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CHAPTER 11 NESTED LOOPS
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Looking at this code, you see two loop constructs. The outer loop works through table_1 and the inner loop works (possibly many times) through table_2. Because of the structure of this pseudo-code, the two tables in a nested loop are commonly referred to as the outer table and the inner table. The outer table is also commonly referred to as the driving table (although I don t think I ve ever heard the inner table referred to as the driven table).
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The terms outer and inner are really only appropriate to nested loop joins. When talking about hash joins, you ought to refer to the build table and probe table; and for merge joins, the terms first table and second table are sufficient. However, you will find that the 10053 trace file always uses the terms outer and inner to identify the first and second tables respectively in a join operation. I will revisit this point in the relevant chapters.
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The execution plan for a nested loop join with an index on the inner table can have two different forms from 9i onward: one when the optimizer uses the index on the inner table for a unique scan, and another when the optimizer uses the index for a range scan. The second form ceases to be an option, however, if the outer table is guaranteed to return a single row. Execution Plan (9.2.0.6 autotrace - unique access on inner table (traditional)) ------------------------------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=324 Card=320 Bytes=11840) 1 0 NESTED LOOPS (Cost=324 Card=320 Bytes=11840) 2 1 TABLE ACCESS (FULL) OF 'DRIVER' (Cost=3 Card=320 Bytes=2560) 3 1 TABLE ACCESS (BY INDEX ROWID) OF 'TARGET' (Cost=2 Card=1 Bytes=29) 4 3 INDEX (UNIQUE SCAN) OF 'T_PK' (UNIQUE) (Cost=1 Card=1) Execution Plan (9.2.0.6 autotrace - range scan on inner table (new option)) --------------------------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=322 Card=319 Bytes=11803) 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TARGET' (Cost=2 Card=1 Bytes=29) 2 1 NESTED LOOPS (Cost=322 Card=319 Bytes=11803) 3 2 TABLE ACCESS (FULL) OF 'DRIVER' (Cost=3 Card=319 Bytes=2552) 4 2 INDEX (RANGE SCAN) OF 'T_PK' (UNIQUE) (Cost=1 Card=1) The second form of the nested loop join appeared in 9i, and represents a cunning optimization which I understand is known as table prefetching that can reduce the logical I/O count (hence latching, and possibly physical I/O count) on larger nested loop joins.
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CHAPTER 11 NESTED LOOPS
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You will note that the way the costs are displayed in the new plan doesn t reflect the modified form of the plan (or the notional saving in resources). The execution plan has simply moved the reference to the second table (line 3 in the traditional plan) to a point outside the nested loop (i.e., to line 1 in the new form of the plan). In principle, you might expect the costs and cardinality of the new plan to look more like the following: Execution Plan (Notional, with full disclosure of prefetch costing) ------------------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=322 Card=319 Bytes=11803) 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TARGET' (Cost=322 Card=319 Bytes=11803) 2 1 SORT (ROWID LIST) (Cost= Card=319 Bytes= ) 3 2 NESTED LOOPS (Cost= Card=319 Bytes= ) 4 3 TABLE ACCESS (FULL) OF 'DRIVER' (Cost=3 Card=319 Bytes=2552) 5 3 INDEX (RANGE SCAN) OF 'T_PK' (UNIQUE) (Cost=1 Card=1) In fact, whichever form you see in the execution plan, the run-time engine may still use the mechanism shown in the traditional plan to date, I have only seen the execution engine use the new mechanism in one very special case. The examples I have picked to show you the plan structure comes from a test case that also happens to show an interesting feature in later versions of 9i both plans came from exactly the same query (see script prefetch_test_02.sql in the online code suite), where the theoretical access to the second table should have been a unique scan for a single row on the primary key index. The reported execution plan actually switched mechanisms as the number of rows in the driving table changed as you can see in the line with the TABLE ACCESS (FULL) OF 'DRIVER', at 320 rows the traditional plan was reported, but at 319 rows the optimizer reported that it would switch to a range scan so that it could use the new mechanism. This test case happens to be an example of the one specific and unrealistic case where the new mechanism actually operates at run time, and if you care to check the logical I/O and latch activity while running the test case, you will find that there are some very clear changes in resource usage. The script is in the online code suite, but you may find that you have to experiment to find the break point every single time you try the test. The exact test is not reproducible, and probably depends on recent activity on your system (script prefetch_test.sql runs the tests on autopilot, and prefetch_test_01.sql demonstrates that the effect depends on CPU costing being enabled). When I examined the 10053 trace file for the two different plans, I was unable to spot any difference anywhere that could explain why the optimizer had changed its choice of plan. The test results were not reproducible on 10g. There are two fairly standard pictures used to represent the nested loop join. Each has its strengths and weaknesses as a tool for explaining what is going on. The first picture, shown in Figure 11-1, simply connects rows from one table with rows from another, using arrows to indicate direction of activity. In a monochrome diagram, this makes it easy to see the connection between rows of one table and their partners in the other table.
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