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

src/backend/commands/explain.c

@@ -1746,7 +1746,7 @@ ExplainNode(PlanState *planstate, List *ancestors,
 		case T_IndexOnlyScan:
 			show_scan_qual(((IndexOnlyScan *) plan)->indexqual,
 						   "Index Cond", planstate, ancestors, es);
-			if (((IndexOnlyScan *) plan)->indexqual)
+			if (((IndexOnlyScan *) plan)->recheckqual)
 				show_instrumentation_count("Rows Removed by Index Recheck", 2,
 										   planstate, es);
 			show_scan_qual(((IndexOnlyScan *) plan)->indexorderby,

src/backend/executor/nodeIndexonlyscan.c

@@ -214,13 +214,11 @@ IndexOnlyNext(IndexOnlyScanState *node)
 
 		/*
 		 * If the index was lossy, we have to recheck the index quals.
-		 * (Currently, this can never happen, but we should support the case
-		 * for possible future use, eg with GiST indexes.)
 		 */
 		if (scandesc->xs_recheck)
 		{
 			econtext->ecxt_scantuple = slot;
-			if (!ExecQualAndReset(node->indexqual, econtext))
+			if (!ExecQualAndReset(node->recheckqual, econtext))
 			{
 				/* Fails recheck, so drop it and loop back for another */
 				InstrCountFiltered2(node, 1);
@@ -555,8 +553,8 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 	 */
 	indexstate->ss.ps.qual =
 		ExecInitQual(node->scan.plan.qual, (PlanState *) indexstate);
-	indexstate->indexqual =
+	indexstate->recheckqual =
-		ExecInitQual(node->indexqual, (PlanState *) indexstate);
+		ExecInitQual(node->recheckqual, (PlanState *) indexstate);
 
 	/*
 	 * If we are just doing EXPLAIN (ie, aren't going to run the plan), stop

src/backend/nodes/copyfuncs.c

@@ -519,6 +519,7 @@ _copyIndexOnlyScan(const IndexOnlyScan *from)
 	 */
 	COPY_SCALAR_FIELD(indexid);
 	COPY_NODE_FIELD(indexqual);
+	COPY_NODE_FIELD(recheckqual);
 	COPY_NODE_FIELD(indexorderby);
 	COPY_NODE_FIELD(indextlist);
 	COPY_SCALAR_FIELD(indexorderdir);

src/backend/nodes/outfuncs.c

@@ -580,6 +580,7 @@ _outIndexOnlyScan(StringInfo str, const IndexOnlyScan *node)
 
 	WRITE_OID_FIELD(indexid);
 	WRITE_NODE_FIELD(indexqual);
+	WRITE_NODE_FIELD(recheckqual);
 	WRITE_NODE_FIELD(indexorderby);
 	WRITE_NODE_FIELD(indextlist);
 	WRITE_ENUM_FIELD(indexorderdir, ScanDirection);

src/backend/nodes/readfuncs.c

@@ -1884,6 +1884,7 @@ _readIndexOnlyScan(void)
 
 	READ_OID_FIELD(indexid);
 	READ_NODE_FIELD(indexqual);
+	READ_NODE_FIELD(recheckqual);
 	READ_NODE_FIELD(indexorderby);
 	READ_NODE_FIELD(indextlist);
 	READ_ENUM_FIELD(indexorderdir, ScanDirection);

src/backend/optimizer/plan/createplan.c

@@ -20,7 +20,6 @@
 #include <math.h>
 
 #include "access/sysattr.h"
-#include "catalog/pg_am.h"
 #include "catalog/pg_class.h"
 #include "foreign/fdwapi.h"
 #include "miscadmin.h"
@@ -189,10 +188,10 @@ static IndexScan *make_indexscan(List *qptlist, List *qpqual, Index scanrelid,
 								 ScanDirection indexscandir);
 static IndexOnlyScan *make_indexonlyscan(List *qptlist, List *qpqual,
 										 Index scanrelid, Oid indexid,
-										 List *indexqual, List *indexorderby,
+										 List *indexqual, List *recheckqual,
+										 List *indexorderby,
 										 List *indextlist,
 										 ScanDirection indexscandir);
-static List *make_indexonly_tlist(IndexOptInfo *indexinfo);
 static BitmapIndexScan *make_bitmap_indexscan(Index scanrelid, Oid indexid,
 											  List *indexqual,
 											  List *indexqualorig);
@@ -623,7 +622,7 @@ create_scan_plan(PlannerInfo *root, Path *best_path, int flags)
 		if (best_path->pathtype == T_IndexOnlyScan)
 		{
 			/* For index-only scan, the preferred tlist is the index's */
-			tlist = copyObject(make_indexonly_tlist(((IndexPath *) best_path)->indexinfo));
+			tlist = copyObject(((IndexPath *) best_path)->indexinfo->indextlist);
 
