PostgreSQL UNIQUE制約の削除

これらは全部ダメ

×　ALTER TABLE [table_name] ALTER COLUMN [column_name] DROP INDEX [index_name];
×　ALTER TABLE [table_name] ALTER COLUMN [column_name] DROP UNIQUE;
×　ALTER TABLE [table_name] DROP INDEX [index_name];
×　DROP INDEX [index_name];

これが正解

○　ALTER TABLE [table_name] DROP CONSTRAINT [column_name];