Jon Nelson <jnelson+pgsql@jamponi.net> writes:
> What influences the calculation of the 'width' value in query plans?
It's generally the sum of the estimated column widths for all the
columns needed at that particular level of the plan.
> Specifically, I have two queries which both query the same set of
> tables via either UNION or UNION ALL based on the presence (or
> absence) of an aggregate function.
Hard to comment about this with such an incomplete view of the situation
--- in particular, data types would be a critical factor, and I also
wonder if you're admitting to all the columns involved.
regards, tom lane