Does Oracle use short-circuit evaluation?

Question

I have an Oracle query that is structured as followed:

SELECT   *
FROM     table
WHERE    X=\'true\' OR
         Y IN (complicated subquery)

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Answer 1

Caveat: Oracle is not my primary area of expertise.

The cost-based optimizer should know that the cost of X = 'true' is less than the sub-query, so it will likely evaluate the simpler alternative first. But the AND and OR conditions in SQL are not short-circuited like && and || are in C and its derivatives.

The sub-query can be one of two forms: correlated and non-correlated.

• A correlated sub-query must be executed many times (which is why they are dangerous to performance) because the correlation means that the sub-query result depends in some way on the row 'currently being evaluated').
• A non-correlated sub-query will only be executed once.

Example correlated sub-query:

SELECT *
  FROM Table1
 WHERE X = 'true'
    OR Y IN (SELECT Z FROM Table2 WHERE Table2.A = Table1.B)

Example non-correlated sub-query:

SELECT *
  FROM Table1
 WHERE X = 'true'
    OR Y IN (SELECT Z FROM Table2 WHERE Table2.A > 13)