MDEV-15563: Allow instant VARCHAR extension from <128 bytes

For up to 127 bytes length, InnoDB would use 1 byte for length, and
that byte would always be less than 128. If the maximum length is
longer than 255 bytes, InnoDB would use a variable-length encoding
for the length, using 1 byte for lengths up to 127 bytes, and
2 bytes for longer lengths.

Thus, 1-byte lengths are always compatible when the maximum size
changes from less than 128 bytes to anything longer.

Field_varstring::is_equal(): Return IS_EQUAL_PACK_LENGTH also when
converting from VARCHAR less than 128 bytes to any longer VARCHAR.

mysql-test/suite/innodb/r/alter_varchar_change.result

@@ -412,6 +412,23 @@ ALTER TABLE t1 MODIFY f2 VARCHAR(300);
 CALL get_table_id("test/t1", @tbl1_id);
 SELECT @tbl1_id = @tbl_id;
 @tbl1_id = @tbl_id
+1
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `f1` int(11) NOT NULL,
+  `f2` varchar(300) DEFAULT NULL,
+  KEY `idx` (`f2`(40))
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;
+CREATE TABLE t1(f1 INT NOT NULL,
+f2 VARCHAR(128),
+INDEX idx(f2(40)))ENGINE=InnoDB;
+CALL get_table_id("test/t1", @tbl_id);
+ALTER TABLE t1 MODIFY f2 VARCHAR(300);
+CALL get_table_id("test/t1", @tbl1_id);
+SELECT @tbl1_id = @tbl_id;
+@tbl1_id = @tbl_id
 0
 SHOW CREATE TABLE t1;
 Table	Create Table

mysql-test/suite/innodb/t/alter_varchar_change.test

@@ -306,6 +306,18 @@ SELECT @tbl1_id = @tbl_id;
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
 
+CREATE TABLE t1(f1 INT NOT NULL,
+		f2 VARCHAR(128),
+		INDEX idx(f2(40)))ENGINE=InnoDB;
+
+CALL get_table_id("test/t1", @tbl_id);
+ALTER TABLE t1 MODIFY f2 VARCHAR(300);
+CALL get_table_id("test/t1", @tbl1_id);
+
+SELECT @tbl1_id = @tbl_id;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
 CREATE TABLE t1(f1 INT NOT NULL,
 		f2 VARCHAR(100),
 		INDEX idx(f2(40)))ENGINE=InnoDB;

sql/field.cc

@@ -7901,17 +7901,21 @@ Field *Field_varstring::new_key_field(MEM_ROOT *root, TABLE *new_table,
 
 uint Field_varstring::is_equal(Create_field *new_field)
 {
+  if (new_field->length < field_length)
+    return IS_EQUAL_NO;
+
   if (new_field->type_handler() == type_handler() &&
       new_field->charset == field_charset &&
       !new_field->compression_method() == !compression_method())
   {
     if (new_field->length == field_length)
       return IS_EQUAL_YES;
-    if (new_field->length > field_length &&
-	((new_field->length <= 255 && field_length <= 255) ||
-	 (new_field->length > 255 && field_length > 255)))
-      return IS_EQUAL_PACK_LENGTH; // VARCHAR, longer variable length
+    if (field_length <= 127 ||
+        new_field->length <= 255 ||
+        field_length > 255)
+      return IS_EQUAL_PACK_LENGTH; // VARCHAR, longer length
   }
+
   return IS_EQUAL_NO;
 }