There is no implicit (automatic) cast from text
or varchar
to integer
(i.e. you cannot pass a varchar
to a function expecting integer
or assign a varchar
field to an integer
one), so you must specify an explicit cast using ALTER TABLE ... ALTER COLUMN ... TYPE ... USING:
ALTER TABLE the_table ALTER COLUMN col_name TYPE integer USING (col_name::integer);
Note that you may have whitespace in your text fields; in that case, use:
ALTER TABLE the_table ALTER COLUMN col_name TYPE integer USING (trim(col_name)::integer);
to strip white space before converting.
This shoud've been obvious from an error message if the command was run in psql
, but it's possible PgAdmin-III isn't showing you the full error. Here's what happens if I test it in psql
on PostgreSQL 9.2:
=> CREATE TABLE test( x varchar );
CREATE TABLE
=> insert into test(x) values ('14'), (' 42 ');
INSERT 0 2
=> ALTER TABLE test ALTER COLUMN x TYPE integer;
ERROR: column "x" cannot be cast automatically to type integer
HINT: Specify a USING expression to perform the conversion.
=> ALTER TABLE test ALTER COLUMN x TYPE integer USING (trim(x)::integer);
ALTER TABLE
Thanks @muistooshort for adding the USING
link.
See also this related question; it's about Rails migrations, but the underlying cause is the same and the answer applies.
If the error still occurs, then it may be related not to column values, but indexes over this column or column default values might fail typecast. Indexes need to be dropped before ALTER COLUMN and recreated after. Default values should be changed appropriately.