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).
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.
- MSSQL Microsoft SQL Server - a good study guide on this topic could be found here : 
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