Re: Statistics mismatch between n_live_tup and actual row count

Andreas Brandl <ml@3.141592654.de> writes:
>> The planner doesn't use n_live_tup;

> I'm just curious: where does the planner take the (approximate) row-count from?

It uses the tuple density estimated by the last vacuum or analyze (viz,
reltuples/relpages) and multiplies that by the current relation size.
There are various reasons for not using n_live_tup, some historical and
some still pretty relevant.

> Might there be a link between n_live_tup drifting and doing unnecessary (blind) updates, which do not change any
informationof a row? 

Possibly.  It's premature to speculate with no test case, but I'm
wondering if HOT updates confuse that arithmetic.  No-op updates
would follow the HOT path as long as there was room on the page...

            regards, tom lane