From 5f03462166c976ca2a229f00b8ae5e2c6c102fbd Mon Sep 17 00:00:00 2001
From: crazycs <crazycs520@gmail.com>
Date: Sun, 28 Apr 2019 10:52:04 +0800
Subject: [PATCH] ddl: add comment for onRebaseAutoID (#10206)

---
 ddl/ddl_api.go | 5 +++++
 ddl/table.go   | 1 +
 2 files changed, 6 insertions(+)

diff --git a/ddl/ddl_api.go b/ddl/ddl_api.go
index d49b228c46677..d8e96e056d206 100644
--- a/ddl/ddl_api.go
+++ b/ddl/ddl_api.go
@@ -1807,6 +1807,11 @@ func (d *ddl) RebaseAutoID(ctx sessionctx.Context, ident ast.Ident, newBase int6
 	if err != nil {
 		return errors.Trace(err)
 	}
+	// If newBase < autoIncID, we need to do a rebase before returning.
+	// Assume there are 2 TiDB servers: TiDB-A with allocator range of 0 ~ 30000; TiDB-B with allocator range of 30001 ~ 60000.
+	// If the user sends SQL `alter table t1 auto_increment = 100` to TiDB-B,
+	// and TiDB-B finds 100 < 30001 but returns without any handling,
+	// then TiDB-A may still allocate 99 for auto_increment column. This doesn't make sense for the user.
 	newBase = mathutil.MaxInt64(newBase, autoIncID)
 	job := &model.Job{
 		SchemaID:   schema.ID,
diff --git a/ddl/table.go b/ddl/table.go
index 225a11208a62b..cec8c7ca07a8e 100644
--- a/ddl/table.go
+++ b/ddl/table.go
@@ -535,6 +535,7 @@ func onRebaseAutoID(store kv.Storage, t *meta.Meta, job *model.Job) (ver int64,
 		job.State = model.JobStateCancelled
 		return ver, errors.Trace(err)
 	}
+	// No need to check `newBase` again, because `RebaseAutoID` will do this check.
 	tblInfo.AutoIncID = newBase
 	tbl, err := getTable(store, schemaID, tblInfo)
 	if err != nil {