Post feedback changes - thanks to all who commented!
Mark Kirkwood wrote:
> I wanted to understand how the planner 'knows' how many rows are likely
> to be emitted in a given stage of a query, and wrote down some examples
> for my own benefit - I then wondered if this would be a good addition to
> the 'Performance Tips' chapter. So... err here it is.
>
> Comments welcome.
>