Bruce Momjian wrote:
Looking at the patch I see:

+       /*
+        * Mark the page as clear of prunable tuples. If we find a tuple which
+        * may become prunable, we shall set the hint again.
+        */
+       PageClearPrunable(page);

I like the idea of the page hint bit, but my question is if there is a
long-running transaction, isn't each SELECT going to try to defragment a
page over and over again because there is still something prunable on
the page?

Maybe that risk could be lowered if instead of a flag, we stored the
minimal global xmin needed to prune at least one tuple.

greetings, Florian Pflug

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