I wrote:
> I'm not sure if the costing change is a bug or not --- the non-bitmap scan
> does seem to be cheaper in reality, but not by a couple orders of
> magnitude as the planner now thinks.

Ah, scratch that, I wasn't looking closely enough.  The 9.4 plan is an
IndexScan whereas 9.5+ uses IndexOnlyScan, which accounts for the cost
differential.  The reason it changed is the remove_unused_subquery_outputs
patch (55d5b3c08279b487), which allows the subquery to know that it
doesn't actually have to return all columns of tenk1, so that an
index-only scan is legal.

                        regards, tom lane

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