Hint (SQL)

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.

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.
Oracle implements hints by using specially-crafted comments in the query that begin with a symbol, thus not affecting SQL compatibility.
EDB Postgres Advanced Server offers hints compatible with those of Oracle.
Microsoft SQL Server offers hints via the keyword