 			/*
 			 * Transfer sortgroupref data to the replacement tlist, if
@@ -2934,7 +2933,8 @@ create_indexscan_plan(PlannerInfo *root,
 	List	   *indexclauses = best_path->indexclauses;
 	List	   *indexorderbys = best_path->indexorderbys;
 	Index		baserelid = best_path->path.parent->relid;
-	Oid			indexoid = best_path->indexinfo->indexoid;
+	IndexOptInfo *indexinfo = best_path->indexinfo;
+	Oid			indexoid = indexinfo->indexoid;
 	List	   *qpqual;
 	List	   *stripped_indexquals;
 	List	   *fixed_indexquals;
@@ -3064,6 +3064,24 @@ create_indexscan_plan(PlannerInfo *root,
 		}
 	}
 
+	/*
+	 * For an index-only scan, we must mark indextlist entries as resjunk if
+	 * they are columns that the index AM can't return; this cues setrefs.c to
+	 * not generate references to those columns.
+	 */
+	if (indexonly)
+	{
+		int			i = 0;
+
+		foreach(l, indexinfo->indextlist)
+		{
+			TargetEntry *indextle = (TargetEntry *) lfirst(l);
+
+			indextle->resjunk = !indexinfo->canreturn[i];
+			i++;
+		}
+	}
+
 	/* Finally ready to build the plan node */
 	if (indexonly)
 		scan_plan = (Scan *) make_indexonlyscan(tlist,
@@ -3071,8 +3089,9 @@ create_indexscan_plan(PlannerInfo *root,
 												baserelid,
 												indexoid,
 												fixed_indexquals,
+												stripped_indexquals,
 												fixed_indexorderbys,
-												make_indexonly_tlist(best_path->indexinfo),
+												indexinfo->indextlist,
 												best_path->indexscandir);
 	else
 		scan_plan = (Scan *) make_indexscan(tlist,
@@ -5444,6 +5463,7 @@ make_indexonlyscan(List *qptlist,
 				   Index scanrelid,
 				   Oid indexid,
 				   List *indexqual,
+				   List *recheckqual,
 				   List *indexorderby,
 				   List *indextlist,
 				   ScanDirection indexscandir)
@@ -5458,6 +5478,7 @@ make_indexonlyscan(List *qptlist,
 	node->scan.scanrelid = scanrelid;
 	node->indexid = indexid;
 	node->indexqual = indexqual;
+	node->recheckqual = recheckqual;
 	node->indexorderby = indexorderby;
 	node->indextlist = indextlist;
 	node->indexorderdir = indexscandir;
@@ -5465,53 +5486,6 @@ make_indexonlyscan(List *qptlist,
 	return node;
 }
 
-/*
- * make_indexonly_tlist
- *
- * Construct the indextlist for an IndexOnlyScan plan node.
- * We must replace any column that can't be returned by the index AM
- * with a null Const of the appropriate datatype.  This is necessary
- * to prevent setrefs.c from trying to use the value of such a column,
- * and anyway it makes the indextlist a better representative of what
- * the indexscan will really return.  (We do this here, not where the
- * IndexOptInfo is originally constructed, because earlier planner
- * steps need to know what is in such columns.)
- */
-static List *
-make_indexonly_tlist(IndexOptInfo *indexinfo)
-{
-	List	   *result;
-	int			i;
-	ListCell   *lc;
-
-	/* We needn't work hard for the common case of btrees. */
-	if (indexinfo->relam == BTREE_AM_OID)
-		return indexinfo->indextlist;
-
-	result = NIL;
-	i = 0;
-	foreach(lc, indexinfo->indextlist)
-	{
-		TargetEntry *indextle = (TargetEntry *) lfirst(lc);
-
-		if (indexinfo->canreturn[i])
-			result = lappend(result, indextle);
-		else
-		{
-			TargetEntry *newtle = makeNode(TargetEntry);
-			Node	   *texpr = (Node *) indextle->expr;
-
-			memcpy(newtle, indextle, sizeof(TargetEntry));
-			newtle->expr = (Expr *) makeNullConst(exprType(texpr),
-												  exprTypmod(texpr),
-												  exprCollation(texpr));
-			result = lappend(result, newtle);
-		}
-		i++;
-	}
-	return result;
-}
-
 static BitmapIndexScan *
 make_bitmap_indexscan(Index scanrelid,
 					  Oid indexid,

src/backend/optimizer/plan/setrefs.c

@@ -1142,8 +1142,26 @@ set_indexonlyscan_references(PlannerInfo *root,
 							 int rtoffset)
 {
 	indexed_tlist *index_itlist;
+	List	   *stripped_indextlist;
+	ListCell   *lc;
 
