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>Chapter 36. Extending <ACRONYM
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="XTYPES"
>36.11. User-defined Types</A
></H1
><P
> As described in <A
HREF="extend-type-system.html"
>Section 36.2</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 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.
</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 = psprintf("(%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. The array type typically
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
> If the internal representation of the data type is variable-length, the
internal representation must follow the standard layout for variable-length
data: the first four bytes must be a <TT
CLASS="TYPE"
>char[4]</TT
> field which is
never accessed directly (customarily named <TT
CLASS="STRUCTFIELD"
>vl_len_</TT
>). You
must use the <CODE
CLASS="FUNCTION"
>SET_VARSIZE()</CODE
> macro to store the total
size of the datum (including the length field itself) in this field
and <CODE
CLASS="FUNCTION"
>VARSIZE()</CODE
> to retrieve it. (These macros exist
because the length field may be encoded depending on platform.)
</P
><P
> For further details see the description of the
<A
HREF="sql-createtype.html"
>CREATE TYPE</A
> command.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="XTYPES-TOAST"
>36.11.1. TOAST Considerations</A
></H2
><P
> If the values of your data type vary in size (in internal form), it's
usually desirable to make the data type <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able (see <A
HREF="storage-toast.html"
>Section 65.2</A
>). You should do this even if the values are always
too small to be compressed or stored externally, because
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> can save space on small data too, by reducing header
overhead.
</P
><P
> To support <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> storage, the C functions operating on the data
type must always 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_DATATYPE_P</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 some appropriate storage
option other than <TT
CLASS="LITERAL"
>plain</TT
>.
</P
><P
> If data alignment is unimportant (either just for a specific function or
because the data type specifies byte alignment anyway) then it's possible
to avoid some of the overhead of <CODE
CLASS="FUNCTION"
>PG_DETOAST_DATUM</CODE
>. You can use
<CODE
CLASS="FUNCTION"
>PG_DETOAST_DATUM_PACKED</CODE
> instead (customarily hidden by
defining a <CODE
CLASS="FUNCTION"
>GETARG_DATATYPE_PP</CODE
> macro) and using the macros
<CODE
CLASS="FUNCTION"
>VARSIZE_ANY_EXHDR</CODE
> and <CODE
CLASS="FUNCTION"
>VARDATA_ANY</CODE
> to access
a potentially-packed datum.
Again, the data returned by these macros is not aligned even if the data
type definition specifies an alignment. If the alignment is important you
must go through the regular <CODE
CLASS="FUNCTION"
>PG_DETOAST_DATUM</CODE
> interface.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Older code frequently declares <TT
CLASS="STRUCTFIELD"
>vl_len_</TT
> as an
<TT
CLASS="TYPE"
>int32</TT
> field instead of <TT
CLASS="TYPE"
>char[4]</TT
>. This is OK as long as
the struct definition has other fields that have at least <TT
CLASS="TYPE"
>int32</TT
>
alignment. But it is dangerous to use such a struct definition when
working with a potentially unaligned datum; the compiler may take it as
license to assume the datum actually is aligned, leading to core dumps on
architectures that are strict about alignment.
</P
></BLOCKQUOTE
></DIV
><P
> Another feature that's enabled by <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> support is the
possibility of having an <I
CLASS="FIRSTTERM"
>expanded</I
> in-memory data
representation that is more convenient to work with than the format that
is stored on disk. The regular or <SPAN
CLASS="QUOTE"
>"flat"</SPAN
> varlena storage format
is ultimately just a blob of bytes; it cannot for example contain
pointers, since it may get copied to other locations in memory.
For complex data types, the flat format may be quite expensive to work
with, so <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides a way to <SPAN
CLASS="QUOTE"
>"expand"</SPAN
>
the flat format into a representation that is more suited to computation,
and then pass that format in-memory between functions of the data type.
</P
><P
> To use expanded storage, a data type must define an expanded format that
follows the rules given in <TT
CLASS="FILENAME"
>src/include/utils/expandeddatum.h</TT
>,
and provide functions to <SPAN
CLASS="QUOTE"
>"expand"</SPAN
> a flat varlena value into
expanded format and <SPAN
CLASS="QUOTE"
>"flatten"</SPAN
> the expanded format back to the
regular varlena representation. Then ensure that all C functions for
the data type can accept either representation, possibly by converting
one into the other immediately upon receipt. This does not require fixing
all existing functions for the data type at once, because the standard
<CODE
CLASS="FUNCTION"
>PG_DETOAST_DATUM</CODE
> macro is defined to convert expanded inputs
into regular flat format. Therefore, existing functions that work with
the flat varlena format will continue to work, though slightly
inefficiently, with expanded inputs; they need not be converted until and
unless better performance is important.
</P
><P
> C functions that know how to work with an expanded representation
typically fall into two categories: those that can only handle expanded
format, and those that can handle either expanded or flat varlena inputs.
The former are easier to write but may be less efficient overall, because
converting a flat input to expanded form for use by a single function may
cost more than is saved by operating on the expanded format.
When only expanded format need be handled, conversion of flat inputs to
expanded form can be hidden inside an argument-fetching macro, so that
the function appears no more complex than one working with traditional
varlena input.
To handle both types of input, write an argument-fetching function that
will detoast external, short-header, and compressed varlena inputs, but
not expanded inputs. Such a function can be defined as returning a
pointer to a union of the flat varlena format and the expanded format.
Callers can use the <CODE
CLASS="FUNCTION"
>VARATT_IS_EXPANDED_HEADER()</CODE
> macro to
determine which format they received.
</P
><P
> The <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> infrastructure not only allows regular varlena
values to be distinguished from expanded values, but also
distinguishes <SPAN
CLASS="QUOTE"
>"read-write"</SPAN
> and <SPAN
CLASS="QUOTE"
>"read-only"</SPAN
> pointers to
expanded values. C functions that only need to examine an expanded
value, or will only change it in safe and non-semantically-visible ways,
need not care which type of pointer they receive. C functions that
produce a modified version of an input value are allowed to modify an
expanded input value in-place if they receive a read-write pointer, but
must not modify the input if they receive a read-only pointer; in that
case they have to copy the value first, producing a new value to modify.
A C function that has constructed a new expanded value should always
return a read-write pointer to it. Also, a C function that is modifying
a read-write expanded value in-place should take care to leave the value
in a sane state if it fails partway through.
</P
><P
> For examples of working with expanded values, see the standard array
infrastructure, particularly
<TT
CLASS="FILENAME"
>src/backend/utils/adt/array_expanded.c</TT
>.
</P
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