Oracle Internals: An Introduction

Library Cache Locks and Pins

The library cache is not one cache, but many. It contains the pseudo code for PL/SQL program units. It contains parse trees and execution plans for shareable SQL statements. It also contains abstract representations in a form called DIANA of the database objects referenced by the SQL statements. The information is needed in this form for PL/SQL program unit compilation and SQL statement parsing and execution, despite the fact that the dictionary cache contains the same information in a different form. The library cache also contains control structures such as synonym translations, dependency tracking information, and library cache locks and pins.

Library cache locks are referred to as breakable parse locks in the Oracle documentation. They are applied to the library cache objects for SQL statements and PL/SQL program units, and recursively to the library cache objects for the database objects on which they depend. Library cache locks are held in shared mode during parse operations and are converted to null mode thereafter. If a DDL statement later modifies the definition of a database object, then the library cache information for that database object and all dependent library cache objects is invalidated by breaking the library cache locks.

Library cache locks can only be broken, however, when the library cache object is not also pinned. A pin is applied to the library cache object for a PL/SQL program unit or SQL statement while it is being compiled, ...