Mike Ault's thoughts on various topics, Oracle related and not. Note: I reserve the right to delete comments that are not contributing to the overall theme of the BLOG or are insulting or demeaning to anyone. The posts on this blog are provided “as is” with no warranties and confer no rights. The opinions expressed on this site are mine and mine alone, and do not necessarily represent those of my employer.
Thursday, August 11, 2005
Cursors! Shared Again!
We soon discovered that it wasn't always a good idea to force bind variables for 100% of statements. When a bind variable was what is known as "unsafe", that is, it directly affects the execution plan depending on its value (such as in a range type query, or when an IN list is used) then it wasn't such a good idea to replace the literals in a statement with a bind variable. Luckily, in 9i, or so we thought, they gave us SIMILAR which supposedly would only substitute when the bind variable was "safe". However, this doesn't seem to be the case.
At a recent client site I had the perturbing problem that the shared pool (at over 500 megabytes) was filled to 100%, but only with a little over 1700 SQL objects! For those familiar with shared pools and the amount of SQL they can hold, you will realize this size pool should be able to hold 30,000 or more SQL areas. Looking at the statspack SQL reports, showed some of the SQL areas had as many as 900 versions. In V$SQLAREA, these versions all get showed as one object so typical monitoring using counts of the SQL areas in V$SQLAREA didn't show this issue.
Further investigation using the V$SQL_SHARED_CURSOR view showed that in the case of one SQL with over 700 versions, only 7 showed any indication that they shouldn't be shared. We attempted to reconcile this using:
and based on some metalink research, setting event "10503 trace name context forever, level 128" which was suposed to correct some issues when character, raw, long raw and long values where replaced with bind variables. We bounced the database and found absolutely no difference. Using a search on v$sql_shared_cursor (searchs against various forms of "high number of SQL versions" proved fruitless) found bug report 3406977.8 which talked about a bug, 3406977, which affects versions 18.104.22.168 through 22.214.171.124 and is fixed in 126.96.36.199 that causes statements like "select * from test where id in (1,2,3);" and "select * from test where id in (2,2,3);" to be made into multiple versions if CURSOR_SHARING is set to FORCE or SIMILAR!
Luckily there is a back port for 188.8.131.52 for both platforms (AIX 5L and Solaris 64 Bit) at the client site that we can apply that (crossing my virtual fingers) will fix the issue. Some of the effects of this also cause high CPU usage (CPU Usage showing as a major amount of the response time, with the result of CPU usage - (CPU recursive + CPU Parse) being a major fraction, in our case CPU Recursive and the result of the equation (known as CPU Other) where both near 50% of the total CPU Usage with CPU parse being at 2%. This large value for the CPU other calculation points at the multi-versioning as being a possible issue. I will keep you all posted on the results of the patch.