Question

I’ve a legacy postgres db that has date columns cast as character(50) fields (don’t ask). I’d like to alter the table and columns to contain actual dates. Because this worked:

select distinct to_date(date_begin, 'YYYY DD MM') from dates;

I naively thought this might work:

alter table dates alter column date_begin type character
using to_date(date_begin, 'YYYY DD MM');

But it does not. Any clues for the clueless?

Answer 1

This just works as intended by the OP. What we have here is a simple thinko/typo.
Read more in the manual about ALTER TABLE.
Demo:

-- DROP SCHEMA x CASCADE;
CREATE SCHEMA x;
CREATE TABLE x.tbl(date_begin character(50));
INSERT INTO x.tbl VALUES ('2011-11-11 11:11'), (NULL), (''), ('1977');
-- NULL and empty string work too
-- even just YYYY works: '1977' .. is converted to '1977-01-01' automatically
-- empty string produce a possibly surprising result: '0001-01-01 BC'

ALTER TABLE x.tbl ALTER COLUMN date_begin TYPE date USING to_date(date_begin, 'YYYY DD MM');

SELECT * FROM x.tbl;

Hint: You wrote type character instead of type date.