Nested CASE-WHEN scoping

While looking at fixing the multiple-evaluation issue in IN and BETWEEN 
discussed a while ago, I realized that the current assumption that only 
one CaseTestExpr placeholder needs to be valid at any given time is not 
true.

Here's a bit contrived example:

CREATE FUNCTION evileq (timestamptz, int4) returns boolean AS $$ SELECT case $2 WHEN length($1::text) THEN true ELSE
falseEND;
 
$$ language sql;
CREATE OPERATOR = (procedure = evileq, leftarg = timestamptz, rightarg = 
int4);

postgres=#  SELECT now() = 29, CASE now() WHEN 29 THEN 'foo' ELSE 'bar' END; ?column? | case
----------+------ t        | bar
(1 row)

Direct call to the operator, "now () = 29" returns true, but when used 
in CASE-WHEN, which implicitly does the same comparison, the result is 
false. Admittedly that's pretty far-fetched, but nevertheless it's a bug.

As part of the BETWEEN/IN fix, I was going to refactor CaseTestExpr and 
CoerceToDomainValue placeholder node types into one generic placeholder 
node. BETWEEN needs three placeholder slots in the worst case [*], and 
now it seems that we need to handle an arbitrary number of simultaneous 
placeholders even for CASE-WHEN.

So I'm going to put the BETWEEN/IN fix aside for now, and refactor the 
placeholder infrastructure to handle several simultaneous placeholders, 
and replace CaseTestExpr and CoerceToDomainValue with it. Actually 
AggRef and WindowFunc nodes look a lot like CaseTestExpr and 
CoerceToDomainValue too, but I'm a bit scared of touching those.

PS. This is all 9.2 material, in case you're wondering. We're talking 
about pretty big patches.

[*] a BETWEEN SYMMETRIC b AND c is handled as "(a <= b AND a >= c) OR (a >= b AND a <= c)", leading to multiple
evaluationof all three operands 
 
if placeholders are not used

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com