"at least for WHERE conditions that are selective" confuses me. Aren't _all_ WHERE clauses selective?
From earlier in the email, selectivity is a scale, the wording here implies "has a meaningful selectivity".
"Fundamentally yes, but you also have to consider how selective the conditions are." - Laurenz
where true - selectivity 0
where not(is_deleted) - selectivity < .01 ish in many cases
Selectivity is evaluating cardinality with an eye on the frequency of the values you are actually going to be filtering on. So low cardinality booleans can be highly selective in usage if you are looking for the rare false in a sea of trues but low selectivity if looking through those trues.
