Trigger a sweep
By default, a sweep is triggered whenever the difference between the oldest interesting transaction and the oldest active exceeds 20,000. That's a settable parameter. Many installations set the interval to zero, which disables automatic sweeping. The more careful installations then schedule a manual sweep periodically when the additional overhead will be tolerable.
Note that the sweep trigger is the difference between the oldest interesting and the oldest active, not the oldest interesting and the next. Record versions created by transactions newer than the oldest active can not be garbage collected, so they don't affect the sweep.
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