Re: planstats.sgml

On 16/02/16 12:46, David G. Johnston wrote:
> On Mon, Feb 15, 2016 at 4:23 PM, Tatsuo Ishii <ishii@postgresql.org
> <mailto:ishii@postgresql.org>>wrote:
>
>     While reading planstats.sgml, I encounted a sentence which I don't
>     understand.
>
>          These numbers are current as of the last <command>VACUUM</> or
>          <command>ANALYZE</> on the table.  The planner then fetches the
>          actual current number of pages in the table (this is a cheap
>     operation,
>          not requiring a table scan).  If that is different from
>          <structfield>relpages</structfield> then
>          <structfield>reltuples</structfield> is scaled accordingly to
>          arrive at a current number-of-rows estimate.  In this case the
>     value of
>          <structfield>relpages</structfield> is up-to-date so the rows
>     estimate is
>          the same as <structfield>reltuples</structfield>.
>
>     I don't understand the last sentence (In this case...). For me it
>     seems it is talking about the case when replages is not different from
>     what the planner fetches from the table. If so, why "In this case"?
>     Isn't "In this case" referrers to the previous sentence (If that is
>     different from...)? Maybe "In this case" should be "Otherwise" or some
>     such?
>
>
> ​The whole sentence is awkward but you are correct in your reading - and
> "otherwise" would be a solid choice.
>

A long while ago when I wrote that, I was expressing the fact that *in 
the example* the numbers matched perfectly, but *in general* the planner 
would scale 'em. It still reads that way to me...but change away if you 
like :-)

> Though iIt seems the whole thing could be simplified to:
>
> These numbers are current as of the last VACUUM or ANALYZE on the
> table.  To account for subsequent changes the planner obtains the actual
> page count for the table and scales pg_class.reltuples by a factor of
> "actual page count" over pg_class.relpages.
>
> The "cheap operation" comment seems gratuitous though could still be
> injected if desired.
>

Well folk interested in performance, like to keep an eye on whether 
these sort of probes cost a lot...

regards

Mark