planner: fix possible inconsistent output cols among union's children

pkg/planner/core/rule_column_pruning.go

@@ -275,6 +275,9 @@ func (p *LogicalUnionAll) PruneColumns(parentUsedCols []*expression.Column, opt
 	if !hasBeenUsed {
 		parentUsedCols = make([]*expression.Column, len(p.schema.Columns))
 		copy(parentUsedCols, p.schema.Columns)
+		for i := range used {
+			used[i] = true
+		}
 	}
 	for _, child := range p.Children() {
 		err := child.PruneColumns(parentUsedCols, opt, p)
@@ -284,16 +287,14 @@ func (p *LogicalUnionAll) PruneColumns(parentUsedCols []*expression.Column, opt
 	}
 
 	prunedColumns := make([]*expression.Column, 0)
-	if hasBeenUsed {
-		// keep the schema of LogicalUnionAll same as its children's
-		used := expression.GetUsedList(p.SCtx(), p.children[0].Schema().Columns, p.schema)
-		for i := len(used) - 1; i >= 0; i-- {
-			if !used[i] {
-				prunedColumns = append(prunedColumns, p.schema.Columns[i])
-				p.schema.Columns = append(p.schema.Columns[:i], p.schema.Columns[i+1:]...)
-			}
+	for i := len(used) - 1; i >= 0; i-- {
+		if !used[i] {
+			prunedColumns = append(prunedColumns, p.schema.Columns[i])
+			p.schema.Columns = append(p.schema.Columns[:i], p.schema.Columns[i+1:]...)
 		}
-		appendColumnPruneTraceStep(p, prunedColumns, opt)
+	}
+	appendColumnPruneTraceStep(p, prunedColumns, opt)
+	if hasBeenUsed {
 		// 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.

tests/integrationtest/r/planner/core/issuetest/planner_issue.result

@@ -180,3 +180,81 @@ LEFT JOIN tmp3 c3 ON c3.id = '1';
 id	id
 1	1
 1	1
+drop table if exists t;
+create table t(a int, b int);
+set @@tidb_max_chunk_size = 32;
+insert into t values(1, 1);
+insert into t select a+1, a+1 from t;
+insert into t select a+2, a+2 from t;
+insert into t select a+4, a+4 from t;
+insert into t select a+8, a+8 from t;
+insert into t select a+16, a+16 from t;
+insert into t select a+32, a+32 from t;
+select a from (select 100 as a, 100 as b union all select * from t) t where b != 0;
+a
+100
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
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+32
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+64
+set @@tidb_max_chunk_size = default;

tests/integrationtest/t/planner/core/issuetest/planner_issue.test

@@ -136,3 +136,18 @@ FROM
   t2 db 
   LEFT JOIN tmp3 c2 ON c2.id = '1' 
   LEFT JOIN tmp3 c3 ON c3.id = '1';
+
+# https://github.com/pingcap/tidb/issues/48755
+drop table if exists t;
+create table t(a int, b int);
+set @@tidb_max_chunk_size = 32;
+# insert into more than 32 rows to the table.
+insert into t values(1, 1);
+insert into t select a+1, a+1 from t;
+insert into t select a+2, a+2 from t;
+insert into t select a+4, a+4 from t;
+insert into t select a+8, a+8 from t;
+insert into t select a+16, a+16 from t;
+insert into t select a+32, a+32 from t;
+select a from (select 100 as a, 100 as b union all select * from t) t where b != 0;
+set @@tidb_max_chunk_size = default;