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CHAPTER 2 TABLESCANS
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TUNING BY CHANGING BLOCK SIZES
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Be very cautious with the option for using different block sizes for different objects the feature was introduced to support transportable tablespaces, not as a tuning mechanism. You may be able to find a few special cases where you can get a positive benefit by changing an object from one block size to another; but in general you may find that a few side effects due to the optimizer changing its arithmetic may outweigh the perceived benefits of your chosen block size.
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One final observation I have included the original results for the 8KB block size in the table; but I have also listed the costs when the table was in a tablespace that used ASSM. Notice that the cost of a tablescan has increased by about 1.5%. Every extent in my table had a couple of blocks taken out for space management bitmaps 2 blocks out of 128 since my extents were 1MB; the extra cost is largely due to the effect of those blocks on the high water mark of the table. Again, if you are advised to move an object into an ASSM tablespace for performance reasons (specifically to avoid contention on inserts), be just a little cautious this is just one of the irritating little side effects of ASSM.
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One of the most serious defects of the optimizer prior to 9i was its assumption that single-block reads and multiblock reads were equally cost effective. This assumption is flawed on two counts. First, multiblock reads often take longer to complete than single-block reads (especially on systems with too few disks, configured with too small a stripe size). Second, a tablescan can use a surprising amount of CPU as each row is tested against some predicate. In 9i, both these flaws are addressed through system statistics. You can collect system statistics over representative periods of time, or you could simply calibrate your hardware (at least the I/O subsystem) for absolute performance figures and then write system statistics into the database. For example, you could issue the following two statements at 9:00 a.m. and noon respectively one Monday morning: execute dbms_stats.gather_system_stats('start') execute dbms_stats.gather_system_stats('stop') The 'start' option takes a starting snapshot of various figures from v$filestat (actually the underlying x$ including some columns that are not exposed in v$filestat) and v$sysstat; the 'stop' takes a second snapshot, works out various statistics about disk and CPU activity over that three hours, and records them in the database. The nature of the data collected and stored is version dependent and is probably still subject to change in 10g, but you should find results in table sys.aux_stats$ that look something like the following from 9i (10g has a few extra rows): select pname, pval1 from sys.aux_stats$
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CHAPTER 2 TABLESCANS
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where sname = 'SYSSTATS_MAIN' ; PNAME PVAL1 ----------- ---------CPUSPEED 559 SREADTIM 1.299 MREADTIM 10.204 MBRC 6 MAXTHR 13938448 SLAVETHR 244736 Accessing the values by querying the base table is not the approved method, of course, and Oracle supplies a PL/SQL API to query, set, and delete system statistics. For example, we can set the basic four system statistics with code like the following, which comes from script set_system_stats.sql in the online code suite: begin dbms_stats.set_system_stats('CPUSPEED',500); dbms_stats.set_system_stats('SREADTIM',5.0); dbms_stats.set_system_stats('MREADTIM',30.0); dbms_stats.set_system_stats('MBRC',12); end; / alter system flush shared_pool; I ve included the flushing of the shared pool in this code fragment as a reminder that when you change the system statistics, existing cursors are not invalidated (as they would be for dependent cursors when you gather statistics on a table or index). You have to flush the shared pool if you want to make sure that existing cursors are reoptimized for the new system statistics.
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Note If you want to use a low-privilege account to collect system statistics, you will need to grant the role gather_system_statistics to the account. This role is defined in $ORACLE_HOME/rdbms/admin/ dbmsstat.sql. There was a bug in many versions of 9.2 that would result in a nonfatal Oracle error if an account tried to modify the system statistics more than once in a session.
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The figures in my anonymous PL/SQL block tell Oracle that A single CPU on my system can perform 500,000,000 standard operations per second. The average single-block read time is 5 milliseconds. The average multiblock read time is 30 milliseconds. The typical multiblock read size is 12 blocks.
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