![]() The USING clause specifies an expression that allows you to convert the old values to the new ones. PostgreSQL allows you to convert the values of a column to the new ones while changing its data type by adding a USING clause as follows: ALTER TABLE table_nameĪLTER COLUMN column_name TYPE new_data_type USING expression Ĭode language: SQL (Structured Query Language) ( sql ) In this syntax, you add a comma ( ,) after each ALTER COLUMN clause. Code language: SQL (Structured Query Language) ( sql ) To change the data types of multiple columns in a single statement, you use multiple ALTER COLUMN clauses like this: ALTER TABLE table_nameĪLTER COLUMN column_name1 TYPE new_data_type,ĪLTER COLUMN column_name2 TYPE new_data_type, ![]() The SET DATA TYPE and TYPE are equivalent. Third, supply the new data type for the column after the TYPE keyword.Second, specify the name of the column that you want to change the data type after the ALTER COLUMN clause.First, specify the name of the table to which the column you want to change after the ALTER TABLE keywords. ![]() Let’s examine the statement in a greater detail: To change the data type of a column, you use the ALTER TABLE statement as follows: ALTER TABLE table_name ALTER COLUMN column_name TYPE new_data_type Code language: CSS ( css ) In fact, VARCHAR and TEXT fields are stored in the same way in Postgres.Summary: this tutorial shows you step by step how to change the data type of a column by using the ALTER TABLE statement. 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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