RFD: Discarded tuple count for SeqScan nodes in EXPLAIN ANALYZE

Hi,

When analyzing the plan of a query I often find myself questioning 
whether an additional index may be sensible, or if it is sensible that a 
SeqScan is used if an index is available.

The current EXPLAIN ANALYZE only shows the number of tuples matching the 
qualifier of an SeqScan Node - for analyzing the above situation it is 
at least equally interesting how many tuples were read and discarded.

Therefore I produced a patch which adds a 'discarded=%f' part to the 
analyze output.
As this is only a RFD the implementation is a bit hackish at the moment 
- the discarded counter is increased in execScan directly instead of a 
helper routine in instrument.c.
Also the discarded count is displayed in other node types as well - for 
some there might be a sensible semantic meaning to it...

Good idea - Bad idea?

Greetings,

Andres