Fix index-only scan plans when not all index columns can be returned.

If an index has both returnable and non-returnable columns, and one of
the non-returnable columns is an expression using a Var that is in a
returnable column, then a query returning that expression could result
in an index-only scan plan that attempts to read the non-returnable
column, instead of recomputing the expression from the returnable
column as intended.

To fix, redefine the "indextlist" list of an IndexOnlyScan plan node
as containing null Consts in place of any non-returnable columns.
This solves the problem by preventing setrefs.c from falsely matching
to such entries.  The executor is happy since it only cares about the
exposed types of the entries, and ruleutils.c doesn't care because a
correct plan won't reference those entries.  I considered some other
ways to prevent setrefs.c from doing the wrong thing, but this way
seems good since (a) it allows a very localized fix, (b) it makes
the indextlist structure more compact in many cases, and (c) the
indextlist is now a more faithful representation of what the index AM
will actually produce, viz. nulls for any non-returnable columns.

This is easier to hit since we introduced included columns, but it's
possible to construct failing examples without that, as per the
added regression test.  Hence, back-patch to all supported branches.

Per bug #17350 from Louis Jachiet.

Discussion: https://postgr.es/m/17350-b5bdcf476e5badbb@postgresql.org

src/test/regress/expected/gist.out

@@ -312,6 +312,34 @@ and p <@ box(point(5,5), point(6, 6));
 (11 rows)
 
 drop index gist_tbl_multi_index;
+-- Test that we don't try to return the value of a non-returnable
+-- column in an index-only scan.  (This isn't GIST-specific, but
+-- it only applies to index AMs that can return some columns and not
+-- others, so GIST with appropriate opclasses is a convenient test case.)
+create index gist_tbl_multi_index on gist_tbl using gist (circle(p,1), p);
+explain (verbose, costs off)
+select circle(p,1) from gist_tbl
+where p <@ box(point(5, 5), point(5.3, 5.3));
+                          QUERY PLAN                           
+---------------------------------------------------------------
+ Index Only Scan using gist_tbl_multi_index on public.gist_tbl
+   Output: circle(p, '1'::double precision)
+   Index Cond: (gist_tbl.p <@ '(5.3,5.3),(5,5)'::box)
+(3 rows)
+
+select circle(p,1) from gist_tbl
+where p <@ box(point(5, 5), point(5.3, 5.3));
+     circle      
+-----------------
+ <(5,5),1>
+ <(5.05,5.05),1>
+ <(5.1,5.1),1>
+ <(5.15,5.15),1>
+ <(5.2,5.2),1>
+ <(5.25,5.25),1>
+ <(5.3,5.3),1>
+(7 rows)
+
 -- Clean up
 reset enable_seqscan;
 reset enable_bitmapscan;

src/test/regress/sql/gist.sql

@@ -142,6 +142,17 @@ and p <@ box(point(5,5), point(6, 6));
 
 drop index gist_tbl_multi_index;
 
+-- Test that we don't try to return the value of a non-returnable
+-- column in an index-only scan.  (This isn't GIST-specific, but
+-- it only applies to index AMs that can return some columns and not
+-- others, so GIST with appropriate opclasses is a convenient test case.)
+create index gist_tbl_multi_index on gist_tbl using gist (circle(p,1), p);
+explain (verbose, costs off)
+select circle(p,1) from gist_tbl
+where p <@ box(point(5, 5), point(5.3, 5.3));
+select circle(p,1) from gist_tbl
+where p <@ box(point(5, 5), point(5.3, 5.3));
+
 -- Clean up
 reset enable_seqscan;
 reset enable_bitmapscan;