One-Shot Plans

Currently, the planner and executor are mostly independent of each
other: the planner doesn't really know when the plan will be executed,
and the executor doesn't know how recently the plan was made.

We can work out the various paths through the traffic cop to see when
a plan will be a "one-shot" - planned and then executed immediately,
then discarded.

In those cases we can take advantage of better optimisations. Most
interestingly, we can evaluate stable functions at plan time, to allow
us to handle partitioning and partial indexes better.

Patch attached. Works...

SET constraint_exclusion = on;
ALTER TABLE <table> ADD CHECK (dt < current_date - 5);
SELECT * FROM <table> WHERE datecolumn >= current_date - 1;
                QUERY PLAN
------------------------------------------
 Result  (cost=0.00..0.01 rows=1 width=0)
   One-Time Filter: false
(2 rows)

WIP in the sense that we might want to change the special case
parameter handling as well.

Comments?

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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