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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:
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. B-tree indexes provide the highest concurrency without deadlock conditions.
Share/exclusive index-level locks are used for read/write access. Locks are released after the command is done.
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.
In short, 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 obviously cannot be used; in that situation, application developers should be aware of the relatively poor concurrent performance of GiST and R-tree indexes.