CREATE_TRANSFORM(7)

1CREATE TRANSFORM(7)      PostgreSQL 12.2 Documentation     CREATE TRANSFORM(7)
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NAME

6       CREATE_TRANSFORM - define a new transform
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SYNOPSIS

9       CREATE [ OR REPLACE ] TRANSFORM FOR type_name LANGUAGE lang_name (
10           FROM SQL WITH FUNCTION from_sql_function_name [ (argument_type [, ...]) ],
11           TO SQL WITH FUNCTION to_sql_function_name [ (argument_type [, ...]) ]
12       );
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DESCRIPTION

15       CREATE TRANSFORM defines a new transform.  CREATE OR REPLACE TRANSFORM
16       will either create a new transform, or replace an existing definition.
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18       A transform specifies how to adapt a data type to a procedural
19       language. For example, when writing a function in PL/Python using the
20       hstore type, PL/Python has no prior knowledge how to present hstore
21       values in the Python environment. Language implementations usually
22       default to using the text representation, but that is inconvenient
23       when, for example, an associative array or a list would be more
24       appropriate.
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26       A transform specifies two functions:
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28       ·   A “from SQL” function that converts the type from the SQL
29           environment to the language. This function will be invoked on the
30           arguments of a function written in the language.
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32       ·   A “to SQL” function that converts the type from the language to the
33           SQL environment. This function will be invoked on the return value
34           of a function written in the language.
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36       It is not necessary to provide both of these functions. If one is not
37       specified, the language-specific default behavior will be used if
38       necessary. (To prevent a transformation in a certain direction from
39       happening at all, you could also write a transform function that always
40       errors out.)
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42       To be able to create a transform, you must own and have USAGE privilege
43       on the type, have USAGE privilege on the language, and own and have
44       EXECUTE privilege on the from-SQL and to-SQL functions, if specified.
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PARAMETERS

47       type_name
48           The name of the data type of the transform.
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50       lang_name
51           The name of the language of the transform.
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53       from_sql_function_name[(argument_type [, ...])]
54           The name of the function for converting the type from the SQL
55           environment to the language. It must take one argument of type
56           internal and return type internal. The actual argument will be of
57           the type for the transform, and the function should be coded as if
58           it were. (But it is not allowed to declare an SQL-level function
59           returning internal without at least one argument of type internal.)
60           The actual return value will be something specific to the language
61           implementation. If no argument list is specified, the function name
62           must be unique in its schema.
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64       to_sql_function_name[(argument_type [, ...])]
65           The name of the function for converting the type from the language
66           to the SQL environment. It must take one argument of type internal
67           and return the type that is the type for the transform. The actual
68           argument value will be something specific to the language
69           implementation. If no argument list is specified, the function name
70           must be unique in its schema.
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NOTES

73       Use DROP TRANSFORM (DROP_TRANSFORM(7)) to remove transforms.
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EXAMPLES

76       To create a transform for type hstore and language plpythonu, first set
77       up the type and the language:
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79           CREATE TYPE hstore ...;
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81           CREATE EXTENSION plpythonu;
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83       Then create the necessary functions:
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85           CREATE FUNCTION hstore_to_plpython(val internal) RETURNS internal
86           LANGUAGE C STRICT IMMUTABLE
87           AS ...;
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89           CREATE FUNCTION plpython_to_hstore(val internal) RETURNS hstore
90           LANGUAGE C STRICT IMMUTABLE
91           AS ...;
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93       And finally create the transform to connect them all together:
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95           CREATE TRANSFORM FOR hstore LANGUAGE plpythonu (
96               FROM SQL WITH FUNCTION hstore_to_plpython(internal),
97               TO SQL WITH FUNCTION plpython_to_hstore(internal)
98           );
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100       In practice, these commands would be wrapped up in an extension.
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102       The contrib section contains a number of extensions that provide
103       transforms, which can serve as real-world examples.
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COMPATIBILITY

106       This form of CREATE TRANSFORM is a PostgreSQL extension. There is a
107       CREATE TRANSFORM command in the SQL standard, but it is for adapting
108       data types to client languages. That usage is not supported by
109       PostgreSQL.
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