LIKELY/UNLIKELY

Usage Notes

SQL normally assumes that terms in the WHERE clause that cannot be used by indices are usually true. If this assumption is incorrect, it could lead to a suboptimal query plan. Use the LIKELY(X) and UNLIKELY(X) SQL functions to provide hints to the query planner about clause terms that are probably not true, which helps the query planner to select the best possible plan.

Use LIKELY/UNLIKELY to optimize evaluation of OR/AND logical expressions. LIKELY/UNLIKELY causes the left side of an expression to be evaluated first. This allows the right side of the query to be skipped when possible. For example, in the clause UNLIKELY(A) AND B, if A evaluates to FALSE, B does not need to be evaluated.

Consider the following:

SELECT COUNT(*) FROM test WHERE UNLIKELY(x IN (7, 8, 9, 10)) AND y > 42;

If x is one of the values 7, 8, 9, or 10, the filter y > 42 is applied. If x is not one of those values, the filter y > 42 is not applied.

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