Hint (SQL) explained

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

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, or keywords.^[1]
• Oracle implements hints by using specially-crafted comments in the query that begin with a symbol, thus not affecting SQL compatibility.^[2]
• EDB Postgres Advanced Server (a proprietary version of PostgreSQL from EnterpriseDB) offers hints compatible with those of Oracle.^{[3] [4]}
• Microsoft SQL Server offers hints via the keyword ^[5]

See also

• Query optimizer
• Query plan

Notes and References

1. MySQL 5.5 Reference Manual: 12.2.9.3 Index Hint Syntax
2. Mike Ault: Oracle SQL Hints Tuning
3. Web site: Postgres Plus Advanced Server Performance and Scalability Guide: Query Optimization Hints . 2013-06-24 . 2014-04-19 . https://web.archive.org/web/20140419011842/http://www.enterprisedb.com/docs/en/9.2/perffeat/Postgres_Plus_Advanced_Server_Performance_Guide-33.htm . dead .
4. Web site: Postgres Plus Advanced Server Oracle Compatibility Developer's Guide: Optimizer Hints . 2013-06-24 . https://web.archive.org/web/20140419012438/http://www.enterprisedb.com/docs/en/9.2/oracompat/Postgres_Plus_Advanced_Server_Oracle_Compatibility_Guide-75.htm . 2014-04-19 . dead .
5. https://docs.microsoft.com/en-us/sql/t-sql/queries/hints-transact-sql-query?view=sql-server-2017 MSSQL 2017 Documentation