In database query operations, various SQL

SQL

SQL is a programming language designed for managing data in relational database management systems ....

implementations use hints as additions to the SQL standard that instruct the database engine

Database engine

A database engine is the underlying software component that a database management system uses to create, read, update and delete data from a database....

 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

Index (database)

A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of slower writes and increased storage space...

 (even if the query optimizer

Query optimizer

The query optimizer is the component of a database management system that attempts to determine the most efficient way to execute a query. The optimizer considers the possible query plans for a given input query, and attempts to determine which of those plans will be the most efficient...

 would decide otherwise).

Implementation

Different database engines use different approaches in implementing hints.

• MySQL
  MySQL
  MySQL officially, but also commonly "My Sequel") is a relational database management system that runs as a server providing multi-user access to a number of databases. It is named after developer Michael Widenius' daughter, My...
  
   uses its own extension to the SQL standard, where a table
  Table (database)
  In relational databases and flat file databases, a table is a set of data elements that is organized using a model of vertical columns and horizontal rows. A table has a specified number of columns, but can have any number of rows...
  
   name may be followed by USE INDEX, FORCE INDEX or IGNORE INDEX keywords.
• Oracle
  Oracle Database
  The Oracle Database is an object-relational database management system produced and marketed by Oracle Corporation....
  
   implements hints by using specially-crafted comments in the query that begin with a + symbol, thus not affecting SQL compatibility.