The Library Cache contains the execution plans and parse trees of SQL and PL/SQL statements. Along with the Dictionary Cache, the Library Cache forms the bulk of the shared pool and is therefore managed internally by Oracle as a part of the pool. The size of the shared pool itself is managed by the SHARED_POOL_SIZE parameter in init.ora. As this parameter goes to the size of the SGA, a balance must be maintained between the efficiency of Library Cache use, and the amount of real memory being allocated to Oracle.

Like the other Cache mechanisms within Oracle, in order to maintain performance a high ratio of memory reads for statements and PL/SQL objects needs to be achieved. The way to monitor this, is to perform the following SQL against the v$librarycache table:

select (sum(pinhits) / sum(pins)) * 100 from v$librarycache

This hit rate should be maintained at around 95+% at all times.