![]() But that also means disk space is not reclaimed, so in order to do that you need to trigger a full table rewrite. When you remove a column from a table, PostgreSQL will automatically remove all of its indexes and. This is an extension of SQL, which disallows zero-column tables. ALTER TABLE DROP COLUMN can be used to drop the only column of a table, leaving a zero-column table. the ability to specify more than one manipulation in a single ALTER TABLE command is an extension. ![]() PostgreSQL way of dropping column is really fast because it involves a catalog update. ALTER TABLE tablename DROP COLUMN columnname. The other forms are PostgreSQL extensions of the SQL standard. Indexes and table constraints involving the column will be automatically dropped as well. DROP COLUMN IF EXISTS This form drops a column from a table. There are several subforms: ADD COLUMN This form adds a new column to the table, using the same syntax as CREATE TABLE. ALTER TABLE usertbl ADD deleteflg character. Description ALTER TABLE changes the definition of an existing table. ![]() However, this approach has a potential drawback: being dropped attributes mentioned in pg_attribute as normal ones, they do count as table attributes and therefore could lower the number of real active attributes you can have in the table. ALTER TABLE usertbl ADD deleteflg character varying(1) NOT NULL. In this particular case it would have been dropped quite easily also from pg_attribute, but imagine a more complex tble where you drop a column in the middle of the attribute list: PostgreSQL would also have to rewrite all the attribute ordering with a quite expensive amount of work. | t ( 2 rows )Īccording to the time spent in VACUUM something good must be happened, and in fact the table space was reduced to the right (or better, the original) amount of space.īut why the dropped column is still mentioned in pg_attribute? attnum > 0 attnum | attname | attisdropped -|-|- 1 | i | f 2 |. If a storage engine does not support an attempted ALTER TABLE operation, a warning may result. Testdb => VACUUM FULL foo VACUUM Time : 20231, 232 ms ( 00 : 20, 231 ) testdb => SELECT pg_size_pretty ( pg_relation_size ( 'foo' ) ) pg_size_pretty - 346 MB ( 1 row ) Time : 1, 519 ms testdb => SELECT attnum, attname, attisdropped FROM pg_attribute a JOIN pg_class c ON c. ALTER TABLE t2 DROP COLUMN c, DROP COLUMN d.
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