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.
- GIN 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. But note that a GIN-indexed
value insertion usually produces several index key insertions
per row, so GIN may do substantial work for a single value's
insertion.
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 or GIN indexes should
be used instead.
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