Though Postgres provides unblocking read/write access to table data, unblocked read/write access is not provided for every index access methods implemented in Postgres.
The various index types are handled as follows:
Share/exclusive index-level locks are used for read/write access. Locks are released after statement is done.
Share/exclusive page-level locks are used for read/write access. Locks are released after page is processed.
Page-level locks produces better concurrency than index-level ones but are subject to deadlocks.
Short-term share/exclusive page-level latches are used for read/write access. Latches are released immediately after the index tuple is inserted/fetched.
Btree indices provide the highest concurrency without deadlock conditions.