cherry pick pingcap#19286 to release-4.0

Signed-off-by: ti-srebot <[email protected]>

ddl/db_integration_test.go

@@ -1820,6 +1820,13 @@ func (s *testIntegrationSuite5) TestChangingDBCharset(c *C) {
 
 	tk.MustExec("ALTER SCHEMA CHARACTER SET = 'utf8mb4' COLLATE = 'utf8mb4_general_ci'")
 	verifyDBCharsetAndCollate("alterdb2", "utf8mb4", "utf8mb4_general_ci")
+
+	// Test changing charset of schema with uppercase name. See https://github.com/pingcap/tidb/issues/19273.
+	tk.MustExec("drop database if exists TEST_UPPERCASE_DB_CHARSET;")
+	tk.MustExec("create database TEST_UPPERCASE_DB_CHARSET;")
+	tk.MustExec("use TEST_UPPERCASE_DB_CHARSET;")
+	tk.MustExec("alter database TEST_UPPERCASE_DB_CHARSET default character set utf8;")
+	tk.MustExec("alter database TEST_UPPERCASE_DB_CHARSET default character set utf8mb4;")
 }
 
 func (s *testIntegrationSuite4) TestDropAutoIncrementIndex(c *C) {

infoschema/builder.go

@@ -187,7 +187,7 @@ func (b *Builder) applyModifySchemaCharsetAndCollate(m *meta.Meta, diff *model.S
 			fmt.Sprintf("(Schema ID %d)", diff.SchemaID),
 		)
 	}
-	newDbInfo := b.copySchemaTables(di.Name.O)
+	newDbInfo := b.copySchemaTables(di.Name.L)
 	newDbInfo.Charset = di.Charset
 	newDbInfo.Collate = di.Collate
 	return nil
@@ -362,6 +362,7 @@ func (b *Builder) copySchemasMap(oldIS *infoSchema) {
 
 // copySchemaTables creates a new schemaTables instance when a table in the database has changed.
 // It also does modifications on the new one because old schemaTables must be read-only.
+// Note: please make sure the dbName is in lowercase.
 func (b *Builder) copySchemaTables(dbName string) *model.DBInfo {
 	oldSchemaTables := b.is.schemaMap[dbName]
 	newSchemaTables := &schemaTables{