![]() ![]() ![]() Alternatively, you can add constraints later (see below) after you've filled in the new column correctly. Keep in mind however that the default value must satisfy the given constraints, or the ADD will fail. In fact all the options that can be applied to a column description in CREATE TABLE can be used here. PostgreSQL ALTER TABLE is also used to rename a table. You can also define constraints on the column at the same time, using the usual syntax:ĪLTER TABLE products ADD COLUMN description text CHECK (description '') postgresql postgresql-9.2 alter-table Share Improve this question Follow asked at 11:16 Andrius 19. PostgreSQL ALTER TABLE statement is used to add, modify, or clear / delete columns in a table. To avoid a potentially lengthy update operation, particularly if you intend to fill the column with mostly nondefault values anyway, it may be preferable to add the column with no default, insert the correct values using UPDATE, and then add any desired default as described below. However, if the default value is volatile (e.g., clock_timestamp()) each row will need to be updated with the value calculated at the time ALTER TABLE is executed. Instead, the default value will be returned the next time the row is accessed, and applied when the table is rewritten, making the ALTER TABLE very fast even on large tables. In fact, VARCHAR and TEXT fields are stored in the same way in Postgres.From PostgreSQL 11, adding a column with a constant default value no longer means that each row of the table needs to be updated when the ALTER TABLE statement is executed. 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. There are many alterable things in postgreSQL heavily discussed in the PostgreSQL Documentation. while leaving the rest of the data untouched. This command enables the user to modify a specific aspect of the table, database, group, etc. Long strings are compressed by the system automatically, so the In SQL, tables, databases, schemas, groups, indexes, servers, and more can be modified using the ALTER command. 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. CREATE OR REPLACE FUNCTION archivedatasets () RETURNS VOID AS BODY DECLARE tablerec record BEGIN FOR tablerec IN SELECT t.tablename FROM informationschema.tables AS t WHERE tableschema 'import' AND quoteident (t.tablename) IN ( SELECT 'tablename' FROM datasets WHERE status IN ('Processed', 'Archived', 'Deleted') ) LOOP. 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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