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config/config.go

@@ -76,6 +76,7 @@ type Config struct {
 	Plugin              Plugin            `toml:"plugin" json:"plugin"`
 	CheckMb4ValueInUTF8 bool              `toml:"check-mb4-value-in-utf8" json:"check-mb4-value-in-utf8"`
 	// TreatOldVersionUTF8AsUTF8MB4 is use to treat old version table/column UTF8 charset as UTF8MB4. This is for compatibility.
+	// Currently not support dynamic modify, because this need to reload all old version schema.
 	TreatOldVersionUTF8AsUTF8MB4 bool `toml:"treat-old-version-utf8-as-utf8mb4" json:"treat-old-version-utf8-as-utf8mb4"`
 }
 

ddl/ddl_api.go

@@ -2444,7 +2444,8 @@ func (d *ddl) AlterTableCharsetAndCollate(ctx sessionctx.Context, ident ast.Iden
 			return errors.Trace(err)
 		}
 	}
-
+	// Old version schema charset maybe modified when load schema if TreatOldVersionUTF8AsUTF8MB4 was enable.
+	// So even if the origCharset equal toCharset, we still need to do the ddl for old version schema.
 	if origCharset == toCharset && origCollate == toCollate && tb.Meta().Version >= model.TableInfoVersion2 {
 		// nothing to do.
 		return nil

domain/domain.go

@@ -179,6 +179,7 @@ func (do *Domain) fetchSchemasWithTables(schemas []*model.DBInfo, m *meta.Meta,
 			done <- err
 			return
 		}
+		// If TreatOldVersionUTF8AsUTF8MB4 was enable, need to convert the old version schema UTF8 charset to UTF8MB4.
 		if config.GetGlobalConfig().TreatOldVersionUTF8AsUTF8MB4 {
 			for _, tbInfo := range tables {
 				infoschema.ConvertOldVersionUTF8ToUTF8MB4IfNeed(tbInfo)