Applies to:Oracle Server - Enterprise Edition - Version: 188.8.131.52 to 184.108.40.206 - Release: 11.1 to 11.2
Information in this document applies to any platform.
A number of customers have seen an increase in the number of child cursors since migrating to Oracle Database 11g Release 2. This can lead to many problems including complete CPU saturation of a machine requiring a database instance bounce or general database performance issues in the form of waits on mutexes and 'library cache lock'.
From Oracle versions 9.0 through 11.1, an oracle instance would limit the number child cursors in the shared pool associated with a single parent to 1024 before it would mark the parent OBSOLETE effectively invalidating it and all the children. Then a new parent with one child would be created and used going forward. But this would only limit the degradation of performance for some environments rather than fix something that could be addressed more effectively through improved application coding. (The attempt to address this from the database side also introduced other issues like bug 5177766). The child limit was removed by Oracle development because it was only masking an application problem at the expense of database performance for better designed applications. In addition, the obsolete code would not work in cases when SQL was wrapped within PL/SQL. The fundamental problem that obsolete code was masking is application code that was written incorrectly with regards to the ability to be shared. For example, it is not written with user binds or the literal characteristics differ to a high degree.
Therefore, setting cursor_sharing = SIMILAR is highly discouraged in Oracle Database 11g Release 2 (and generally has not been recommended for most environments even in earlier versions) for several reasons:
1) This parameter is generally overly restrictive in what it actually allows to be shared. SIMILAR tells oracle to try and share cursors by replacing all literals with binds for legacy applications, but directs that sharing only be performed when all the replaced literal values were exactly the same (in the case of predicates referencing columns with histograms or using inequality operators such as BETWEEN, <, and !=)
2) This parameter seems to bypass a lot of the improvements made with Oracle Database 11g’s Adaptive Cursor Sharing feature and other abilities in the Cost Based Optimizer code to make better decisions on what execution plans should and should not be shared.
3) Having many child cursors all associated with 1 parent cursor could perform much worse than having many parent cursors that would be seen with having the default setting of cursor_sharing = EXACT (or FORCE). The scenario of many thousands of child cursors associated with 1 parent results in a potential bottleneck for searches for matching cursors (soft parsing) within the library cache.
The cursor_sharing parameter was introduced as a workaround for legacy applications that could not scale because they had not yet been redesigned to use bind variables. It has been presumed that most applications have been redesigned since then. If you are still using such an application, our recommendation is to set cursor_sharing = FORCE. This setting maximizes cursor sharing while leveraging the Adaptive Cursor Sharing framework to generate multiple execution plans based on different literal value ranges if necessary.
NOTE:261020.1 - High Version Count with CURSOR_SHARING = SIMILAR or FORCE
NOTE:377847.1 - Unsafe Literals or Peeked Bind Variables