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In database query operations, various SQL implementations use hints as additions to the SQL standard that instruct the database engine on how to execute the query. For example, a hint may tell the engine to use as little memory as possible (even if the query will run slowly), or to use or not to use an index (even if the query optimizer would decide otherwise).

Implementation

Different database engines use different approaches in implementing hints.

• MySQL uses its own extension to the SQL standard, where a table name may be followed by USE INDEX, FORCE INDEX or IGNORE INDEX keywords.
• Oracle implements hints by using specially-crafted comments in the query that begin with a + symbol, thus not affecting SQL compatibility.
• Postgres Plus® Advanced Server (a proprietary version of PostgreSQL from EnterpriseDB) offers hints compatible with those of Oracle.

See also

• Query optimizer
• Query plan

References

1. MySQL 5.5 Reference Manual: 12.2.9.3 Index Hint Syntax
2. Mike Ault: Oracle SQL Hints Tuning
3. Postgres Plus Advanced Server Performance and Scalability Guide: Query Optimization Hints
4. Postgres Plus Advanced Server Oracle Compatibility Developer's Guide: Optimizer Hints

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