PostgreSQL

Drop the index without locking out concurrent selects, inserts, updates, and deletes on the index’s table. A normal DROP INDEX acquires an ACCESS EXCLUSIVE lock on the table, blocking other accesses until the index drop can be completed. With this option, the command instead waits until conflicting transactions have completed. + There are several caveats to be aware of when using this option. Only one index name can be specified, and the CASCADE option is not supported. (Thus, an index that supports a UNIQUE or PRIMARY KEY constraint cannot be dropped this way.) Also, regular DROP INDEX commands can be performed within a transaction block, but DROP INDEX CONCURRENTLY cannot. + For temporary tables, DROP INDEX is always non-concurrent, as no other session can access them, and non-concurrent index drop is cheaper.

Do not throw an error if the index does not exist. A notice is issued in this case.

The name (optionally schema-qualified) of an index to remove.

Automatically drop objects that depend on the index, and in turn all objects that depend on those objects (see Section 5.13).

Refuse to drop the index if any objects depend on it. This is the default.