CREATE CONVERSION — define a new encoding conversion
CREATE [ DEFAULT ] CONVERSIONnameFORsource_encodingTOdest_encodingFROMfunction_name
   CREATE CONVERSION defines a new conversion between
   character set encodings.  Also, conversions that
   are marked DEFAULT can be used for automatic encoding
   conversion between
   client and server. For this purpose, two conversions, from encoding A to
   B and from encoding B to A, must be defined.
 
   To be able to create a conversion, you must have EXECUTE privilege
   on the function and CREATE privilege on the destination schema.
  
DEFAULT       The DEFAULT clause indicates that this conversion
       is the default for this particular source to destination
       encoding. There should be only one default encoding in a schema
       for the encoding pair.
      
nameThe name of the conversion. The conversion name can be schema-qualified. If it is not, the conversion is defined in the current schema. The conversion name must be unique within a schema.
source_encodingThe source encoding name.
dest_encodingThe destination encoding name.
function_nameThe function used to perform the conversion. The function name can be schema-qualified. If it is not, the function will be looked up in the path.
The function must have the following signature:
conv_proc(
    integer,  -- source encoding ID
    integer,  -- destination encoding ID
    cstring,  -- source string (null terminated C string)
    internal, -- destination (fill with a null terminated C string)
    integer   -- source string length
) RETURNS void;   Use DROP CONVERSION to remove user-defined conversions.
  
The privileges required to create a conversion might be changed in a future release.
   To create a conversion from encoding UTF8 to
   LATIN1 using myfunc:
CREATE CONVERSION myconv FOR 'UTF8' TO 'LATIN1' FROM myfunc;
    CREATE CONVERSION
    is a PostgreSQL extension.
    There is no CREATE CONVERSION
    statement in the SQL standard, but a CREATE TRANSLATION
    statement that is very similar in purpose and syntax.