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CHAPTER 1 WHAT DO YOU MEAN BY COST
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According to the CPU costing model: Cost = ( #SRds * sreadtim + #MRds * mreadtim + #CPUCycles / cpuspeed ) / sreadtim where #SRDs - number of single block reads #MRDs - number of multi block reads #CPUCycles - number of CPU Cycles sreadtim - single block read time mreadtim - multi block read time cpuspeed - CPU cycles per second Translated, this says the following: The cost is the time spent on single-block reads, plus the time spent on multiblock reads, plus the CPU time required, all divided by the time it takes to do a single-block read. Which means the cost is the total predicted execution time for the statement, expressed in units of the single-block read time.
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REPORTING CPUSPEED
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Although the manual indicates that the cpuspeed is reported in cycles per second, there are two possible errors in the statement. The simple error is that the values that appear suggest the unit of measure is supposed to be millions of cycles per second (i.e., CPU speed in MHz). Even then, the number always seems to fall short of expectations in my case by a factor of anything between 5 and 30 on various machines I ve tested. The more subtle error then is that the value may actually be a measure of millions of standardized oracle operations per second, where a standardized oracle operation is some special subroutine designed to burn CPU. (A 10053 trace file from 10.2 offers corroborative evidence for this.) Whether the number represents cycles per second or operations per second, the difference is only a simple scaling factor. The mechanism involved in using the cpuspeed is unchanged.
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Why does Oracle choose such an odd time unit for the cost, rather than simply the number of centiseconds I think it s purely for backward compatibility. The cost under 8i (and 9i before you enabled full CPU costing) was just the count of the number of I/O requests, with no distinction made between single-block and multiblock I/Os. So, for backward compatibility, if the new code reports the time in units of the single-block read time, the number produced for the cost for a typical (lightweight, index-based) OLTP query will not change much as you upgrade from 8i to 9i. A little extra thought about this formula will also tell you that when you enable CPU costing, the cost of a tablescan will tend to go up by a factor that is roughly (mreadtim / sreadtim). So 9i with CPU costing will tend to favor indexed access paths a little more than 8i did because 9i
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CHAPTER 1 WHAT DO YOU MEAN BY COST
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recognizes (correctly) that multiblock reads could take longer than single-block reads. If you are planning to upgrade from 8i to 9i (or 8i to 10g), make sure you enable CPU costing from day one of your regression testing there will be some surprises waiting for you. A final consideration when examining this formula is that there is no explicit mention of any components relating to the time spent on the I/O that can result from merge joins, hash joins, or sorting. In all three cases, Oracle uses direct path writes and reads with sizes that usually have nothing to do with the normal multiblock read size so neither mreadtim nor sreadtim would seem to be entirely appropriate.
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Transformation and Costing
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There is an important aspect of optimization that is often overlooked and can easily lead to confusion, especially as you work through different versions of Oracle. Before doing any cost calculation, Oracle may transform your SQL into an equivalent statement possibly one that isn t even legally acceptable SQL and then work out the cost for that equivalent statement. Depending on the version of Oracle, there are transformations that (a) cannot be done, (b) are always done if possible, and (c) are done, costed, and discarded. Consider, for example, the following fragments of SQL (the full script, view_merge_01.sql, is available with the online code suite for this chapter): create or replace view avg_val_view as select id_par, avg(val) avg_val_t1 from t2 group by id_par ; select t1.vc1, avg_val_t1 from t1, avg_val_view where and ; t1.vc2 = lpad(18,32) avg_val_view.id_par = t1.id_par
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You will note that avg_val_view is an aggregate view of the table t2. The query then joins t1 to t2 on the column that is driving the aggregation. In this case, Oracle could use one of two possible mechanisms to produce the correct result set: instantiate the aggregate view and then join the view to table t1, or merge the view definition into the query and transform it. From a 9i system, here are the two possible execution plans:
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