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CHAPTER 12 HASH JOINS
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delta = -106 (see below)
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As the hash_area_size increases, the cost of the hash drops steadily by a few units, but every now and again it experiences a sharp drop (as highlighted by the delta lines in the preceding listing). These step changes result from the optimizer allowing for a larger cluster size as the hash_area_size increases. If you compare the 10104 trace with the 10053 trace, you will usually find that each step change does actually correspond (closely, but not perfectly the run-time engine doesn t follow the exact model used in the calculations) to the cluster size increasing by a single block. So the old model does allow for different sized I/Os and a similar effect is visible in the numbers when you enable CPU costing, even if you stick with the manual hash_area_size. Since that s the case, you might start to wonder how things change when you start to adjust the system statistics after all, the most significant effect of system statistics is to tell the optimizer the size and relative time for multiblock reads. So which system statistics affect the cost of the join Unsurprisingly, if you change the CPU speed, then the cost of the join changes though, as you might expect, it won t normally change very much. The cost also changes if you change the relative values of mreadtim and sreadtim. Hash joins do a lot of multiblock reads and writes (albeit direct path ones), so the I/O response time should make a difference to the cost, and there are some indications that the optimizer simply uses the mreadtim as the time for each of its cluster-sized I/Os, irrespective of the actual current size of the cluster. The value of the MBRC statistic also has an impact but apparently only some of the time. I haven t been able to work this out yet, but I believe it is introducing yet another complication into the decision about number of partitions and the cluster size, and since you can only see the run-time values for this settings, not the optimizer predictions, it is very hard to work out why the costs have changed with the value of MBRC when they do change, and why they haven t when they don t. So CPU costing has some relevance, and there is good sense in the stepped values we see when using automatic workarea sizes. Nevertheless, the long intervals of constant cost you see with the automatic workarea sizing are still a bit of a puzzle. The answer only becomes clear when you examine the 10104 trace, and realize that the optimizer is making memory-based decisions in a radically different way. As we saw earlier on, the first values reported in the 10104 trace as follows:
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CHAPTER 12 HASH JOINS
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Original memory: 581632 Memory after all overhead: 710649 Memory for slots: 688128
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-- 568 KB -- 694 KB -- 672 KB
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I ve picked this set of three values from a trace file that had the pga_aggregate_target set to 11,440KB to highlight a couple of points. First, that the hash join did, indeed, start with 5% of the pga_aggregate_target, but almost immediately decided that more was necessary. So it seems the 5% is not an absolutely hard limit for hash joins. The second point emerges when I extract the Memory for slots from several other trace files with pga_aggregate_target varying from 10MB to 40MB every single one up 33,200KB (32.4MB) has the same 672KB, after which Oracle jumps to using 1,440KB for its slots. This strategy is a reflection of the costing we saw with automatic workarea sizing enabled the optimizer gets the same cost time after time because it is planning to use the same amount of memory time after time, irrespective of what you tell it is available. When you switch to the modern technology, it really doesn t matter what you have set the pga_aggregate_target to, the optimizer is going to work out a sensible amount of memory for doing the join in an efficient fashion, and is only going to vary the memory demands in large steps, because basically it s only large changes that make a worthwhile difference. In this example, the big switch occurred because Oracle moved from using 7 blocks per partition to 15 blocks per partition it s the sort of change that might make the extra use of memory give a reasonable return in terms of I/O performance.
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