If a schema name is given then the type is created in the specified
schema. Otherwise it is created in the current schema. The type
name must be distinct from the name of any existing type or domain
in the same schema. (Because tables have associated data types,
the type name must also be distinct from the name of any existing
table in the same schema.)
Base Types
The second form of CREATE TYPE creates a new base type
(scalar type). The parameters may appear in any order, not only that
illustrated above, and most are optional. You must register
two or more functions (using CREATE FUNCTION) before
defining the type. The support functions
input_function and
output_function
are required, while the functions
receive_function,
send_function and
analyze_function
are optional. Generally these functions have to be coded in C
or another low-level language.
The input_function
converts the type's external textual representation to the internal
representation used by the
operators and functions defined for the type.
output_function
performs the reverse transformation. The input function may be
declared as taking one argument of type cstring,
or as taking three arguments of types
cstring, oid, integer.
The first argument is the input text as a C string, the second
argument is the element type's OID in case this is an array type
(or the type's own OID for a composite type),
and the third is the typmod of the destination column, if known.
The input function should return a value of the data type itself.
The output function may be
declared as taking one argument of the new data type, or as taking
two arguments of which the second is type oid.
The second argument is again the array element type OID for array types
or the type OID for composite types.
The output function should return type cstring.
The optional receive_function
converts the type's external binary representation to the internal
representation. If this function is not supplied, the type cannot
participate in binary input. The binary representation should be
chosen to be cheap to convert to internal form, while being reasonably
portable. (For example, the standard integer data types use network
byte order as the external binary representation, while the internal
representation is in the machine's native byte order.) The receive
function should perform adequate checking to ensure that the value is
valid.
The receive function may be declared as taking one argument of type
internal, or two arguments of types internal
and oid. It must return a value of the data type itself.
(The first argument is a pointer to a StringInfo buffer
holding the received byte string; the optional second argument is the
element type OID in case this is an array type, or the type's own OID for a
composite type.) Similarly, the optional
send_function converts
from the internal representation to the external binary representation.
If this function is not supplied, the type cannot participate in binary
output. The send function may be
declared as taking one argument of the new data type, or as taking
two arguments of which the second is type oid.
The second argument is again the array element type OID for array types
or the type OID for composite types.
The send function must return type bytea.
You should at this point be wondering how the input and output functions
can be declared to have results or arguments of the new type, when they have
to be created before the new type can be created. The answer is that the
input function must be created first, then the output function (and
the binary I/O functions if wanted), and finally the data type.
EnterpriseDB will first see the name of the new
data type as the return type of the input function. It will create a
"shell" type, which is simply a placeholder entry in
the system catalog, and link the input function definition to the shell
type. Similarly the other functions will be linked to the (now already
existing) shell type. Finally, CREATE TYPE replaces the
shell entry with a complete type definition, and the new type can be used.
The optional analyze_function
performs type-specific statistics collection for columns of the data type.
By default, ANALYZE will attempt to gather statistics using
the type's "equals" and "less-than" operators, if there
is a default b-tree operator class for the type. For non-scalar types
this behavior is likely to be unsuitable, so it can be overridden by
specifying a custom analysis function. The analysis function must be
declared to take a single argument of type internal, and return
a boolean result. The detailed API for analysis functions appears
in src/include/commands/vacuum.h.
While the details of the new type's internal representation are only
known to the I/O functions and other functions you create to work with
the type, there are several properties of the internal representation
that must be declared to EnterpriseDB.
Foremost of these is
internallength.
Base data types can be fixed-length, in which case
internallength is a
positive integer, or variable length, indicated by setting
internallength
to VARIABLE. (Internally, this is represented
by setting typlen to -1.) The internal representation of all
variable-length types must start with a 4-byte integer giving the total
length of this value of the type.
The optional flag PASSEDBYVALUE indicates that
values of this data type are passed by value, rather than by
reference. You may not pass by value types whose internal
representation is larger than the size of the Datum type
(4 bytes on most machines, 8 bytes on a few).
The alignment parameter
specifies the storage alignment required for the data type. The
allowed values equate to alignment on 1, 2, 4, or 8 byte boundaries.
Note that variable-length types must have an alignment of at least
4, since they necessarily contain an int4 as their first component.
The storage parameter
allows selection of storage strategies for variable-length data
types. (Only plain is allowed for fixed-length
types.) plain specifies that data of the type
will always be stored in-line and not compressed.
extended specifies that 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
strategy 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.)
A default value may be specified, in case a user wants columns of the
data type to default to something other than the null value.
Specify the default with the DEFAULT key word.
(Such a default may be overridden by an explicit DEFAULT
clause attached to a particular column.)
To indicate that a type is an array, specify the type of the array
elements using the ELEMENT key word. For example, to
define an array of 4-byte integers (int4), specify
ELEMENT = int4. More details about array types
appear below.
To indicate the delimiter to be used between values in the external
representation of arrays of this type, delimiter can be
set to a specific character. The default delimiter is the comma
(,). Note that the delimiter is associated
with the array element type, not the array type itself.
Array Types
Whenever a user-defined base data type is created,
EnterpriseDB automatically creates an
associated array type, whose name consists of the base type's
name prepended with an underscore. The parser understands this
naming convention, and translates requests for columns of type
foo[] into requests for type _foo.
The implicitly-created array type is variable length and uses the
built-in input and output functions array_in and
array_out.
You might reasonably ask why there is an ELEMENT
option, if the system makes the correct array type automatically.
The only case where it's useful to use ELEMENT is when you are
making a fixed-length type that happens to be internally an array of a number of
identical things, and you want to allow these things to be accessed
directly by subscripting, in addition to whatever operations you plan
to provide for the type as a whole. For example, type name
allows its constituent char elements to be accessed this way.
A 2-D point type could allow its two component numbers to be
accessed like point[0] and point[1].
Note that
this facility only works for fixed-length types whose internal form
is exactly a sequence of identical fixed-length fields. A subscriptable
variable-length type must have the generalized internal representation
used by array_in and array_out.
For historical reasons (i.e., this is clearly wrong but it's far too
late to change it), subscripting of fixed-length array types starts from
zero, rather than from one as for variable-length arrays.