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31.11. User-Defined Types
As described in Section 31.2, PostgreSQL can be extended to support new data types. This section describes how to define new base types, which are data types defined below the level of the SQL language. Creating a new base type requires implementing functions to operate on the type in a low-level language, usually C.
The examples in this section can be found in complex.sql and complex.c in the src/tutorial directory of the source distribution. See the README file in that directory for instructions about running the examples.
A user-defined type must always have input and output functions. These functions determine how the type appears in strings (for input by the user and output to the user) and how the type is organized in memory. The input function takes a null-terminated character string as its argument and returns the internal (in memory) representation of the type. The output function takes the internal representation of the type as argument and returns a null-terminated character string. If we want to do anything more with the type than merely store it, we must provide additional functions to implement whatever operations we'd like to have for the type.
Suppose we want to define a type complex that represents complex numbers. A natural way to represent a complex number in memory would be the following C structure:
We will need to make this a pass-by-reference type, since it's too large to fit into a single Datum value.
As the external string representation of the type, we choose a string of the form (x,y).
The input and output functions are usually not hard to write, especially the output function. But when defining the external string representation of the type, remember that you must eventually write a complete and robust parser for that representation as your input function. For instance:
The output function can simply be:
You should be careful to make the input and output functions inverses of each other. If you do not, you will have severe problems when you need to dump your data into a file and then read it back in. This is a particularly common problem when floating-point numbers are involved.
Optionally, a user-defined type can provide binary input and output routines. Binary I/O is normally faster but less portable than textual I/O. As with textual I/O, it is up to you to define exactly what the external binary representation is. Most of the built-in data types try to provide a machine-independent binary representation. For complex, we will piggy-back on the binary I/O converters for type float8:
To define the complex type, we need to create the user-defined I/O functions before creating the type:
Notice that the declarations of the input and output functions must reference the not-yet-defined type. This is allowed, but will draw warning messages that may be ignored. The input function must appear first.
Finally, we can declare the data type:
When you define a new base type, PostgreSQL automatically provides support for arrays of that type. For historical reasons, the array type has the same name as the base type with the underscore character (_) prepended.
Once the data type exists, we can declare additional functions to provide useful operations on the data type. Operators can then be defined atop the functions, and if needed, operator classes can be created to support indexing of the data type. These additional layers are discussed in following sections.
If the values of your data type might exceed a few hundred bytes in size (in internal form), you should make the data type TOAST-able (see Section 49.2). To do this, the internal representation must follow the standard layout for variable-length data: the first four bytes must be an int32 containing the total length in bytes of the datum (including itself). The C functions operating on the data type must be careful to unpack any toasted values they are handed, by using PG_DETOAST_DATUM. (This detail is customarily hidden by defining type-specific GETARG macros.) Then, when running the CREATE TYPE command, specify the internal length as variable and select the appropriate storage option.
For further details see the description of the CREATE TYPE command.
