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To provide some bias against using a large index when a small one would do as well, genericcostestimate adds a "fudge factor", which for a long time was random_page_cost * index_pages/10000. However, this can grow to be the dominant term in indexscan cost estimates when the index involved is large enough, a behavior that was never intended. Change to a ln(1 + n/10000) formulation, which has nearly the same behavior up to a few hundred pages but tails off significantly thereafter. (A log curve seems correct on first principles, since what we're trying to account for here is index descent costs, which are typically logarithmic.) Per bug #7619 from Niko Kiirala. Possibly this change should get back-patched, but I'm hesitant to mess with cost estimates in stable branches.
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