[#29256] YSQL: replace hacky varno logic

Summary:
Commit de8c392 fixes a problem by
modifying varnos during Explain output formatting, which is hacky.
Commit 141703a fixes a different
problem facing a similar situation, but it applies the fix in the right
place, set_plan_refs, except that it disables the fix in case there are
no index quals to pushdown.  That doesn't seem to be a valid condition
to do this fix, so bring it out of that condition.  Then, remove the
hacky varno modification code in explain.c as it is no longer needed.

Also consider commit a1a5160
yb_copy_replace_varnos_mutator also doing varno modifications.  That can
be investigated separately.

Jira: DB-19033

Test Plan:
On Almalinux 8:

    ./yb_build.sh fastdebug --gcc13 daemons initdb --skip-pg-parquet \
      --java-test TestPgRegressDistinctPushdown

Close: #29256

Reviewers: amartsinchyk, patnaik.balivada

Reviewed By: amartsinchyk

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D48031

Author: jaki

src/postgres/src/backend/commands/explain.c

@@ -183,7 +183,6 @@ static void YbExplainDistinctPrefixLen(PlanState *planstate, List *indextlist,
 									   ExplainState *es, List *ancestors);
 static void show_ybtidbitmap_info(YbBitmapTableScanState *planstate,
 								  ExplainState *es);
-static Node *yb_fix_indexpr_mutator(Node *node, int *newvarno);
 
 typedef enum YbStatLabel
 {
@@ -6620,7 +6619,6 @@ YbExplainDistinctPrefixLen(PlanState *planstate, List *indextlist,
 		bool		useprefix;
 		int			keyno;
 		ListCell   *tlelc;
-		Index		scanrelid;
 
 		initStringInfo(&distinct_prefix_key_buf);
 
@@ -6629,25 +6627,21 @@ YbExplainDistinctPrefixLen(PlanState *planstate, List *indextlist,
 										   planstate->plan,
 										   ancestors);
 		useprefix = (list_length(es->rtable) > 1 || es->verbose);
-		scanrelid = ((Scan *) planstate->plan)->scanrelid;
 
 		keyno = 0;
 		foreach(tlelc, indextlist)
 		{
 			TargetEntry *indextle;
-			Node	   *indexpr;
 			char	   *exprstr;
 
 			if (keyno >= yb_distinct_prefixlen)
 				break;
 
 			indextle = (TargetEntry *) lfirst(tlelc);
 
-			/* Fix the varno of prefix to scanrelid after making a copy. */
-			indexpr = yb_fix_indexpr_mutator((Node *) indextle->expr,
-											 (void *) &scanrelid);
 			/* Deparse the expression, showing any top-level cast */
-			exprstr = deparse_expression(indexpr, context, useprefix, true);
+			exprstr = deparse_expression((Node *) indextle->expr, context,
+										 useprefix, true);
 			resetStringInfo(&distinct_prefix_key_buf);
 			appendStringInfoString(&distinct_prefix_key_buf, exprstr);
 			/* Emit one property-list item per key */
@@ -6659,27 +6653,3 @@ YbExplainDistinctPrefixLen(PlanState *planstate, List *indextlist,
 		ExplainPropertyList("Distinct Keys", result, es);
 	}
 }
-
-static Node *
-yb_fix_indexpr_mutator(Node *node, int *newvarno)
-{
-	if (node == NULL)
-		return NULL;
-
-	if (nodeTag(node) == T_Var)
-	{
-		Var		   *var = palloc(sizeof(Var));
-
-		/* Copy old var into a new one and adjust varno */
-		*var = *((Var *) node);
-		if (!IS_SPECIAL_VARNO(var->varno))
-			var->varno = *newvarno;
-		if (var->varnosyn > 0)
-			var->varnosyn = *newvarno;
-
-		return (Node *) var;
-	}
-
-	return expression_tree_mutator(node, yb_fix_indexpr_mutator,
-								   (void *) newvarno);
-}

src/postgres/src/backend/optimizer/plan/setrefs.c

@@ -612,8 +612,6 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 				/*
 				 * YB: Index quals has to be fixed to refer to index columns,
 				 * not main table columns, so we need to index the indextlist.
-				 * Also, indextlist has to be converted, as ANALYZE may use it.
-				 * Skip that if we don't have index pushdown quals.
 				 */
 				if (splan->yb_idx_pushdown.quals)
 				{
@@ -634,11 +632,15 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 									   INDEX_VAR,
 									   rtoffset,
 									   NUM_EXEC_TLIST(plan));
-					splan->indextlist =
-						fix_scan_list(root, splan->indextlist, rtoffset,
-									  NUM_EXEC_TLIST(plan));
 					pfree(index_itlist);
 				}
+				/*
+				 * YB: Also, indextlist has to be converted as ANALYZE may use
+				 * it.
+				 */
+				splan->indextlist =
+					fix_scan_list(root, splan->indextlist, rtoffset,
+								  NUM_EXEC_TLIST(plan));
 				splan->indexqual =
 					fix_scan_list(root, splan->indexqual,
 								  rtoffset, 1);