Fix index-only scan plans, take 2.

Commit 4ace45677 failed to fix the problem fully, because the
same issue of attempting to fetch a non-returnable index column
can occur when rechecking the indexqual after using a lossy index
operator.  Moreover, it broke EXPLAIN for such indexquals (which
indicates a gap in our test cases :-().

Revert the code changes of 4ace45677 in favor of adding a new field
to struct IndexOnlyScan, containing a version of the indexqual that
can be executed against the index-returned tuple without using any
non-returnable columns.  (The restrictions imposed by check_index_only
guarantee this is possible, although we may have to recompute indexed
expressions.)  Support construction of that during setrefs.c
processing by marking IndexOnlyScan.indextlist entries as resjunk
if they can't be returned, rather than removing them entirely.
(We could alternatively require setrefs.c to look up the IndexOptInfo
again, but abusing resjunk this way seems like a reasonably safe way
to avoid needing to do that.)

This solution isn't great from an API-stability standpoint: if there
are any extensions out there that build IndexOnlyScan structs directly,
they'll be broken in the next minor releases.  However, only a very
invasive extension would be likely to do such a thing.  There's no
change in the Path representation, so typical planner extensions
shouldn't have a problem.

As before, back-patch to all supported branches.

Discussion: https://postgr.es/m/3179992.1641150853@sss.pgh.pa.us
Discussion: https://postgr.es/m/17350-b5bdcf476e5badbb@postgresql.org

2 files changed, 20 insertions, 6 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 3c6fecd2e1e..a9a45f566c4 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1454,7 +1454,7 @@ typedef struct IndexScanState
 /* ----------------
  *	 IndexOnlyScanState information
  *
- *		indexqual		   execution state for indexqual expressions
+ *		recheckqual		   execution state for recheckqual expressions
  *		ScanKeys		   Skey structures for index quals
  *		NumScanKeys		   number of ScanKeys
  *		OrderByKeys		   Skey structures for index ordering operators
@@ -1473,7 +1473,7 @@ typedef struct IndexScanState
 typedef struct IndexOnlyScanState
 {
 	ScanState	ss;				/* its first field is NodeTag */
-	ExprState  *indexqual;
+	ExprState  *recheckqual;
 	struct ScanKeyData *ioss_ScanKeys;
 	int			ioss_NumScanKeys;
 	struct ScanKeyData *ioss_OrderByKeys;
diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index b61a9493a1d..90f02ce6fdd 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -418,15 +418,28 @@ typedef struct IndexScan
  * index-only scan, in which the data comes from the index not the heap.
  * Because of this, *all* Vars in the plan node's targetlist, qual, and
  * index expressions reference index columns and have varno = INDEX_VAR.
- * Hence we do not need separate indexqualorig and indexorderbyorig lists,
- * since their contents would be equivalent to indexqual and indexorderby.
+ *
+ * We could almost use indexqual directly against the index's output tuple
+ * when rechecking lossy index operators, but that won't work for quals on
+ * index columns that are not retrievable.  Hence, recheckqual is needed
+ * for rechecks: it expresses the same condition as indexqual, but using
+ * only index columns that are retrievable.  (We will not generate an
+ * index-only scan if this is not possible.  An example is that if an
+ * index has table column "x" in a retrievable index column "ind1", plus
+ * an expression f(x) in a non-retrievable column "ind2", an indexable
+ * query on f(x) will use "ind2" in indexqual and f(ind1) in recheckqual.
+ * Without the "ind1" column, an index-only scan would be disallowed.)
+ *
+ * We don't currently need a recheckable equivalent of indexorderby,
+ * because we don't support lossy operators in index ORDER BY.
  *
  * To help EXPLAIN interpret the index Vars for display, we provide
  * indextlist, which represents the contents of the index as a targetlist
  * with one TLE per index column.  Vars appearing in this list reference
  * the base table, and this is the only field in the plan node that may
- * contain such Vars.  Note however that index columns that the AM can't
- * reconstruct are replaced by null Consts in indextlist.
+ * contain such Vars.  Also, for the convenience of setrefs.c, TLEs in
+ * indextlist are marked as resjunk if they correspond to columns that
+ * the index AM cannot reconstruct.
  * ----------------
  */
 typedef struct IndexOnlyScan
@@ -437,6 +450,7 @@ typedef struct IndexOnlyScan
 	List	   *indexorderby;	/* list of index ORDER BY exprs */
 	List	   *indextlist;		/* TargetEntry list describing index's cols */
 	ScanDirection indexorderdir;	/* forward or backward or don't care */
+	List	   *recheckqual;	/* index quals in recheckable form */
 } IndexOnlyScan;
 
 /* ----------------