PostgreSQL 8.1.3 Documentation
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12.5. Locking and Indexes

Though PostgreSQL provides nonblocking read/write access to table data, nonblocking read/write access is not currently offered for every index access method implemented in PostgreSQL. The various index types are handled as follows:

B-tree and GiST indexes: Short-term share/exclusive page-level locks are used for read/write access. Locks are released immediately after each index row is fetched or inserted. These index types provide the highest concurrency without deadlock conditions.
Hash indexes: Share/exclusive hash-bucket-level locks are used for read/write access. Locks are released after the whole bucket is processed. Bucket-level locks provide better concurrency than index-level ones, but deadlock is possible since the locks are held longer than one index operation.
R-tree indexes: Share/exclusive index-level locks are used for read/write access. Locks are released after the entire command is done.

Currently, B-tree indexes offer the best performance for concurrent applications; since they also have more features than hash indexes, they are the recommended index type for concurrent applications that need to index scalar data. When dealing with non-scalar data, B-trees are not useful, and GiST indexes should be used instead. R-tree indexes are deprecated and are likely to disappear entirely in a future release.

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PostgreSQL Manual

12.5. Locking and Indexes

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