PostgreSQL 7.1.3 Documentation
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# 9.7. Locking and Indices

Though Postgres provides nonblocking read/write access to table data, nonblocking read/write access is not currently offered for every index access method implemented in Postgres.

The various index types are handled as follows:

GiST and R-Tree indices

  Share/exclusive index-level locks are used for read/write access. Locks are released after statement is done.

Hash indices

  Share/exclusive page-level locks are used for read/write access. Locks are released after page is processed.

  Page-level locks provide better concurrency than index-level ones but are subject to deadlocks.

Btree indices

  Short-term share/exclusive page-level locks are used for read/write access. Locks are released immediately after each index tuple is fetched/inserted.

  Btree indices provide the highest concurrency without deadlock conditions.

In short, btree indices are the recommended index type for concurrent applications.

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