How to change VARCHAR type to DATETIME using ALTER in Postgresql?

Question

How can I change VARCHAR() type to DATETIME using ALTER in Postgresql? And my table column has already data like: "2013-12-08 16:09:07 "

Answer 1

You want the USING clause to ALTER TABLE ... ALTER COLUMN ... TYPE , and the to_timestamp function .

ALTER TABLE mytable 
  ALTER COLUMN thecolumn 
   TYPE TIMESTAMP WITH TIME ZONE 
     USING to_timestamp(thecolumn, 'YYYY-MM-DD HH24:MI:SS');

In this case as the data looks like it's already a valid timestamp you can probably simplify it with a cast instead:

ALTER TABLE mytable 
  ALTER COLUMN thecolumn 
   TYPE TIMESTAMP WITH TIME ZONE 
     USING to_timestamp(thecolumn::timestamp with time zone);

You will note that I've used the type name "timestamp with time zone" instead of "datetime". That's because in PostgreSQL, datetime is just an alias for timestamp without time zone ... but in most cases you actually want to use timestamp with time zone instead. To learn more about timestamps, see the manual .