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If the number of reloads in the output shown in Listing 22-4 is high, you need to make sure that the objects are pinned using the following command: SQL> EXECUTE SYS.DBMS_SHARED_POOL.KEEP(object_name,object_type); You can use the following statements to pin a package first in the shared pool and then remove it, if necessary: SQL> EXECUTE SYS.DBMS_SHARED_POOL.KEEP(NEW_EMP.PKG, PACKAGE); SQL> EXECUTE SYS.DBMS_SHARED_POOL.UNKEEP(NEW_EMP.PKG,PACKAGE); Of course, if you shut down and restart your database, the shared pool won t retain the pinned objects. That s why most DBAs use scripts with all the objects they want to pin in the shared pool and schedule them to run right after every database start. Most of the objects usually are small, so there s no reason to be too conservative about how many you pin. For example, I pin all my packages, including Oracle-supplied PL/SQL packages. Look at the following example, which gives you an idea about the total memory taken up by a large number of packages. This query shows the total number of packages in my database: SQL> SELECTCOUNT(*) 2 FROM V$DB_OBJECT_CACHE 3* WHERE type='PACKAGE'; COUNT(*) --------------167 SQL> The following query shows the total amount of memory needed to pin all my packages in the shared pool: SQL> SELECT SUM(sharable_mem) 2 FROM V$DB_OBJECT_CACHE 3* WHERE type='PACKAGE'; SUM(SHARABLE_MEM) ----------------4771127 SQL> As you can see, pinning every single package in my database takes up less than 5MB of a total of several hundred megabytes of memory allocated to the shared pool.

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When users request data, Oracle reads the data from the disks (in terms of Oracle blocks) and stores it in the buffer cache so it may access the data easily if necessary. As the need for the data diminishes, eventually Oracle removes the data from the buffer cache to make room for newer data. Note that some operations don t use the buffer cache (SGA); rather, they read directly into the PGA area. Direct sort operations and parallel reads are examples of such operations.

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As with the shared pool component, the best way to manage the buffer cache is to choose automatic SGA management However, if you choose to manage the SGA manually, you can use a process of trial and error to set the buffer cache size You assign an initial amount of memory to the pool and watch the buffer cache hit ratios to see how often the application can retrieve the data from memory, as opposed to going to disk The terminology used for calculating the buffer hit ratio can be somewhat confusing on occasion Here are the key terms you need to understand: Physical reads: These are the data blocks that Oracle reads from disk Reading data from disk is much more expensive than reading data that s already in Oracle s memory.

When you issue a query, Oracle always first tries to retrieve the data from memory the database buffer cache and not disk DB block gets: This is a read of the buffer cache, to retrieve a block in current mode This most often happens during data modification when Oracle has to be sure that it s updating the most recent version of the block So, when Oracle finds the required data in the database buffer cache, it checks whether the data in the blocks is up to date If a user changes the data in the buffer cache but hasn t committed those changes yet, new requests for the same data can t show these interim changes If the data in the buffer blocks is up to date, each such data block retrieved is counted as a DB block get.

Consistent gets: This is a read of the buffer cache, to retrieve a block in consistent mode This may include a read of undo segments to maintain the read consistency principle (see 6 for more information about read consistency) If Oracle finds that another session has updated the data in that block since the read began, then it will apply the new information from the undo segments Logical reads: Every time Oracle is able to satisfy a request for data by reading it from the database buffer cache, you get a logical read Thus logical reads include both DB block gets and consistent gets Buffer gets: This term refers to the number of database cache buffers retrieved This value is the same as the logical reads described earlier.