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CHAPTER 9 QUERY TRANSFORMATION
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the implementation of this outer join approach, except it discards an outer row the moment it finds a match (thus avoiding redundant joins to the inner table).
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Anti-join Anomaly
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In much the same way that the optimizer can transform in subqueries to exists subqueries, it can also transform not in subqueries into not exists subqueries. But sometimes it won t, for no apparent reason. Compare the following two queries given the definition of the tables (see book_subq.sql in the online code suite) they are logically equivalent: select from where book_key books NOT EXISTS ( select from where ) ; select from where book_key books book_key NOT IN ( select from ) null sales sales.book_key = books.book_key
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book_key sales
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; In 10g the optimizer will use a hash anti-join to execute the first query. Moreover,10g will transform the second query into the first query and do a hash anti-join on that as well. On the other hand, 9i behaves differently and in a totally counterintuitive way. It will transform the second query into the first query, and execute a hash anti-join. But it won t perform a hash anti-join on the first query unless it is hinted (and the hash anti-join is much cheaper than the filter operation that appears by default). You may recall in the table of optimizer parameters earlier on that we saw a 9i parameter called _unnest_notexists_sq, with a default value of single. Strangely, this query seems to be described perfectly by that parameter but the single existence subquery is not unnested.
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ANTI-JOIN BUGS IN 9I
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There are several bugs listed on MetaLink relating to anti-joins in 9i, typically reported as fixed in 10.2. Generally these bugs are of the wrong results category, so be a little cautious about execution plans that report antijoins. It s worth getting a list of the current bugs with their rediscovery information just in case.
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The moral of this story is that the optimizer can do all sorts of clever transformations between different ways of writing queries so it s a nice idea to write the query in a way that makes sense to you as the first approach; but sometimes a transformation that you might
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CHAPTER 9 QUERY TRANSFORMATION
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expect simply doesn t happen so sometimes you have to be prepared to write your queries in a way that helps the optimizer along a bit.
Nulls and Not In
We still have to address the issue of nulls blocking the anti-join (and associated) transformations. There are two ways of understanding the issue. The first is the informal observation that the anti-join is approximately equivalent to the outer join show in the previous section but the outer join does an equality comparison between the dept_no columns of the emp and dept tables, and nulls always produce problems when you start comparing them in any way other than is null or is not null. The second, slightly more formal, approach is to note the comment in the SQL Reference manual that colX not in ('A', 'B', 'C') is equivalent to colX != 'A' and colX != 'B' and colX != 'C' The string of ANDs means that every one of the individual conditions in the list must be checked and must evaluate to true for the entire expression to evaluate to true. If a single condition evaluates to false or null, then the expression is not true. As an example of the errors that can occur if you forget this, script notin.sql in the online code suite sets up a case with the following results: select * from t1 where n1 = 99; N1 V1 ---------- -------------------99 Ninety-nine 1 row selected. select * from t2 where n1 = 99; no rows selected select from where * t1 t1.n1 not in ( select t2.n1 from t2 )
; no rows selected The first query shows that there is a row in table t1 with n1 = 99. The second query shows that there are no corresponding rows with n1 = 99 in table t2. The last query shows that there
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