Indexing every WHERE column is not PostgreSQL optimization

One PostgreSQL indexing mistake I see often: “The query filters on A, B and C, so let’s create an index on A, B, C.” That may work, but it may also be the wrong index. For composite B-tree indexes, PostgreSQL cares about predicate type, column order, selectivity, table size, and the actual execution plan. In this post, I explain why equality predicates usually belong before range predicates, why n_distinct from statistics matters, and why a theoretically good index is useless if the planner neve