.May be I missed ?

The parent cursor contains the SQL statement text only, but no execution
plan.
Execution plans are found in child cursors. Child cursors are also called
versions.

With Regards,
Sreejith

--
Sreejith S Nair
Associate Systems Architect | AOS DBA Team
IBS Software Services Private Ltd.
2nd Floor, IBS Campus, Technopark Campus, Trivandrum - 695 581, Kerala,
India
((Direct) +91 471 661 4707 ) +91 808 648 5523
*sreejith.sreekantan@xxxxxxxxxx
8 www.ibsplc.com





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