CREATE TYPE

Description

CREATE TYPE allows the user to register a new user data type with Postgres for use in the current data base. The user who defines a type becomes its owner. typename is the name of the new type and must be unique within the types defined for this database.

CREATE TYPE requires the registration of two functions (using create function) before defining the type. The representation of a new base type is determined by input_function, which converts the type's external representation to an internal representation usable by the operators and functions defined for the type. Naturally, output_function performs the reverse transformation. Both the input and output functions must be declared to take one or two arguments of type "opaque".

New base data types can be fixed length, in which case internallength is a positive integer, or variable length, in which case Postgres assumes that the new type has the same format as the Postgres-supplied data type, "text". To indicate that a type is variable length, set internallength to VARIABLE. The external representation is similarly specified using the externallength keyword.

To indicate that a type is an array and to indicate that a type has array elements, indicate the type of the array element using the element keyword. For example, to define an array of 4-byte integers ("int4"), specify

ELEMENT = int4

To indicate the delimiter to be used on arrays of this type, delimiter can be set to a specific character. The default delimiter is the comma (",").

A default value is optionally available in case a user wants some specific bit pattern to mean "data not present." Specify the default with the DEFAULT keyword.

The optional arguments send_function and receive_function are used when the application program requesting Postgres services resides on a different machine. In this case, the machine on which Postgres runs may use a format for the data type different from that used on the remote machine. In this case it is appropriate to convert data items to a standard form when sending from the server to the client and converting from the standard format to the machine specific format when the server receives the data from the client. If these functions are not specified, then it is assumed that the internal format of the type is acceptable on all relevant machine architectures. For example, single characters do not have to be converted if passed from a Sun-4 to a DECstation, but many other types do.

The optional flag, PASSEDBYVALUE, indicates that operators and functions which use this data type should be passed an argument by value rather than by reference. Note that you may not pass by value types whose internal representation is more than four bytes.

The storage keyword allows selection of storage strategies for variable-length data types (only plain is allowed for fixed-length types). plain disables TOAST for the data type: it will always be stored in-line and not compressed. extended gives full TOAST capability: the system will first try to compress a long data value, and will move the value out of the main table row if it's still too long. external allows the value to be moved out of the main table, but the system will not try to compress it. main allows compression, but discourages moving the value out of the main table. (Data items with this storage method may still be moved out of the main table if there is no other way to make a row fit, but they will be kept in the main table preferentially over extended and external items.)

For new base types, a user can define operators, functions and aggregates using the appropriate facilities described in this section.

Examples

This command creates the box data type and then uses the type in a table definition:

CREATE TYPE box (INTERNALLENGTH = 8,
    INPUT = my_procedure_1, OUTPUT = my_procedure_2);
CREATE TABLE myboxes (id INT4, description box);
   

This command creates a variable length array type with integer elements:

CREATE TYPE int4array (INPUT = array_in, OUTPUT = array_out,
    INTERNALLENGTH = VARIABLE, ELEMENT = int4);
CREATE TABLE myarrays (id int4, numbers int4array);
   

This command creates a large object type and uses it in a table definition:

CREATE TYPE bigobj (INPUT = lo_filein, OUTPUT = lo_fileout,
    INTERNALLENGTH = VARIABLE);
CREATE TABLE big_objs (id int4, obj bigobj);
   

Compatibility