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'\" t
.\" Title: CREATE TRANSFORM
.\" Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\" Date: 2024
.\" Manual: PostgreSQL 16.2 Documentation
.\" Source: PostgreSQL 16.2
.\" Language: English
.\"
.TH "CREATE TRANSFORM" "7" "2024" "PostgreSQL 16.2" "PostgreSQL 16.2 Documentation"
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.nh
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.\" * MAIN CONTENT STARTS HERE *
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.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))
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