I have a question about the ALTER TABLE command on a really large table (almost 30 millions rows).
One of its columns is a varchar(255) and I would like to resize it to a varchar(40).
Basically, I would like to change my column by running the following command:
ALTER TABLE mytable ALTER COLUMN mycolumn TYPE varchar(40);
I have no problem if the process is very long but it seems my table is no more readable during the ALTER TABLE command.
Is there a smarter way? Maybe add a new column, copy values from the old column, drop the old column and finally rename the new one?
Note: I use PostgreSQL 9.0.
There's a description of how to do this at Resize a column in a PostgreSQL table without changing data. You have to hack the database catalog data. The only way to do this officially is with ALTER TABLE, and as you've noted that change will lock and rewrite the entire table while it's running.
Make sure you read the Character Types section of the docs before changing this. All sorts of weird cases to be aware of here. The length check is done when values are stored into the rows. If you hack a lower limit in there, that will not reduce the size of existing values at all. You would be wise to do a scan over the whole table looking for rows where the length of the field is >40 characters after making the change. You'll need to figure out how to truncate those manually--so you're back some locks just on oversize ones--because if someone tries to update anything on that row it's going to reject it as too big now, at the point it goes to store the new version of the row. Hilarity ensues for the user.
VARCHAR is a terrible type that exists in PostgreSQL only to comply with its associated terrible part of the SQL standard. If you don't care about multi-database compatibility, consider storing your data as TEXT and add a constraint to limits its length. Constraints you can change around without this table lock/rewrite problem, and they can do more integrity checking than just the weak length check.