I checked the Tom Kyte site. A soft parse comprises two operations. One is a simple
syntax check;
e.g. "select from dual;" would fail this soft parse as it is missing a column list or
a literal.
The other portion of a soft parse what he calls a semantics check is checking to see
if the tables and columns exist, that the person has the proper permissions, that
there are no ambiguities.
select deptno from emp, dept
where emp.deptno = dept.deptno
/
would fail this type of parse. My Kyte's definition of a soft parse jibes nicely
with the one I used earlier. I didn't include the syntactical error portion as the
statements in question are all valid SQL. However it is just as important. Semantic
and syntactical checks are done; i.e.., a soft
parse is done before the cache is checked.
Quoting from the article
"The next step in the parse operation is to see if the statement we are currently
parsing has already in fact been processed by some other session. If it has ?
we may be in luck here, we can skip the next two steps in the process, that of
optimization and row source generation. If we can skip these next two steps in
the process, we have done what is known as a Soft Parse.
----------------------------------------------------------------------------------------
While writing this it has suddenly dawned on me what Suhen was talking about when said
cursor_sharing = 'FORCE' avoids a hard parse at the cost of a soft.
If this is set
select * from emp where ename = 'KING';
will be soft parsed.
It will be changed to
select * from emp where ename = :bind_variable;
This statement will undergo soft parsing again.
If the statement can be found in cache; then no hard parsing is needed. The
generation of the second SQL statement replacing the literal with a bind variables
increases the likelihood of not having to hard parse.
--------------------------------------------------------------------------------------------------
Now about session_cached_cursors. First checking the hits
1 select a.name, b.value
2 from v$sysstat a, v$sesstat b
3 where a.statistic# = b.statistic#
4 and a.statistic# = 191
5* and b.sid = 8
SQL> /
NAME VALUE
---------------------------------------------------------------- ---------
session cursor cache hits 10
----------------------------------------------------------------------------------------
running the statement
1* select ename from scott.emp where empno = :v_empno
SQL> /
ENAME
----------
MILL
If I run the query to ge the session cached cursors statement. I see it has been
incremented.
NAME VALUE
---------------------------------------------------------------- ---------
session cursor cache hits 11
now if I revoke the permissions on the table.
================================================================================================
I get
SQL> /
select ename from scott.emp where empno = :v_empno
*
ERROR at line 1:
ORA-01031: insufficient privileges
-------------------------------------------------------------------------------------------
Sure seems like the statement is undergoing a semantics check despite the
availability of a cached cursor.
The article posted by Tom Kyte, does not state that session_cached_cursors avoids
soft parses. It says they make finding the cursor less expensive. Particularly the
expense of latching the shared pool and the library cache.
He runs a query 1000 times. Once without it being cached and again with it being
cached and finds
NAME RUN1 RUN2 DIFF
---------------------------------------- ---------- ---------- ----------
LATCH.shared pool 2142 1097 -1045
LATCH.library cache 17361 2388 -14973
==============================================================================================
The lesser latch count is for the query using session_cached cursors.
Session_Cached_Cursors do save on resources and are important to scalability. But I
have yet to see something which proves they stop soft parsing.
I saw Steve' Orr's contribution
"An entry is created
for the session's cursor cache and future cursor CLOSEs are ignored. Once in
the session cursor cache the SQL statement does not need to be reparsed.
This gives a significant performance boost!
Giving credit where due: The above was "inspired" from pages 277-280 in
"Scaling Oracle8i" by James Morle."
I have posted material which refutes the above.
Again how does one avoid the soft parsing?
-----Original Message-----
Sent: Wednesday, July 24, 2002 11:43 PM
To: Multiple recipients of list ORACLE-L
Ian,
When coding you should parse once and execute the query many times rather than
loop
parse
bind
execute
close
end;
It can be seen that a parse operation is done on each iteration through the
loop. You may have avoided hard parsing but the program is still soft
parsing. It has to check the shared pool for the query executed each time.
When coding u should rather
parse
loop
bind
execute
end;
close;
So you would be parsing once and executing the query several times.
Therefore reduction on latch contention which makes your application more
scalable and hence better performance.
Check out
http://asktom.oracle.com/pls/ask/f?p=4950:8:1092060::NO::F4950_P8_DISPLAYID,F4950_P8_CRITERIA:2588723819082,%7Bsoft%7D%20and%20%7Bparsing%7D
Also see Bjorn's paper on bind variables
Cheers
Suhen
> Please define soft parsing. Oracle needs to check that the user
> submitting a SQL statement has permissions to run it. It has to do this
> every time a statement is run, bind variables or not. I thought the
> processing of the statement to check permissions to be soft parsing. But,
> perhaps I'm misinformed.
>
> When "cursor-sharing" converts a statement to use bind variables it would
> save on hard parsing, if a match were found the pool; also, it could lessen
> the number of statements present in the pool.
>
> Ian MacGregor
> Stanford Linear Accelerator Center
> [EMAIL PROTECTED]
>
> -----Original Message-----
> Sent: Wednesday, July 24, 2002 9:23 PM
> To: Multiple recipients of list ORACLE-L
>
>
> Mike, Kirti,
>
> Try page 441
>
> CURSOR_SHARING=FORCE does improve badly written applications that use lots
> of literals.
> However coding should be done using bind variables in almost all occasions.
>
> CURSOR_SHARING=FORCE reduces the hard parsing.
>
> What CURSOR_SHARING=FORCE does is rewrites all queries to use bind
> variables before parsing.
>
> eg. select ename from emp where empno = 10;
> rewritten as
> select ename from emp where empno =:SYS_B_0
> or in 8.1.6 , 8.1.7
> select name from emp where empno =:"SYS_B_0"
>
> So it substitutes the literal with bind variables but incurs the cost of
> soft parsing the statement.
> Soft Parsing too frequently limits the scalability of applications and
> sacrifices optimal performance which could have been achieved in the first
> place if written using bind variables.
>
> Parse once and execute as many times as we like.
>
> Also check out Bjorn's paper on bind variables and cursor sharing at
> http://technet.oracle.com/deploy/performance/pdf/cursor.pdf
>
> So CURSOR sharing is not the "silver bullet" as one may expect.
>
> Regards
> Suhen
>
> On Thu, 25 Jul 2002 10:23, you wrote:
> > Mike,
> > What is the version of the database? Some versions of 8.1.7 had a few
> > bugs when this parameter was set to FORCE. I suggest searching Metalink.
> > But it does work as advertised in later releases. I would also recommend
> > reviewing Tom Kytes' book to read about his views in using this parameter
> > at the instance level (my boss is reading my copy, so I can't give you
> > page #s).
> >
> > - Kirti
> >
> > -----Original Message-----
> > Sent: Wednesday, July 24, 2002 6:08 PM
> > To: Multiple recipients of list ORACLE-L
> >
> >
> > Has anyone set Cursor Sharing to Force ?
> > I have a new system that we have to support
> > and there is alot literals filling up the
> > pool. I have never changed this parameter
> > from the default as many seemed to think the
> > jury was still out on it. However, due to
> > my situation, I figured I would try it out.
> > If anyone has any experience with this one
> > I would be curious to know what happened.
> >
> > Mike
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