Re: pgstattuple: fix free space calculation

On Thu, 22 Aug 2024 at 10:11, Frédéric Yhuel <frederic(dot)yhuel(at)dalibo(dot)com>
wrote:

> Hello,
>
> I think that pgstattuple should use PageGetExactFreeSpace() instead of
> PageGetHeapFreeSpace() or PageGetFreeSpace(). The latter two compute the
> free space minus the space of a line pointer. They are used like this in
> the rest of the code (heapam.c):
>
> pagefree = PageGetHeapFreeSpace(page);
>
> if (newtupsize > pagefree) { we need a another page for the tuple }
>
> ... so it makes sense to take the line pointer into account in this
> context.
>
> But it in the pgstattuple context, I think we want the exact free space.
>
> I have attached a patch.
>
> Best regards,
> Frédéric

I agree with the approach here.
A minor comment here is to change the comments in code referring to the
PageGetHeapFreeSpace.

--- a/contrib/pgstattuple/pgstatapprox.c
+++ b/contrib/pgstattuple/pgstatapprox.c
@@ -111,7 +111,7 @@ statapprox_heap(Relation rel, output_type *stat)
* treat them as being free space for our purposes.
*/
if (!PageIsNew(page))
- stat->free_space += PageGetHeapFreeSpace(page);
+ stat->free_space += PageGetExactFreeSpace(page);
--
Regards,
Rafia Sabih