planner: fix inconsistent schema between UnionAll and child operator

planner/core/integration_test.go

@@ -4713,6 +4713,12 @@ func (s *testIntegrationSerialSuite) TestPushDownGroupConcatToTiFlash(c *C) {
 
 func (s *testIntegrationSuite) TestIssue27797(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
+	origin := tk.MustQuery("SELECT @@session.tidb_partition_prune_mode")
+	originStr := origin.Rows()[0][0].(string)
+	defer func() {
+		tk.MustExec("set @@session.tidb_partition_prune_mode = '" + originStr + "'")
+	}()
+	tk.MustExec("set @@session.tidb_partition_prune_mode = 'static'")
 	tk.MustExec("use test")
 	tk.MustExec("drop table if exists t27797")
 	tk.MustExec("create table t27797(a int, b int, c int, d int) " +
@@ -4897,3 +4903,19 @@ func (s *testIntegrationSuite) TestIssue30094(c *C) {
 		"  └─TableFullScan 10000.00 cop[tikv] table:t30094 keep order:false, stats:pseudo",
 	))
 }
+
+func (s *testIntegrationSuite) TestIssue29705(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	origin := tk.MustQuery("SELECT @@session.tidb_partition_prune_mode")
+	originStr := origin.Rows()[0][0].(string)
+	defer func() {
+		tk.MustExec("set @@session.tidb_partition_prune_mode = '" + originStr + "'")
+	}()
+	tk.MustExec("set @@session.tidb_partition_prune_mode = 'static'")
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t;")
+	tk.MustExec("create table t(id int) partition by hash(id) partitions 4;")
+	tk.MustExec("insert into t values(1);")
+	result := tk.MustQuery("SELECT COUNT(1) FROM ( SELECT COUNT(1) FROM t b GROUP BY id) a;")
+	result.Check(testkit.Rows("1"))
+}

planner/core/rule_column_pruning.go

@@ -241,6 +241,25 @@ func (p *LogicalUnionAll) PruneColumns(parentUsedCols []*expression.Column) erro
 				p.schema.Columns = append(p.schema.Columns[:i], p.schema.Columns[i+1:]...)
 			}
 		}
+		// It's possible that the child operator adds extra columns to the schema.
+		// Currently, (*LogicalAggregation).PruneColumns() might do this.
+		// But we don't need such columns, so we add an extra Projection to prune this column when this happened.
+		for i, child := range p.Children() {
+			if p.schema.Len() < child.Schema().Len() {
+				// Copy and use the Union's schema as the Projection's Exprs and schema.
+				newCols := make([]expression.Column, len(p.schema.Columns))
+				exprs := make([]expression.Expression, len(p.schema.Columns))
+				for j, col := range p.schema.Columns {
+					newCols[j] = *col
+					exprs[j] = &newCols[j]
+				}
+				proj := LogicalProjection{Exprs: exprs, AvoidColumnEvaluator: true}.Init(p.ctx, p.blockOffset)
+				proj.SetSchema(p.schema.Clone())
+
+				proj.SetChildren(child)
+				p.children[i] = proj
+			}
+		}
 	}
 	return nil
 }