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What About Index Full Scans
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There are occasions when the optimizer will decide that the optimum access path is to read the entire index in correct index order, starting with the leaf block at one end of the index and following the leaf pointers until it gets to the leaf block at the other end of the index.
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FIRST_ROWS OPTIMIZATION
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One of the options for the optimizer_mode in 8i was first_rows. This option still exists for backward compatibility in 9i and 10g, but is deprecated. One of the critical features of first_rows optimization (compared to the newer first_rows_N optimization) was the existence of a few rules for overriding the normal costing behavior. One such rule was that if there was an index that could be used to avoid a sort, then the optimizer would use it apparently regardless of how much more expensive the path might be. So a query that acquired five rows and sorted them under all_rows might switch to acquiring 1,000,000 rows, and discarding all but five of them under first_rows if this meant the sort could be avoided. So one side effect of first_rows optimization was the relatively frequent appearance of full scans of indexes. (In fact, you could change this behavior by adjusting the hidden parameter _sort_elimination_cost_ratio, as the default value made the behavior rather extreme.)
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One possible cause for this behavior would be to avoid a sort for an order by clause, another might be that almost all the data in the table had been deleted and an indexed access to just the
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CHAPTER 4 SIMPLE B-TREE ACCESS
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few rows remaining would be quicker than scanning a very large number of empty table blocks. Consider this example (btree_cost_03.sql in the online code suite): alter session set "_optimizer_skip_scan_enabled"=false; select /*+ index(t1) */ small_vc from t1 where n2 = 2 order by n1 ; Execution Plan (8.1.7.4, 9.2.0.6 and 10.1.0.4) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=1601 Card=500 Bytes=8500) 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'T1' (Cost=1601 Card=500 Bytes=8500) 2 1 INDEX (FULL SCAN) OF 'T1_I1' (NON-UNIQUE) (Cost=1113 Card=500) Notice how there is no clue in this example that the order by clause is present in the query. More complex execution plans may be a little more helpful, with explicit lines like sort (order by) nosort. How does the cost work in this case Informally, we have no restrictions on the index until after we have reached the leaf blocks, so we could expect the effective index selectivity to be 1.00 (100%). When we examine the leaf blocks, we can identify the entries where n2 = 2, a single predicate with a selectivity of 0.05, so we could expect this to be the effective table selectivity. So let s put these numbers into the formula and check: Cost = 2 + (1 * 1111) + (0.05 * 9745) = 2 + 1111 + 487.25 = 1113 + 488 = 1601
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-- round() or ceil() -- I've chosen ceil()
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The numbers do work out properly but only because I ve fiddled them a little bit. Usually it s looked as if ceil() is used on costs in 8i, and round() in 9i and 10g. The error in this example is small, so I m not going to worry too much about it. (For commonly occurring cases, I might fuss to nail down the error; for the fringe examples I m happy with a 99% fit to the model, especially since I know that the kernel code has a number of adjustments for special cases.)
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And Index-only Queries
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What should we do with the baseline formula when the query doesn t need to visit the table at all Again, we can start with an informal argument, and check to see if the arithmetic works. If we aren t going to visit the table, then perhaps we just ignore the last component the bit that represents the visit to the table (btree_cost_03.sql in the online code suite).
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