[londons_explore]

For many queries, even setting X=1 would probably have big benefits. If it takes far longer than expected to find the first result, it's probably time for a new plan.

Implementing only the X=1 case would also dramatically simplify the design of such a feature. Limiting it to read only queries would also make everything much simpler.

[davidrowley]

I agree. The primary area where bad estimates bite us is estimating some path will return 1 row. When we join that Nested Loop looks like a great option. What could be faster to join to 1 row?! It just does not go well when 1 row turns into more than 1. We'd realise that the 1-row estimate we wrong by the time we got to row 2, so queuing 1 row would likely cover the majority of cases.