CREATE CONVERSION — define a new encoding conversion
CREATE [ DEFAULT ] CONVERSIONnameFORsource_encodingTOdest_encodingFROMfunction_name
   CREATE CONVERSION defines a new conversion between
   two character set encodings.
  
   Conversions that are marked DEFAULT can be used for
   automatic encoding conversion between client and server.  To support that
   usage, 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
    boolean   -- if true, don't throw an error if conversion fails
) RETURNS integer;
The return value is the number of source bytes that were successfully converted. If the last argument is false, the function must throw an error on invalid input, and the return value is always equal to the source string length.
   Neither the source nor the destination encoding can
   be SQL_ASCII, as the server's behavior for cases
   involving the SQL_ASCII “encoding” is
   hard-wired.
  
   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.