I think it would be really nice to have a "strict mode" (for app testing), where PostgreSQL returns an error if an index would improve the query asymptotically

I think it would be really nice to have a "strict mode" (for app testing), where PostgreSQL returns an error if an index would improve the query asymptotically and it doesn't exist (only based on the query itself, not statistics). And a "CREATE INDICES FOR <sql>" command to create the indices (for app upgrades), plus an automatic index creation mode (for interactive and development use). In general, the system should be architected so that asymptotically suboptimal execution never happens.