21a39de580
btcostestimate() makes an estimate of the number of index tuples that will be visited based on knowledge of which index clauses can actually bound the scan within nbtree. However, it forgot to account for partial indexes in this calculation, with the result that the cost of the index scan could be significantly overestimated for a partial index. Fix that by merging the predicate with the abbreviated indexclause list, in the same way as we do with the full list to estimate how many heap tuples will be visited. Also, slightly increase the "fudge factor" that's meant to give preference to smaller indexes over larger ones. While this is applied to all indexes, it's most important for partial indexes since it can be the only factor that makes a partial index look cheaper than a similar full index. Experimentation shows that the existing value is so small as to easily get swamped by noise such as page-boundary-roundoff behavior. I'm tempted to kick it up more than this, but will refrain for now. Per report from Ruben Blanco. These are long-standing issues, but given the lack of prior complaints I'm not going to risk changing planner behavior in back branches by back-patching.
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