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CHAPTER 13 SORTING AND MERGE JOINS
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---- Direct Read Statistics -----Size of read slots for merge phase Number of read slots for merge phase Size of read slots for output Number of read slots for output Number of direct sync reads Number of blocks read synchronously Number of direct async reads Number of blocks read asynchronously
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-- only in multipass -- only in multipass 16384 64 435 461 558 1104
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Notice how the number of direct sync reads (435) in the 10032 statistics matches the number of direct read waits reported in the 10046 trace file. Moreover, the total number of reads (435 + 558) reported in the 10032 trace matches the number of reads reported in the v$tempstat file statistics (993). It s a pity that the number of direct writes in the 10032 trace (268) doesn t quite match the number of writes reported in v$tempstat (269), but the 9i trace was missing a line about async writes that appeared in the 8i and 10g traces, so perhaps the discrepancy is a simple accounting error somewhere in the async code. Going back to the 10046 trace, the last four lines from the trace show Oracle rereading blocks from the sort segment to merge them. The p2 value listed in these four lines is the starting block number in the file for the read, and shows that Oracle is jumping all over the place in the file; but there were 26 consecutive read waits at the start of the list, corresponding to the 26 lines saying Merging run at mmmm for nn blocks in the 10033 trace. The p3 value is the number of blocks read. After starting with two blocks for each read (the limit set by the sort_multiblock_read_count), all subsequent reads in the trace file were just one block each and they left gaps all over the place in the sort runs that had previously been written, which is probably the effect of having lots of read slots for output available for async read requests. Given the reports of asynchronous writes and reads in the 10032 trace, it is no surprise that we can t get the numbers to match up. In fact, we even find that the number of read waits in the 10046 trace shows quite clearly that sometimes we end up waiting for async reads, as well as sync reads.
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Once you know how sorting works, it is possible to look at the numbers that the optimizer produces to estimate the cost of a sort, and try to invent a rationale that would explain how those numbers have been generated. Of course, there are four different scenarios to deal with if you want complete coverage:
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CHAPTER 13 SORTING AND MERGE JOINS
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CPU costing disabled CPU costing disabled CPU costing enabled CPU costing enabled
(8i style) (default 9i style) (common 9i style) (strategic 9i and default 10g style)
Then you have to worry about the problem of generalization: Is there anything different for sort (aggregate), what about sorting to create an index; and is the cost of sorting in a sort/merge join in any way different from a simple sort Finally, of course, there are the O/S issues does the costing mechanism make any allowance for the differences between 32-bit and 64-bit platforms or the availability of asynchronous I/O And before I say anything about how the optimizer calculates the cost of sorts, I would like to draw your attention to a hidden parameter that appeared in 9i, _new_sort_cost_estimate. The default value for this parameter is true, and the description reads enables the use of new cost estimate for sort. Inferring the actual algorithms of the optimizer by checking the results from controlled experiments is virtually impossible especially when there are so many changes between releases, and so many little tweaks that might be due to version, might be bugs, or might be special cases. I m not claiming a complete understanding of the costs for sorts and merge joins but the following notes may help you deal with problems.
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