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SQL Anywhere 11.0.0 » SQL Anywhere Server - SQL Usage » Query Optimizer » Query optimization and execution

 

Semantic query transformations

To operate efficiently, SQL Anywhere usually rewrites your query, possibly in several steps, into a new form. It ensures that the new version computes the same result, even though it expresses the query in a new way. In other words, SQL Anywhere rewrites your queries into semantically equivalent, but syntactically different, forms.

SQL Anywhere can perform a number of different rewrite operations. If you read the access plans, you frequently find that they do not correspond to a literal interpretation of your original statement. For example, the optimizer tries as much as possible to rewrite subqueries with joins. It is important to realize that the optimizer rewrites your SQL statements to make them more efficient.

Some of the rewrite optimizations performed during the Query Rewrite phase can be observed in the results returned by the REWRITE function. See REWRITE function [Miscellaneous].

Example

Unlike the SQL language definition, some languages mandate strict behavior for AND and OR operations. Some guarantee that the condition on the left-hand side will be evaluated first. If the truth of the entire condition can then be determined, the compiler guarantees that the condition on the right-hand side will not be evaluated.

This arrangement lets you combine conditions that would otherwise require two nested IF statements into one. For example, in C you can test whether a pointer is NULL before you use it as follows. The nested conditions in the first statement can be replaced using the syntax shown in the second statement below:

if ( X != NULL ) {
   if ( X->var != 0 ) {
      ... statements ...
   }
}
if ( X != NULL && X->var != 0 ) {
      ... statements ...
}

Unlike C, SQL has no such rules concerning execution order. SQL Anywhere is free to rearrange the order of such conditions as it sees fit. The original and reordered forms are semantically equivalent because the SQL language specification makes no distinction between one order or another. In particular, a query optimizer is completely free to reorder predicates in a WHERE, HAVING, or ON clause.


Types of semantic transformations