-	index_itlist = build_tlist_index(plan->indextlist);
+	/*
+	 * Vars in the plan node's targetlist, qual, and recheckqual must only
+	 * reference columns that the index AM can actually return.  To ensure
+	 * this, remove non-returnable columns (which are marked as resjunk) from
+	 * the indexed tlist.  We can just drop them because the indexed_tlist
+	 * machinery pays attention to TLE resnos, not physical list position.
+	 */
+	stripped_indextlist = NIL;
+	foreach(lc, plan->indextlist)
+	{
+		TargetEntry *indextle = (TargetEntry *) lfirst(lc);
+
+		if (!indextle->resjunk)
+			stripped_indextlist = lappend(stripped_indextlist, indextle);
+	}
+
+	index_itlist = build_tlist_index(stripped_indextlist);
 
 	plan->scan.scanrelid += rtoffset;
 	plan->scan.plan.targetlist = (List *)
@@ -1160,6 +1178,13 @@ set_indexonlyscan_references(PlannerInfo *root,
 					   INDEX_VAR,
 					   rtoffset,
 					   NUM_EXEC_QUAL((Plan *) plan));
+	plan->recheckqual = (List *)
+		fix_upper_expr(root,
+					   (Node *) plan->recheckqual,
+					   index_itlist,
+					   INDEX_VAR,
+					   rtoffset,
+					   NUM_EXEC_QUAL((Plan *) plan));
 	/* indexqual is already transformed to reference index columns */
 	plan->indexqual = fix_scan_list(root, plan->indexqual,
 									rtoffset, 1);

src/backend/optimizer/plan/subselect.c

@@ -2336,6 +2336,8 @@ finalize_plan(PlannerInfo *root, Plan *plan,
 		case T_IndexOnlyScan:
 			finalize_primnode((Node *) ((IndexOnlyScan *) plan)->indexqual,
 							  &context);
+			finalize_primnode((Node *) ((IndexOnlyScan *) plan)->recheckqual,
+							  &context);
 			finalize_primnode((Node *) ((IndexOnlyScan *) plan)->indexorderby,
 							  &context);
 

src/include/nodes/execnodes.h

@@ -1490,7 +1490,7 @@ 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
@@ -1509,7 +1509,7 @@ 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;

src/include/nodes/plannodes.h

@@ -420,15 +420,28 @@ 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
+ * contain such Vars.  Also, for the convenience of setrefs.c, TLEs in
- * reconstruct are replaced by null Consts in indextlist.
+ * indextlist are marked as resjunk if they correspond to columns that
+ * the index AM cannot reconstruct.
  * ----------------
  */
 typedef struct IndexOnlyScan
@@ -436,6 +449,7 @@ typedef struct IndexOnlyScan
 	Scan		scan;
 	Oid			indexid;		/* OID of index to scan */
 	List	   *indexqual;		/* list of index quals (usually OpExprs) */
+	List	   *recheckqual;	/* index quals in recheckable form */
 	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 */

src/test/regress/expected/gist.out

@@ -340,6 +340,37 @@ where p <@ box(point(5, 5), point(5.3, 5.3));
  <(5.3,5.3),1>
 (7 rows)
 
+-- Similarly, test that index rechecks involving a non-returnable column
+-- are done correctly.
+explain (verbose, costs off)
+select p from gist_tbl where circle(p,1) @> circle(point(0,0),0.95);
+                                      QUERY PLAN                                       
+---------------------------------------------------------------------------------------
+ Index Only Scan using gist_tbl_multi_index on public.gist_tbl
+   Output: p
+   Index Cond: ((circle(gist_tbl.p, '1'::double precision)) @> '<(0,0),0.95>'::circle)
+(3 rows)
+
+select p from gist_tbl where circle(p,1) @> circle(point(0,0),0.95);
+   p   
+-------
+ (0,0)
+(1 row)
+
+-- This case isn't supported, but it should at least EXPLAIN correctly.
+explain (verbose, costs off)
+select p from gist_tbl order by circle(p,1) <-> point(0,0) limit 1;
+                                     QUERY PLAN                                     
+------------------------------------------------------------------------------------
+ Limit
+   Output: p, ((circle(p, '1'::double precision) <-> '(0,0)'::point))
+   ->  Index Only Scan using gist_tbl_multi_index on public.gist_tbl
+         Output: p, (circle(p, '1'::double precision) <-> '(0,0)'::point)
+         Order By: ((circle(gist_tbl.p, '1'::double precision)) <-> '(0,0)'::point)
+(5 rows)
+
+select p from gist_tbl order by circle(p,1) <-> point(0,0) limit 1;
+ERROR:  lossy distance functions are not supported in index-only scans
 -- Clean up
 reset enable_seqscan;
 reset enable_bitmapscan;

src/test/regress/sql/gist.sql

@@ -153,6 +153,17 @@ 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));
 
+-- Similarly, test that index rechecks involving a non-returnable column
+-- are done correctly.
+explain (verbose, costs off)
+select p from gist_tbl where circle(p,1) @> circle(point(0,0),0.95);
+select p from gist_tbl where circle(p,1) @> circle(point(0,0),0.95);
+
+-- This case isn't supported, but it should at least EXPLAIN correctly.
+explain (verbose, costs off)
+select p from gist_tbl order by circle(p,1) <-> point(0,0) limit 1;
+select p from gist_tbl order by circle(p,1) <-> point(0,0) limit 1;
+
 -- Clean up
 reset enable_seqscan;
 reset enable_bitmapscan;