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Don Burleson Blog 







ordered_predicates tips

Oracle Database Tips by Donald BurlesonMarch 1,  2016

Question: What does the ordered_predicates hint do?

Answer:  The ordered_predicates hint is specified in the WHERE clause of a query and is used to specify the order in which Boolean predicates should be evaluated.

Also, see my notes on the related ordered hint.

 In the absence of ordered_predicates, Oracle uses the following steps to evaluate the order of SQL predicates:

  1. Subqueries are evaluated before the outer Boolean conditions in the WHERE clause.
  2. All Boolean conditions without built-in functions or subqueries are evaluated in reverse from the order they are found in the WHERE clause, with the last predicate being evaluated first.
  3. Boolean predicates with built-in functions of each predicate are evaluated in increasing order of their estimated evaluation costs.

You can override these default evaluation rules using the ordered_predicates hint, and your WHERE clause items will then be evaluated in the order that they appear in the query.

The ordered_predicates hint is commonly used in cases where a PL/SQL function is used in the WHERE clause of a query. It is also very useful in cases where you know the most restrictive predicates and you want Oracle to evaluate these first.

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