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Message Posted: Fri, 27 Jun 2003 @ 22:09:09 GMT

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Strange though they may seem at first, Explains are a great tool for analyzing plans. They tell you just what the amps are doing. For join steps, what type of join is being used, what the "geography" of the input tables are, what the join terms are, what single-table terms may be applied in the step and how many rows are expected to be produced by the join.

There is some information on explains in chapter 4 of the Performance Optimization manual.

The step you were asking about is a deadlock prevention mechanism. What it does is force all lock request for a given table to go to a single amp. When you don't see steps like this for a table, it means the lock requests for that table will all be access locks (access locks never block other lock requests so no deadlock prevention is required).

Locks are placed at the beginning of a plan and released at the very end as part of end transaction processing.

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