vacuum and query plans?

hello,
a bit of a theoretical question if you dont mind..
i ve seen lately more and more occurences of queries having huge planning
times (>500ms) while their execution time is very low (<10ms). at that
point, if a vacuum is executed on the table involved (without analyze) then
the planning time goes back to 'normal'. could someone elaborate on the
connection between planning time and what vacuum does? i am asking bc i
would have thought that an analyze would have resolved the issue (with
increased sampling for example) but it didnt. only vacuum does resolve it.

thank you in advance


(the version is 9.4.x)



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