Rearrange the implementation of index-only scans.

This commit changes index-only scans so that data is read directly from the
index tuple without first generating a faux heap tuple.  The only immediate
benefit is that indexes on system columns (such as OID) can be used in
index-only scans, but this is necessary infrastructure if we are ever to
support index-only scans on expression indexes.  The executor is now ready
for that, though the planner still needs substantial work to recognize
the possibility.

To do this, Vars in index-only plan nodes have to refer to index columns
not heap columns.  I introduced a new special varno, INDEX_VAR, to mark
such Vars to avoid confusion.  (In passing, this commit renames the two
existing special varnos to OUTER_VAR and INNER_VAR.)  This allows
ruleutils.c to handle them with logic similar to what we use for subplan
reference Vars.

Since index-only scans are now fundamentally different from regular
indexscans so far as their expression subtrees are concerned, I also chose
to change them to have their own plan node type (and hence, their own
executor source file).

1 files changed, 2 insertions, 2 deletions

diff --git a/src/backend/executor/nodeNestloop.c b/src/backend/executor/nodeNestloop.c
index 49b880d0caf..d6433c7f537 100644
--- a/src/backend/executor/nodeNestloop.c
+++ b/src/backend/executor/nodeNestloop.c
@@ -147,8 +147,8 @@ ExecNestLoop(NestLoopState *node)
 				ParamExecData *prm;
 
 				prm = &(econtext->ecxt_param_exec_vals[paramno]);
-				/* Param value should be an OUTER var */
-				Assert(nlp->paramval->varno == OUTER);
+				/* Param value should be an OUTER_VAR var */
+				Assert(nlp->paramval->varno == OUTER_VAR);
 				Assert(nlp->paramval->varattno > 0);
 				prm->value = slot_getattr(outerTupleSlot,
 										  nlp->paramval->varattno,