On 12/27/2014 12:16 AM, Alvaro Herrera wrote:
> Tom Lane wrote:
>
>> The argument that autovac workers need fresher stats than anything else
>> seems pretty dubious to start with. Why shouldn't we simplify that down
>> to "they use PGSTAT_STAT_INTERVAL like everybody else"?
>
> The point of wanting fresher stats than that, eons ago, was to avoid a
> worker vacuuming a table that some other worker vacuumed more recently
> than PGSTAT_STAT_INTERVAL. I realize now that the semantics we really
> want was something like "stats no older than XYZ" where the given value
> is the timestamp at which we start checking; if we get anything newer
> than that it would be okay, but we currently reject it because of lack
> of a more appropriate API. (If it takes more than PGSTAT_STAT_INTERVAL
> to get the stats back, a regular backend would ask for fresher stats,
> but to an autovac worker they would be good enough as long as they are
> newer than its recheck start time.)
>
> Nowadays we can probably disregard the whole issue, since starting a new
> vacuum just after the prior one finished should not cause much stress to
> the system thanks to the visibility map.
Vacuuming is far from free, even if the visibility map says that most
pages are visible to all: you still scan all indexes, if you remove any
dead tuples at all.
- Heikki
