From e0cf097968d7fd1a1aee950c65c16b51de4d3462 Mon Sep 17 00:00:00 2001
From: Ti Chi Robot <ti-community-prow-bot@tidb.io>
Date: Mon, 26 Aug 2024 17:39:16 +0800
Subject: [PATCH] planner: make sure mpp join task's hashCols are all contained
 of its plan's schema (#52836) (#55655)

close pingcap/tidb#42587
---
 planner/core/rule_eliminate_projection.go | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/planner/core/rule_eliminate_projection.go b/planner/core/rule_eliminate_projection.go
index a9a1ad250b5b4..a1a998b8c0b92 100644
--- a/planner/core/rule_eliminate_projection.go
+++ b/planner/core/rule_eliminate_projection.go
@@ -130,7 +130,11 @@ func doPhysicalProjectionElimination(p PhysicalPlan) PhysicalPlan {
 	}
 	child := p.Children()[0]
 	if childProj, ok := child.(*PhysicalProjection); ok {
-		childProj.SetSchema(p.Schema())
+		// when current projection is an empty projection(schema pruned by column pruner), no need to reset child's schema
+		// TODO: avoid producing empty projection in column pruner.
+		if p.Schema().Len() != 0 {
+			childProj.SetSchema(p.Schema())
+		}
 	}
 	return child
 }