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Concurrency support (http://issues.apache.org/jira/browse/HIVE-1293) is a must in databases and their use cases are well understood. At a minimum, we want to support concurrent readers and writers whenever possible.
It would be useful to add a mechanism to discover the current locks which have been acquired. There is no immediate requirement to add an API to explicitly acquire any locks, so all locks would be acquired implicitly.

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For some operations, locks are hierarchical in nature - for eg. for some partition operations, the table
is also locked (for eg. to make sure that the table cannot be dropped while a new partition is being created).

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