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CHAPTER 4 MEMORY STRUCTURES
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crutch in this area. The only real way to solve this issue, however, is to use reusable SQL in the first place. Even on the largest of large systems, I find that there are typically at most 10,000 to 20,000 unique SQL statements. Most systems execute only a few hundred unique queries. The following real-world example demonstrates just how bad things can get if you use the Shared pool poorly. I was asked to work on a system where the standard operating procedure was to shut down the database each and every night, to wipe out the SGA and restart it clean. The reason for doing this was that the system was having issues during the day whereby it was totally CPU-bound and, if the database were left to run for more than a day, performance would really start to decline. They were using a 1GB Shared pool inside of a 1.1GB SGA. This is true: 0.1GB dedicated to block buffer cache and other elements and 1GB dedicated to caching unique queries that would never be executed again. The reason for the cold start was that if they left the system running for more than a day, they would run out of free memory in the Shared pool. At that point, the overhead of aging structures out (especially from a structure so large) was such that it overwhelmed the system and performance was massively degraded (not that performance was that great anyway, since they were managing a 1GB Shared pool). Additionally, the people working on this system constantly wanted to add more and more CPUs to the machine, due to the fact that hard-parsing SQL is so CPU intensive. By correcting the application and allowing it to use bind variables, not only did the physical machine requirements drop (they then had many times more CPU power than they needed), but also the allocation of memory to the various pools was reversed. Instead of a 1GB Shared pool, they had less then 100MB allocated and they never used it all over many weeks of continuous uptime. One last comment about the Shared pool and the parameter SHARED_POOL_SIZE. In Oracle9i and before, there is no direct relationship between the outcome of the query ops$tkyte@ORA9IR2> select sum(bytes) from v$sgastat where pool = 'shared pool'; SUM(BYTES) ---------100663296 and the SHARED_POOL_SIZE parameter ops$tkyte@ORA9IR2> show parameter shared_pool_size NAME TYPE VALUE ------------------------------------ ----------- -----------------------------shared_pool_size big integer 83886080 other than the fact that the SUM(BYTES) FROM V$SGASTAT will always be larger than the SHARED_ POOL_SIZE. The Shared pool holds many other structures that are outside the scope of the corresponding parameter. The SHARED_POOL_SIZE is typically the largest contributor to the Shared pool as reported by the SUM(BYTES), but it is not the only contributor. For example, the parameter CONTROL_FILES contributes 264 bytes per file to the miscellaneous section of the Shared pool. It is unfortunate that the Shared pool in V$SGASTAT and the parameter SHARED_POOL_SIZE are named as they are, since the parameter contributes to the size of the Shared pool, but it is not the only contributor.
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CHAPTER 4 MEMORY STRUCTURES
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In Oracle 10g and above, however, you should see a one-to-one correspondence between the two, assuming you are using manual SGA memory management (i.e., you have set the SHARED_POOL_SIZE parameter yourself): ops$tkyte@ORA10G> select sum(bytes)/1024/1024 mbytes 2 from v$sgastat where pool = 'shared pool'; MBYTES ---------128 ops$tkyte@ORA10G> show parameter shared_pool_size; NAME TYPE VALUE ------------------------------------ ----------- -----------------------------shared_pool_size big integer 128M This is a relatively important change as you go from Oracle9i and before to 10g. In Oracle 10g, the SHARED_POOL_SIZE parameter controls the size of the Shared pool, whereas in Oracle9i and before, it was just the largest contributor to the Shared pool. You would want to review your 9i and before actual Shared pool size (based on V$SGASTAT) and use that figure to set your SHARED_POOL_SIZE parameter in Oracle 10g and above. The various other components that used to add to the size of the Shared pool now expect that memory to have been allocated for them by you.
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