From d6e48505592ae900ac61b17203bc146fa7d77144 Mon Sep 17 00:00:00 2001
From: qiuyesuifeng <cuiqiu.bupt@gmail.com>
Date: Tue, 10 Nov 2015 12:21:53 +0800
Subject: [PATCH 1/3] ddl: fix column value exist backfill bug.

---
 ddl/column.go | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/ddl/column.go b/ddl/column.go
index df24f97617..3d14cbd25c 100644
--- a/ddl/column.go
+++ b/ddl/column.go
@@ -366,10 +366,13 @@ func (d *ddl) backfillColumnData(t table.Table, columnInfo *model.ColumnInfo, ha
 			}
 
 			backfillKey := t.RecordKey(handle, &column.Col{ColumnInfo: *columnInfo})
-			_, err = txn.Get(backfillKey)
+			backfillValue, err := txn.Get(backfillKey)
 			if err != nil && !kv.IsErrNotFound(err) {
 				return errors.Trace(err)
 			}
+			if backfillValue != nil {
+				return nil
+			}
 
 			value, _, err := tables.GetColDefaultValue(nil, columnInfo)
 			if err != nil {

From 3256ce28272407a37e5f32207e41562bebbf7f1c Mon Sep 17 00:00:00 2001
From: qiuyesuifeng <cuiqiu.bupt@gmail.com>
Date: Tue, 10 Nov 2015 13:41:41 +0800
Subject: [PATCH 2/3] ddl: address comment.

---
 ddl/column.go | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/ddl/column.go b/ddl/column.go
index 3d14cbd25c..c1365a6989 100644
--- a/ddl/column.go
+++ b/ddl/column.go
@@ -329,6 +329,12 @@ func (d *ddl) onDropColumn(t *meta.Meta, job *model.Job) error {
 	}
 }
 
+// How to backfill column data in reorganization state?
+//  1, Generate a snapshot with special version.
+//  2, Traverse the snapshot, get every row in the table.
+//  3, For one row, if the row has been already deleted, skip to next row.
+//  4, If not deleted, check whether column data has existed, if existed, skip to next row.
+//  5, If column data doesn't exist, backfill the column with default value and then continue to handle next row.
 func (d *ddl) backfillColumn(t table.Table, columnInfo *model.ColumnInfo, version uint64, seekHandle int64) error {
 	for {
 		handles, err := d.getSnapshotRows(t, version, seekHandle)

From c210aafc9fcb2114e92b163967f9d07a06ff28f2 Mon Sep 17 00:00:00 2001
From: qiuyesuifeng <cuiqiu.bupt@gmail.com>
Date: Tue, 10 Nov 2015 17:21:46 +0800
Subject: [PATCH 3/3] ddl: address comment.

---
 ddl/column.go | 10 +++++-----
 ddl/index.go  | 10 +++++-----
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/ddl/column.go b/ddl/column.go
index 1643a5f956..276b0b08d4 100644
--- a/ddl/column.go
+++ b/ddl/column.go
@@ -305,11 +305,11 @@ func (d *ddl) onDropColumn(t *meta.Meta, job *model.Job) error {
 }
 
 // How to backfill column data in reorganization state?
-//  1, Generate a snapshot with special version.
-//  2, Traverse the snapshot, get every row in the table.
-//  3, For one row, if the row has been already deleted, skip to next row.
-//  4, If not deleted, check whether column data has existed, if existed, skip to next row.
-//  5, If column data doesn't exist, backfill the column with default value and then continue to handle next row.
+//  1. Generate a snapshot with special version.
+//  2. Traverse the snapshot, get every row in the table.
+//  3. For one row, if the row has been already deleted, skip to next row.
+//  4. If not deleted, check whether column data has existed, if existed, skip to next row.
+//  5. If column data doesn't exist, backfill the column with default value and then continue to handle next row.
 func (d *ddl) backfillColumn(t table.Table, columnInfo *model.ColumnInfo, reorgInfo *reorgInfo) error {
 	seekHandle := reorgInfo.Handle
 	version := reorgInfo.SnapshotVer
diff --git a/ddl/index.go b/ddl/index.go
index ebfc1d1d75..8c397656b9 100644
--- a/ddl/index.go
+++ b/ddl/index.go
@@ -332,11 +332,11 @@ func fetchRowColVals(txn kv.Transaction, t table.Table, handle int64, indexInfo
 const maxBatchSize = 1024
 
 // How to add index in reorganization state?
-//  1, Generate a snapshot with special version.
-//  2, Traverse the snapshot, get every row in the table.
-//  3, For one row, if the row has been already deleted, skip to next row.
-//  4, If not deleted, check whether index has existed, if existed, skip to next row.
-//  5, If index doesn't exist, create the index and then continue to handle next row.
+//  1. Generate a snapshot with special version.
+//  2. Traverse the snapshot, get every row in the table.
+//  3. For one row, if the row has been already deleted, skip to next row.
+//  4. If not deleted, check whether index has existed, if existed, skip to next row.
+//  5. If index doesn't exist, create the index and then continue to handle next row.
 func (d *ddl) addTableIndex(t table.Table, indexInfo *model.IndexInfo, reorgInfo *reorgInfo) error {
 	seekHandle := reorgInfo.Handle
 	version := reorgInfo.SnapshotVer