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Home -> Community -> Mailing Lists -> Oracle-L -> RE: RE: shared pool latch
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> what is the difference between a child cursor and a parent cursor?
Every cursor (aka sql statements) gets two cursors in the library cache. One parent and one child cursor. This is the implementation for cursor multi versioning.
Let us consider a case: A SQL statement was executed first time, then it would get a parent and child. If the same sql statement was executed again (with no change to the body of SQL keeping the same hash_value ), with a different non-shareable attributes, then a new child cursor will be added to this parent cursor. As you can see, you can have many child cursors associated with a parent cursor. Non-shareable attributed includes : such as different optimizer_mode, different session parameters etc.
Here is an example.
Executed following SQL statements, First one with a different optimizer_mode and what follows is the corresponding library cache dump. As you can see there are two child cursors for the statement "select sysdate from dual" and one child for the statement "select sysdate+1 from dual". Understandably +1 is converted to +"SYS_B0" due to cursor_sharing testing I was doing..
Second statement "select sysdate from dual" is not shareable as the optimizer_mode is different. But the second "select sysdate+1 from dual" is shareable and so only one child.
select sysdate from dual;
alter session set optimizer_mode=first_rows;
select sysdate from dual;
select sysdate+1 from dual;
select sysdate+1 from dual;
lwt=385eb9428[385eb9428,385eb9428] ltm=385eb9438[385eb9438,385eb9438] pwt=385eb9458[385eb9458,385eb9458] ptm=385eb94e8[385eb94e8,385eb94e8] ref=385eb9408[385eb9408,385eb9408] lnd=385eb9500[385eb9500,385eb9500]LIBRARY OBJECT: object=385eb9068
lwt=385eaf4d8[385eaf4d8,385eaf4d8] ltm=385eaf4e8[385eaf4e8,385eaf4e8] pwt=385eaf508[385eaf508,385eaf508] ptm=385eaf598[385eaf598,385eaf598] ref=385eaf4b8[385eaf4b8,385eaf4b8] lnd=385eaf5b0[385eaf5b0,385eaf5b0]LIBRARY OBJECT: object=385eaf1c0
-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of ryan.gaffuri_at_cox.net
Sent: Wednesday, March 31, 2004 1:38 PM
To: oracle-l_at_freelists.org
Subject: Re: RE: shared pool latch
Let me try to keep this alive.. Responses in line..
HTH
Thanks
Riyaj "Re-yas" Shamsudeen
Certified Oracle DBA
-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Sultan Syed
Sent: Wednesday, March 31, 2004 4:21 AM
To: oracle freelists.org
Subject: shared pool latch
Hi guys
The following questions are confusing me,please could someone give
explanation
on the below
1)Is librarycache latch held during the whole process of parse,execute
and fetch phases ,
or it is held upto the time searching for the object in hash table and
release.
Library cache latch is held while searching for a parent cursor and subsequent child cursor with similar attributes with parent cursor pinned. Also library cache latch is held when creating a new library cache object if the child cursor is not_found or found_but_not_shareable. Since, parent cursor is pinned while searching for a child cursor, I am not sure whether the search for child cursor needs to hold on to the library cache latch or not. But if a new parent cursor is to be created then the library cache latch must be held.
Execute and Fetch uses library cache pin and library cache locks to maintain the objects from undergoing concurrent modification.
2)session_cached_cursor caches the child cursor or parent cursors also. As far as I know, session_cached_cursor is searched for pinned child cursor. I don't know whether the parent is also kept in the session cached cursor or not.
3)While doing hard parse which latch is acquired ,shared pool or library
cache latch or both,
if it is both which latch will be acquired first?
library cache latch is already held while requesting for shared pool latch. Shared pool latch is acquired to request/release space from the shared pool free memory area and released immediately after that. Request for library cache latch can never be made while holding the shared pool latch (at least up to 9i) as latch level semantics will prevent that ( as shared pool latch gets are always in willing-to-wait mode).
Thanks in advance
Syed
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