SQL statements can, intentionally or not, require the mixing of different data types in the same expression. PostgreSQL has extensive facilities for evaluating mixed-type expressions.
In many cases a user does not need to understand the details of the type conversion mechanism. However, implicit conversions done by PostgreSQL can affect the results of a query. When necessary, these results can be tailored by using explicit type conversion.
This chapter introduces the PostgreSQL type conversion mechanisms and conventions. Refer to the relevant sections in Глава 8 and Глава 9 for more information on specific data types and allowed functions and operators.