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CHAPTER 2 TABLESCANS
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alter table pt1 exchange partition p0999 with table load_table including indexes without validation ; The problem is that this doesn t bring table-level statistics up to date (in fact, there have been many reports in the past about table-level statistics vanishing when partition maintenance took place). After you ve done a partition exchange, you need to have a mechanism that brings the table-level statistics up to date preferably without using an excessive amount of machine resources. You really have to know your data, and the most up-to-date version of the dbms_stats package, to do this efficiently. The problem of partitions and table-level statistics echoes on down the chain to subpartitions. If you want to query exactly one subpartition of one partition, then the optimizer uses the statistics for that one subpartition. If you want to query several subpartitions from a single partition, the optimizer switches to the partition-level statistics for that one partition. If your query gets any messier in its selection, the optimizer will use the table-level statistics.
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Summary
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The cost of a tablescan is largely the cost of assumed multiblock reads. To calculate the cost, the optimizer divides the number of used blocks in the table (below the high water mark) by a number representing the assumed size of the multiblock read. (It was only when I wrote this line for the summary that I thought how nice it would be if the parameter included the word size instead of count and was expressed in KB.) In 8i, this calculated count of the required number of multiblock reads is the cost. In 9i, the introduction of system statistics allows the result to be adjusted by factors representing typical sizes and relative speeds of multiblock reads, and adding in the CPU cost of visiting the blocks and acquiring data from every row in the blocks. And 10g holds some clues that at some future date the optimizer will also factor in recent cache history, which is likely to result in a reduced value for the cost. For index fast full scans, the number of leaf blocks in the index, rather than the number of blocks below the high water mark, seems to be the driving value used in the calculation. In many cases, this will give a reasonable result. There are a couple of scenarios, though, where the number of leaf blocks can be much smaller than the number of blocks below the high water mark, and this would result in the cost of an index fast full scan being seriously underestimated. Partitioned tables are likely to be a problem. The optimizer can use the statistics from a single partition if the partition can be identified at parse time; otherwise it uses the table-level statistics. And the same type of strategy appears with subpartitions of a single partition. In many cases, you may find that the only way to get the optimizer to produce sensible plans is to refresh the table-level statistics as you do partition maintenance. This may prove to be a resourceintensive approach.
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CHAPTER 2 TABLESCANS
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Test Cases
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The files in the download for this chapter are shown in Table 2-6. Table 2-6. 2 Test Cases
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plan_run81.sql plan_run92.sql tablescan_01.sql calc_mbrc.sql tablescan_01a.sql tablescan_01b.sql set_system_stats.sql tablescan_02.sql tablescan_03.sql tablescan_04.sql cpu_costing.sql parallel.sql parallel_2.sql index_ffs.sql hack_stats.sql partition.sql setenv.sql
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Script to report all columns of the plan_table in 8.1 Script to report all columns of the plan_table in 9.2 Builds simple test data set for tablescan costs Script to generate adjusted dbf_mbrc values Impact of block size on cost for a table fixed at 10,000 blocks Impact of multiple block sizes on cost for a table fixed at 80MB Sample of code to set system statistics Original test case modified to investigate normal CPU costing Original test case modified to investigate noworkload CPU costing Effects of noworkload CPU costing with multiple block sizes Basic demonstration of predicates moving Reuses the first tablescan example for parallel testing Version of parallel.sql with system statistics enabled Simple demonstration of fast full scan costs on an index General-purpose script for modifying statistics on an object Demonstration of how partitioned tablescans are costed Sets the standardized test environment for SQL*Plus
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