Preconditions and postconditions
The Rules Engine uses preconditions and postconditions in some optimisation techniques. These are used in corrective UPDATE constraints.
A constraint has a precondition for a given column if, in the WHERE clause of the statement, the column is compared to a constant, or to an expression evaluating to a constant, or to NULL or to NOT NULL. Clauses with subqueries and clauses containing OR are never preconditions. There may be more than one precondition: in this case the preconditions are linked by AND.
A constraint has a postcondition for a given column if that column is SET to a constant value, or to an expression evaluating to a constant, or to NULL. A SET clause setting the column to a subquery is never a postcondition. There may be several postconditions: in this case the SET clause contains two or more parts separated by commas.
Consider the following SQL statement:
In this SQL statement, the clauses:
ColA IS NULL
ColC = 'Y'
are preconditions, while the clause:
ColB IS 'N'
is a postcondition.
NOTE: In SQL, you can write more than one SET statement using commas as separators as in the example, but you can use the following construct against Oracle only:
Before corrective UPDATE constraints are evaluated, the Rules Engine looks for any preconditions and postconditions of the constraints: