Oracle Metric library cache Oracle Tips by Burleson Consulting

The Oracle Documentation notes on the library cache:

"The library cache Oracle metric holds executable forms of SQL cursors, PL/SQL programs, and Java classes. This section focuses on tuning as it relates to cursors, PL/SQL programs, and Java classes. These are collectively referred to as application code.

When application code is run, Oracle attempts to reuse existing code if it has been executed previously and can be shared. If the parsed representation of the statement does exist in the library cache and it can be shared, then Oracle reuses the existing code. This is known as a soft parse, or a library cache hit.

If Oracle is unable to use existing code, then a new executable version of the application code must be built. This is known as a hard parse, or a library cache miss."

Oracle MetaLink documents also point out that "The library cache latches (and the library cache pin latch in Oracle 7.1) protect operations within the library cache itself."

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http://www.rampant-books.com/book_2005_1_awr_proactive_tuning.htm