planner: do not reset child projection's schema (#52836) (#55109)

close #42587

pkg/planner/core/rule_eliminate_projection.go

@@ -132,7 +132,11 @@ 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())
+		}
 	}
 	for i, col := range p.Schema().Columns {
 		if p.SCtx().GetSessionVars().StmtCtx.ColRefFromUpdatePlan.Has(int(col.UniqueID)) && !child.Schema().Columns[i].Equal(nil, col) {