[boomer918]

Partial indexes can flip query plans if the covered part becomes so small that it won't be represented when sampled by the stats collector. The planner could then decide that the index scan isn't worth it and could try an alternative less efficient index if one exists.

[tbrock]

Yeah and sadly using the index in those scenarios could be even more worth it due to the high selectivity it has.

Is PG smart enough to avoid that if the query patterns are frequently or exclusively covered by the index?