Drop an index in postgresql
You could try DROP INDEX [ CONCURRENTLY ] name


CONCURRENTLY

Drop the index without locking out concurrent selects, inserts,
updates, and deletes on the index's table. A normal DROP INDEX
acquires 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.
Here's the documentation for postgres 9.2:
http://www.postgresql.org/docs/9.2/static/sql-dropindex.html


Note: This feature is not available before postgres 9.2.


Here's the documentation for postgres 9.1:
http://www.postgresql.org/docs/9.1/static/sql-dropindex.html
This is similar to ONLINE = ON option in SQL Server:
http://technet.microsoft.com/en-us/library/ms176118.aspx