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>Chapter 33. Extending <ACRONYM
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><H1
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><A
NAME="XTYPES"
>33.11. User-Defined Types</A
></H1
><A
NAME="AEN37631"
></A
><P
> As described in <A
HREF="extend-type-system.html"
>Section 33.2</A
>,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>
language. Creating a new base type requires implementing functions
to operate on the type in a low-level language, usually C.
</P
><P
> The examples in this section can be found in
<TT
CLASS="FILENAME"
>complex.sql</TT
> and <TT
CLASS="FILENAME"
>complex.c</TT
>
in the <TT
CLASS="FILENAME"
>src/tutorial</TT
> directory of the source distribution.
See the <TT
CLASS="FILENAME"
>README</TT
> file in that directory for instructions
about running the examples.
</P
><P
> <A
NAME="AEN37644"
></A
>
<A
NAME="AEN37646"
></A
>
A user-defined type must always have input and output
functions.<A
NAME="AEN37648"
></A
><A
NAME="AEN37651"
></A
>
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.
</P
><P
> Suppose we want to define a type <TT
CLASS="TYPE"
>complex</TT
> that represents
complex numbers. A natural way to represent a complex number in
memory would be the following C structure:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct Complex {
double x;
double y;
} Complex;</PRE
><P>
We will need to make this a pass-by-reference type, since it's too
large to fit into a single <TT
CLASS="TYPE"
>Datum</TT
> value.
</P
><P
> As the external string representation of the type, we choose a
string of the form <TT
CLASS="LITERAL"
>(x,y)</TT
>.
</P
><P
> 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:
</P><PRE
CLASS="PROGRAMLISTING"
>PG_FUNCTION_INFO_V1(complex_in);
Datum
complex_in(PG_FUNCTION_ARGS)
{
char *str = PG_GETARG_CSTRING(0);
double x,
y;
Complex *result;
if (sscanf(str, " ( %lf , %lf )", &x, &y) != 2)
ereport(ERROR,
(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
errmsg("invalid input syntax for complex: \"%s\"",
str)));
result = (Complex *) palloc(sizeof(Complex));
result->x = x;
result->y = y;
PG_RETURN_POINTER(result);
}</PRE
><P>
The output function can simply be:
</P><PRE
CLASS="PROGRAMLISTING"
>PG_FUNCTION_INFO_V1(complex_out);
Datum
complex_out(PG_FUNCTION_ARGS)
{
Complex *complex = (Complex *) PG_GETARG_POINTER(0);
char *result;
result = (char *) palloc(100);
snprintf(result, 100, "(%g,%g)", complex->x, complex->y);
PG_RETURN_CSTRING(result);
}</PRE
><P>
</P
><P
> 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.
</P
><P
> 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
<TT
CLASS="TYPE"
>complex</TT
>, we will piggy-back on the binary I/O converters
for type <TT
CLASS="TYPE"
>float8</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>PG_FUNCTION_INFO_V1(complex_recv);
Datum
complex_recv(PG_FUNCTION_ARGS)
{
StringInfo buf = (StringInfo) PG_GETARG_POINTER(0);
Complex *result;
result = (Complex *) palloc(sizeof(Complex));
result->x = pq_getmsgfloat8(buf);
result->y = pq_getmsgfloat8(buf);
PG_RETURN_POINTER(result);
}
PG_FUNCTION_INFO_V1(complex_send);
Datum
complex_send(PG_FUNCTION_ARGS)
{
Complex *complex = (Complex *) PG_GETARG_POINTER(0);
StringInfoData buf;
pq_begintypsend(&buf);
pq_sendfloat8(&buf, complex->x);
pq_sendfloat8(&buf, complex->y);
PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
}</PRE
><P>
</P
><P
> Once we have written the I/O functions and compiled them into a shared
library, we can define the <TT
CLASS="TYPE"
>complex</TT
> type in SQL.
First we declare it as a shell type:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TYPE complex;</PRE
><P>
This serves as a placeholder that allows us to reference the type while
defining its I/O functions. Now we can define the I/O functions:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION complex_in(cstring)
RETURNS complex
AS '<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>'
LANGUAGE C IMMUTABLE STRICT;
CREATE FUNCTION complex_out(complex)
RETURNS cstring
AS '<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>'
LANGUAGE C IMMUTABLE STRICT;
CREATE FUNCTION complex_recv(internal)
RETURNS complex
AS '<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>'
LANGUAGE C IMMUTABLE STRICT;
CREATE FUNCTION complex_send(complex)
RETURNS bytea
AS '<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>'
LANGUAGE C IMMUTABLE STRICT;</PRE
><P>
</P
><P
> Finally, we can provide the full definition of the data type:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TYPE complex (
internallength = 16,
input = complex_in,
output = complex_out,
receive = complex_recv,
send = complex_send,
alignment = double
);</PRE
><P>
</P
><P
> When you define a new base type,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> automatically provides support
for arrays of that
type.<A
NAME="AEN37680"
></A
> For historical reasons, the array type
has the same name as the base type with the underscore character
(<TT
CLASS="LITERAL"
>_</TT
>) prepended.
</P
><P
> 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.
</P
><P
> <A
NAME="AEN37686"
></A
>
If the values of your data type might exceed a few hundred bytes in
size (in internal form), you should make the data type
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able (see <A
HREF="storage-toast.html"
>Section 52.2</A
>).
To do this, the internal
representation must follow the standard layout for variable-length
data: the first four bytes must be an <TT
CLASS="TYPE"
>int32</TT
> 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 <CODE
CLASS="FUNCTION"
>PG_DETOAST_DATUM</CODE
>.
(This detail is customarily hidden by defining type-specific
<CODE
CLASS="FUNCTION"
>GETARG</CODE
> macros.) Then,
when running the <TT
CLASS="COMMAND"
>CREATE TYPE</TT
> command, specify the
internal length as <TT
CLASS="LITERAL"
>variable</TT
> and select the appropriate
storage option.
</P
><P
> For further details see the description of the
<A
HREF="sql-createtype.html"
><I
>CREATE TYPE</I
></A
> command.
</P
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