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JQLs support an additional timeout:
A JQL returning N issues will build N hierarchies by using pulses of work foe each root issue. This creates a sequence of pulses that might take a long while. This makes the thread might also take a lot of work (minutes) to be completed. The JQL timeout is checked every time between the start and the end of the sequences of pulses. If the timeout is reached (W), the JQL will by atomátically aborted freeing the thread. In that way, Jira administrators can avoid too long working threads.
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