It's not about indexing, though most casual users don't know that FKs are not indexed automatically. Postgres should be smart enough to know when an index is ap

It's not about indexing, though most casual users don't know that FKs are not indexed automatically. Postgres should be smart enough to know when an index is appropriate for a foreign key, but I respect the fact that the core team wants people to make explicit well-reasoned decisions to index instead of doing it automatically. Regardless of indexing, what actually matters is cardinality estimates. Foreign keys are almost always a reasonable hint to the SQL engine that there will be a large number of joins along foreign keys, and thus should be able to provide cross-table cardinality estimates.