Re: "analyze" putting wrong reltuples in pg_class

Is there any way we can warn users when their fsm parameters are too
small?

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Tom Lane wrote:
> Ron Mayer <ron(at)intervideo(dot)com> writes:
> > logs2=# select pgstattuple('e_ip_full');
> > NOTICE: physical length: 293.84MB live tuples: 1697755 (169.26MB, 57.60%)
> > dead tuples: 0 (0.00MB, 0.00%) free/reusable space: 110.84MB (37.72%)
> > overhead: 4.67%
> > pgstattuple
> > -------------
> > 0
> > (1 row)
>
> 38% overhead space is awfully high. I am betting that your max_fsm_pages
> configuration parameter needs to be kicked up --- it would seem that
> your system is failing to reclaim free space effectively. (Check the
> mail list archives for recent discussions of this point.)
>
> What I think is happening is that the free space is not evenly
> distributed but is concentrated near the start of the table. This
> causes ANALYZE to make a faulty estimate of the average number of live
> tuples per page, because its initial scan will see mostly free space
> and not very many live tuples on the first few hundred pages. So it
> extrapolates a too-small estimate for the total number of tuples.
>
> It would probably be good at some point to make ANALYZE more robust,
> but your immediate problem is too much wasted space. I'd recommend
> bumping up max_fsm_pages to some reasonable fraction of your total
> database size, and then doing a VACUUM FULL to get back the space leaked
> so far.
>
> regards, tom lane
>
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