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CHAPTER 9 QUERY TRANSFORMATION
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are no rows in table t1 that do not have a corresponding row in table t2 despite the fact that we ve clearly just reported one. The problem is that there is a row in table t2 where n1 is null as soon as this occurs, the final query can never return any data because every row from table t1 that is tested gets to this null row and tests the condition t1.n1 = null (which is not the same as t1.n1 is null), which never returns true. The solution, of course, is to include a not null predicate at both ends of the test: select from where and * t1 t1.n1 is not null t1.n1 not in ( select t2.n1 from t2 where t2.n1 is not null )
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; N1 V1 ---------- -------------------99 Ninety-nine 1 row selected. Because of the problem of comparisons with null, the optimizer is only able to transform the standard not in subquery if the not null predicates or constraints are in place. Otherwise, the only possible execution plan is the filter option; moreover, the clever optimization that we saw for the filter option is not (currently) implemented for the case where the filter is there to deal with a not in. This is another reason for defining database constraints. If you know that a column should never be null, then tell the database about it, otherwise bad data (in the form of a null) may get into that column one day, and make any existing not in subqueries suddenly return the wrong answer for no apparent reason. Just to complete the catalog, you might note that the (apparently opposite) expression colX in ('A', 'B', 'C') is equivalent to colX = 'A' or colX = 'B' or colX = 'C' The string of ORs means that just one of the individual tests in the list needs evaluate to true (after which Oracle need check no further) for the entire expression to evaluate to true. Apart from the benefit of being able to take a shortcut on the test, this does mean that the presence of a null somewhere in the test is not a problem. This shows us that, counterintuitively, the two operators in and not in are not exact opposites of each other.
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The ordered Hint
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As the optimizer becomes increasingly clever with subqueries, you may find that code that used to work efficiently suddenly starts to misbehave because you ve told it to! Hints even the
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CHAPTER 9 QUERY TRANSFORMATION
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simplest ones can be very dangerous, and here s an example showing how things can go badly wrong (see script ordered.sql in the online code suite). It starts with a query that needed a little bit of hinting in 8i to make the join between t1 and t3 take place in the right order: select /*+ ordered push_subq */ t1.v1 from t1, t3 where and t3.n1 = t1.n1 exists ( select from where and ) exists ( select from where and )
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t2.id t2 t2.n1 = 15 t2.id = t1.id
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t4.id t4 t4.n1 = 15 t4.id = t3.id
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; This happens to work perfectly in 8i, with the optimizer producing the following plan: Execution Plan (8.1.7.4) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=103 Card=1812 Bytes=45300) 1 0 FILTER 2 1 HASH JOIN (Cost=103 Card=1812 Bytes=45300) 3 2 TABLE ACCESS (FULL) OF 'T1' (Cost=51 Card=1000 Bytes=16000) 4 3 TABLE ACCESS (BY INDEX ROWID) OF 'T2' (Cost=2 Card=1 Bytes=8) 5 4 INDEX (UNIQUE SCAN) OF 'T2_PK' (UNIQUE) (Cost=1 Card=1) 6 2 TABLE ACCESS (FULL) OF 'T3' (Cost=51 Card=1000 Bytes=9000) 7 1 TABLE ACCESS (BY INDEX ROWID) OF 'T4' (Cost=2 Card=1 Bytes=8) 8 7 INDEX (UNIQUE SCAN) OF 'T4_PK' (UNIQUE) (Cost=1 Card=1) Unfortunately, when you upgrade to 9i, the optimizer still obeys your hints and you haven t put enough of them into the SQL to deal with the new, improved options that 9i uses. Remember that query transformation takes place before the other stages of optimization. The optimizer will unnest the two subqueries in your original statement. Unfortunately, subquery unnesting seems to take place from the bottom up, and the resulting inline views are inserted from the top down in your from clause. This means the table order in the transformed query is now t4, t2, t1, t3 and then there s that ordered hint to apply! (The push_subq hint is ignored, because after 9i has finished its transformation, there are no outstanding subqueries to push.) So the new execution plan thanks to your hinting is as follows:
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