1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
|
'\" t
.\" Title: CREATE TRANSFORM
.\" Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\" Date: 2024
.\" Manual: PostgreSQL 15.7 Documentation
.\" Source: PostgreSQL 15.7
.\" Language: English
.\"
.TH "CREATE TRANSFORM" "7" "2024" "PostgreSQL 15.7" "PostgreSQL 15.7 Documentation"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
CREATE_TRANSFORM \- define a new transform
.SH "SYNOPSIS"
.sp
.nf
CREATE [ OR REPLACE ] TRANSFORM FOR \fItype_name\fR LANGUAGE \fIlang_name\fR (
FROM SQL WITH FUNCTION \fIfrom_sql_function_name\fR [ (\fIargument_type\fR [, \&.\&.\&.]) ],
TO SQL WITH FUNCTION \fIto_sql_function_name\fR [ (\fIargument_type\fR [, \&.\&.\&.]) ]
);
.fi
.SH "DESCRIPTION"
.PP
\fBCREATE TRANSFORM\fR
defines a new transform\&.
\fBCREATE OR REPLACE TRANSFORM\fR
will either create a new transform, or replace an existing definition\&.
.PP
A transform specifies how to adapt a data type to a procedural language\&. For example, when writing a function in PL/Python using the
hstore
type, PL/Python has no prior knowledge how to present
hstore
values in the Python environment\&. Language implementations usually default to using the text representation, but that is inconvenient when, for example, an associative array or a list would be more appropriate\&.
.PP
A transform specifies two functions:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
A
\(lqfrom SQL\(rq
function that converts the type from the SQL environment to the language\&. This function will be invoked on the arguments of a function written in the language\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
A
\(lqto SQL\(rq
function that converts the type from the language to the SQL environment\&. This function will be invoked on the return value of a function written in the language\&.
.RE
.sp
It is not necessary to provide both of these functions\&. If one is not specified, the language\-specific default behavior will be used if necessary\&. (To prevent a transformation in a certain direction from happening at all, you could also write a transform function that always errors out\&.)
.PP
To be able to create a transform, you must own and have
USAGE
privilege on the type, have
USAGE
privilege on the language, and own and have
EXECUTE
privilege on the from\-SQL and to\-SQL functions, if specified\&.
.SH "PARAMETERS"
.PP
\fItype_name\fR
.RS 4
The name of the data type of the transform\&.
.RE
.PP
\fIlang_name\fR
.RS 4
The name of the language of the transform\&.
.RE
.PP
\fIfrom_sql_function_name\fR[(\fIargument_type\fR [, \&.\&.\&.])]
.RS 4
The name of the function for converting the type from the SQL environment to the language\&. It must take one argument of type
internal
and return type
internal\&. The actual argument will be of the type for the transform, and the function should be coded as if it were\&. (But it is not allowed to declare an SQL\-level function returning
internal
without at least one argument of type
internal\&.) The actual return value will be something specific to the language implementation\&. If no argument list is specified, the function name must be unique in its schema\&.
.RE
.PP
\fIto_sql_function_name\fR[(\fIargument_type\fR [, \&.\&.\&.])]
.RS 4
The name of the function for converting the type from the language to the SQL environment\&. It must take one argument of type
internal
and return the type that is the type for the transform\&. The actual argument value will be something specific to the language implementation\&. If no argument list is specified, the function name must be unique in its schema\&.
.RE
.SH "NOTES"
.PP
Use
\fBDROP TRANSFORM\fR
to remove transforms\&.
.SH "EXAMPLES"
.PP
To create a transform for type
hstore
and language
plpython3u, first set up the type and the language:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE TYPE hstore \&.\&.\&.;
CREATE EXTENSION plpython3u;
.fi
.if n \{\
.RE
.\}
.sp
Then create the necessary functions:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE FUNCTION hstore_to_plpython(val internal) RETURNS internal
LANGUAGE C STRICT IMMUTABLE
AS \&.\&.\&.;
CREATE FUNCTION plpython_to_hstore(val internal) RETURNS hstore
LANGUAGE C STRICT IMMUTABLE
AS \&.\&.\&.;
.fi
.if n \{\
.RE
.\}
.sp
And finally create the transform to connect them all together:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE TRANSFORM FOR hstore LANGUAGE plpython3u (
FROM SQL WITH FUNCTION hstore_to_plpython(internal),
TO SQL WITH FUNCTION plpython_to_hstore(internal)
);
.fi
.if n \{\
.RE
.\}
.sp
In practice, these commands would be wrapped up in an extension\&.
.PP
The
contrib
section contains a number of extensions that provide transforms, which can serve as real\-world examples\&.
.SH "COMPATIBILITY"
.PP
This form of
\fBCREATE TRANSFORM\fR
is a
PostgreSQL
extension\&. There is a
\fBCREATE TRANSFORM\fR
command in the
SQL
standard, but it is for adapting data types to client languages\&. That usage is not supported by
PostgreSQL\&.
.SH "SEE ALSO"
.PP
CREATE FUNCTION (\fBCREATE_FUNCTION\fR(7)),
CREATE LANGUAGE (\fBCREATE_LANGUAGE\fR(7)),
CREATE TYPE (\fBCREATE_TYPE\fR(7)),
DROP TRANSFORM (\fBDROP_TRANSFORM\fR(7))
|
