<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >User-defined Types</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.13 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Extending SQL" HREF="extend.html"><LINK REL="PREVIOUS" TITLE="User-defined Aggregates" HREF="xaggr.html"><LINK REL="NEXT" TITLE="User-defined Operators" HREF="xoper.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2015-06-13T20:07:41"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.13 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="User-defined Aggregates" HREF="xaggr.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="extend.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 35. Extending <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM ></TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="User-defined Operators" HREF="xoper.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="XTYPES" >35.11. User-defined Types</A ></H1 ><P > As described in <A HREF="extend-type-system.html" >Section 35.2</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 = (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. 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 values of your data type vary in size (in internal form), you should make the data type <ACRONYM CLASS="ACRONYM" >TOAST</ACRONYM >-able (see <A HREF="storage-toast.html" >Section 56.2</A >). You should do this even if the data 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 do this, 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 <CODE CLASS="FUNCTION" >SET_VARSIZE()</CODE > to store the size of the datum in this field and <CODE CLASS="FUNCTION" >VARSIZE()</CODE > to retrieve it. 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 the appropriate storage option. </P ><P > If the 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 > For further details see the description of the <A HREF="sql-createtype.html" >CREATE TYPE</A > command. </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="xaggr.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="xoper.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >User-defined Aggregates</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="extend.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >User-defined Operators</TD ></TR ></TABLE ></DIV ></BODY ></HTML >