Postgresql alter table column size8/9/2023 ![]() ![]() Trigger to take care of ongoing changes from the applicationsĮLSEIF (NEW.a is null and OLD.a is not null) THENĮLSEIF (NEW.a is not null and OLD.a is null) THEN It can be executed over multiple hours or days as needed.ĪLTER TABLE PRU ADD COLUMN A1 INTEGER, ADD COLUMN A1_CHANGED BOOLEAN The advantages of this method is you have more control over the process. You might have to stop your application to perform this type of long running operation.Īnother approach to change the datatype of the column could be to This exclusive lock could generate errors in the application. This method is the easiest one, but could generate high contention due to required exclusive lock for the table. SELECT * FROM pg_stat_statements WHERE query like '%optionA%' ![]() We could review stats from the command above with following query: Generate rows until 2M, by looping the following statement:ĪLTER /*optionA*/ TABLE PRU ALTER COLUMN A TYPE INTEGER USING A::INTEGER If you'd like to follow along with an example of this scenario, let's first create a table and generate data for it. Let's say we want to change the type of column A to Integer. In the second column called A we have integer data currently saved as Text type. One is a column called id with type bigserial. Suppose we have a table PRU with two columns. In fact, VARCHAR and TEXT fields are stored in the same way in Postgres.Due to performance and locking reasons, changing a datatype column using ALTER COLUMN can be a long-running operation. The size specification in VARCHAR is only used to check the size of the values which are inserted, it does not affect the disk layout. Stored in background tables so that they do not interfere with rapid Physical requirement on disk might be less. Long strings are compressed by the system automatically, so the Longer strings have 4 bytes of overhead instead of 1. Plus the actual string, which includes the space padding in the case The storage requirement for a short string (up to 126 bytes) is 1 byte Postgres, unlike some other databases, is smart enough to only use just enough space to fit the string (even using compression for longer strings), so even if your column is declared as VARCHAR(255) - if you store 40-character strings in the column, the space usage will be 40 bytes + 1 byte of overhead. THERE'S NO NEED TO RESIZE THE COLUMN IN YOUR CASE! Constraints you can change around without this table lock/rewrite problem, and they can do more integrity checking than just the weak length check. If you don't care about multi-database compatibility, consider storing your data as TEXT and add a constraint to limits its length. VARCHAR is a terrible type that exists in PostgreSQL only to comply with its associated terrible part of the SQL standard. 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. 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. If you hack a lower limit in there, that will not reduce the size of existing values at all. The length check is done when values are stored into the rows. All sorts of weird cases to be aware of here. Make sure you read the Character Types section of the docs before changing this. 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. You have to hack the database catalog data. There's a description of how to do this at Resize a column in a PostgreSQL table without changing data. ![]()
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