proposal - patch: psql - sort_by_size

Hi

I returned to possibility to sort output of \d* and \l by size. There was
more a experiments in this area, but without success. Last patch was
example of over engineering, and now, I try to implement this feature
simply how it is possible. I don't think so we need too complex solution -
if somebody needs specific report, then it is not hard to run psql with
"-E" option, get and modify used query (and use a power of SQL). But
displaying databases objects sorted by size is very common case.

This proposal is based on new psql variable "SORT_BY_SIZE". This variable
will be off by default. The value of this variable is used only in verbose
mode (when the size is displayed - I don't see any benefit sort of size
without showing size). Usage is very simple and implementation too:

\dt -- sorted by schema, name
\dt+ -- still sorted by schema, name

\set SORT_BY_SIZE on
\dt -- sorted by schema, name (size is not calculated and is not visible)
\dt+ -- sorted by size

\dt+ public.* -- sorted by size from schema public

Comments, notes?

Regards

Pavel