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In various SQL implementations, a hint is an addition to the SQL standard that instructs the database engine on how to execute the query. For example, a hint may tell the engine to use or not to use an index (even if the query optimizer would decide otherwise).

• Implementation

• See also

• References

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.cite-ref-1[1]

• Oracle implements hints by using specially-crafted comments in the query that begin with a + symbol, thus not affecting SQL compatibility.cite-ref-2[2]

• EDB Postgres Advanced Server (a proprietary version of PostgreSQL from EnterpriseDB) offers hints compatible with those of Oracle.cite-ref-3[3]cite-ref-4[4]

• Microsoft SQL Server offers hints via the OPTION keyword cite-ref-5[5]

• Query optimizer

• Query plan

cite-note-11. ↑ MySQL 5.5 Reference Manual: 12.2.9.3 Index Hint Syntax

cite-note-22. ↑ Mike Ault: Oracle SQL Hints Tuning

cite-note-33. ↑ "Postgres Plus Advanced Server Performance and Scalability Guide: Query Optimization Hints". Archived from the original on 2014-04-19. Retrieved 2013-06-24.

cite-note-44. ↑ "Postgres Plus Advanced Server Oracle Compatibility Developer's Guide: Optimizer Hints". Archived from the original on 2014-04-19. Retrieved 2013-06-24.

cite-note-55. ↑ MSSQL 2017 Documentation