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Because the shared pool contains the dictionary cache and the library cache, performance can suffer greatly if it is not large enough. How do I determine the hit ratio in the shared pool?<p><B>Technique</B><p>The shared pool is one of the three main memory structures in the system global area (SGA). It is comprised of the data dictionary cache and the library cache. The data dictionary cache buffers in memory data dictionary objects that contain data about tables, indexes, users, and all the rest of the database抯 objects.<p>Every database action requires information from the data dictionary. If that information is not in the data dictionary cache section of memory, then Oracle8 must get it from disk. If this happens consistently, then database performance suffers.<p>The shared pool also contains the library or SQL cache. The library cache buffers previously executed queries, so they need not be reloaded and re-parsed if a user calls them again. If a SQL statement is executed repeatedly but cannot reside in the SQL cache because it is too small, performance improvements are possible. <p>The UTLESTAT report contains sections focusing on the library and dictionary caches. Listings 15.4 and 15.5 show excerpts from those sections of the report summarizing library and data dictionary cache performance, respectively.<p>Listing 15.4 The library cache performance section of the UTLESTAT report<p><code><b>SVRMGR> Rem Select Library cache statistics. The pin hit rate should be high.<p>SVRMGR> select namespace library,<p>2> gets,<p>3> round(decode(gethits,0,1,gethits)/decode(gets,0,1,gets),3)<p>4> gethitratio,<p>5> pins,⌨️ 快捷键说明
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