User-Defined Data Type Conversion

Description

There are three data type conversion modes for PostgreSQL: implicit conversion, assignment conversion, and explicit conversion. They correspond to i (Implicit), a (Assignment), and e (Explicit) in the pg_cast system catalog.

• Implicit conversion: a conversion from low bytes to high bytes of the same data type, for example, from int to bigint
• Assignment conversion: a conversion from high bytes to low bytes of the same data type, for example, from smallint to int
• Explicit conversion: a conversion between different data types

How to Use

1. Before converting data types, you can run the following command to check whether RDS for PostgreSQL supports data type conversion:

   select * from pg_catalog.pg_cast ;
   oid  | castsource | casttarget | castfunc | castcontext | castmethod 
   -------+------------+------------+----------+-------------+------------
    11277 |         20 |         21 |      714 | a           | f
    11278 |         20 |         23 |      480 | a           | f
    11279 |         20 |        700 |      652 | i           | f
    11280 |         20 |        701 |      482 | i           | f
   ......

2. Run the following command to check whether int4 can be converted to text:

   select * from pg_catalog.pg_cast where castsource = 'int4'::regtype and casttarget = 'bool'::regtype;
     oid  | castsource | casttarget | castfunc | castcontext | castmethod 
   -------+------------+------------+----------+-------------+------------
    11311 |         23 |         16 |     2557 | e           | f
   (1 row)

   The conversion is supported, and the conversion type is implicit conversion.

   If no built-in conversion functions are available, customize a conversion function to support the conversion. For details, see User-Defined Data Type Conversion.

User-Defined Data Type Conversion

• Use double colons (::) to perform a forcible conversion.

  select '10'::int,'2023-10-05'::date;
   int4 |    date    
  ------+------------
     10 | 2023-10-05
  (1 row)

• Use the CAST function to convert the type.

  select CAST('10' as int),CAST('2023-10-05' as date);
   int4 |    date    
  ------+------------
     10 | 2023-10-05
  (1 row)

• Customize a data type conversion.

  For details, see https://www.postgresql.org/docs/14/sql-createcast.html.

  

  Adding a custom type conversion will affect the existing execution plans of RDS for PostgreSQL. Therefore, customizing type conversions are not recommended.

  • Conversion between time and character types

    CREATE CAST(varchar as date) WITH INOUT AS IMPLICIT;

  • Conversion between boolean types and numeric types

    create cast(boolean as numeric) with INOUT AS IMPLICIT;

  • Conversion between numeric types and character types

    create cast(varchar as numeric) with INOUT AS IMPLICIT;

  Example: Convert text to date.

  create or replace function public.text_to_date(text) returns date as 
  $$          
    select to_date($1,'yyyy-mm-dd');  
  $$ 
  language sql strict;  
  
  create cast (text as date) with function public.text_to_date(text) as implicit;  
  
  select text '2023-09-09' + 1;
    ?column?  
  ------------
   2023-09-10
  (1 row)