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CHAPTER 17 A CURE FOR MADNESS
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SELECT * FROM R1 WHERE NUM > 0 ; is defined as follows: 1. Evaluate R1. 2. Restrict the result of the previous step to just those rows satisfying NUM > 0. 3. Project the result of the previous step over all of its columns (which is effectively a no op, of course). In other words, the inner subquery must be evaluated before the outer WHERE and SELECT clauses are executed (hence my unequivocal no to the question Is this rewriting on the part of the optimizer valid ). But there s still a little more to be said.
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Part of the reason I wanted to discuss Gennick s question is that it illustrates very well my thesis that model vs. implementation is one of the great logical differences. I have argued this point in many places and on many occasions; in fact, I recently published a paper with the title On the Logical Difference Between Model and Implementation ( 5 in the present book). In most of my discussions of this topic, however, I have concentrated on the relational model specifically (contrasting it with implementations perhaps I should say would-be implementations of that model). In the case of Gennick s question, however, the relational model is involved only somewhat incidentally; rather, the model we re dealing with is a formal language definition (of the language SQL, to be precise), and the implementation is what Gennick refers to in his paper as the database engine (also known as the optimizer). In other words, Gennick s question is a language question, not a relational one.2 Any formal language definition, if it s worth its salt, will specify the precise syntax and semantics of the language in question. The pertinent portion of the SQL language definition reads as follows (I m quoting here from the SQL:1992 version of that standard): <table expression> ::= <from clause> [ <where clause> ] [ <group by clause> ] [ <having clause> ] The result of a <table expression> is a derived table in which the descriptor of the i-th column is the same as the descriptor of the i-th column of the table specified by the <from clause> ... If all optional clauses are omitted, then the result of the <table expression> is the same as the result of the <from clause>. Otherwise, each specified clause is applied to the result of the previously specified clause and the result of the <table expression> is the result of the application of the last specified clause.
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2. But see the section Further Thoughts, later.
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CHAPTER 17 A CURE FOR MADNESS
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In the case at hand, therefore (in which there is a WHERE clause but no GROUP BY or HAVING clause), we see that (a) we have the syntax correct3 and (b) the semantics are as previously stated: First execute the FROM clause, then execute the WHERE clause. That s the formal model. That s what the query means. Now, the algorithm given in the formal definition first execute the FROM clause, then execute the WHERE clause, in the case at hand is purely conceptual in nature. In effect, the standard is just saying If you execute this algorithm, then you ll get the right answer. But that if is a pretty big if ! There s no implication that the implementation (the database engine ) has to do exactly what the formal definition says. In fact, the implementation is free to use any algorithm it chooses, just so long as whatever algorithm it does use is guaranteed to give the same answer as the specified conceptual algorithm. And, of course, there are often very good reasons usually performance reasons for using a different algorithm, thereby (for example) executing clauses in a different order or otherwise rewriting the original query. But the implementation is free to do such things only if it can be proved that the algorithm used is logically equivalent to the conceptual one specified in the formal definition. In Gennick s example, however, the optimizer has clearly used an algorithm that is not logically equivalent to the conceptual algorithm specified in the SQL standard, and the implementation is incorrect.
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