Most of what you say I agree with. But if this outer query is run only once with version A there is a caveat with where it says “where id = X”.

This cannot match more than one X at a time. So that forces the inner query to be run once, as we can only have one id, and running it twice may produce 2.

I am not sure though to be honest.

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We are too deep for me to reply now, but to your next comment I didn't mean only one row, but only one id. It is easy for a small difference in word choice to get things wrong.

I think if it did return two rows; ie limit 2- the query with = will fail. Hmmm maybe that will happen even with limit 1 under certain plans. I wouldn’t trust it.