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.
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.