Postgres has it, but it didn't used to, and it's still got caveats. Beyond Postgres, indexing random values is fundamentally harder than indexing sorted ones,

Postgres has it, but it didn't used to, and it's still got caveats. Beyond Postgres, indexing random values is fundamentally harder than indexing sorted ones, whether you've got a hashmap or btree or something else. The often-cited O(1) lookup complexity of a hashmap assumes everything easily fits in uniform-access memory.