|PostgreSQL 7.4.7 Documentation|
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The columns of the information schema views use special data types that are defined in the information schema. These are defined as simple domains over ordinary built-in types. You should not use these types for work outside the information schema, but your applications must be prepared for them if they select from the information schema.
These types are:
A nonnegative integer.
A character string (without specific maximum length).
A character string. This type is used for SQL identifiers, the type character_data is used for any other kind of text data.
A domain over the type timestamp
Every column in the information schema has one of these four types.
Boolean (true/false) data is represented in the information schema by a column of type character_data that contains either YES or NO. (The information schema was invented before the type boolean was added to the SQL standard, so this convention is necessary to keep the information schema backward compatible.)