MDEV-31025 Redundant table alter fails when fixed column

			stored externally

row_merge_buf_add(): Has strict assert that fixed length mismatch
shouldn't happen while rebuilding the redundant row format table

btr_index_rec_validate(): Fixed size column can be stored externally.
So sum of inline stored length and external stored length of the
column should be equal to total column length

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

@@ -129,5 +129,25 @@ SELECT ROW_FORMAT FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='t1';
 ROW_FORMAT
 Dynamic
 DROP TABLE t1;
+#
+#  MDEV-31025 Redundant table alter fails when fixed column
+#		stored externally
+#
+set @old_sql_mode = @@sql_mode;
+SET @@sql_mode='';
+CREATE TABLE t1(pk INT,c CHAR(255),c2 CHAR(255),c3 CHAR(255),
+c4 char(255), c5 char(255), c6 char(255),
+c7 char(255), c8 char(255), primary key(pk)
+)Engine=InnoDB character set utf32 ROW_FORMAT=REDUNDANT;
+INSERT INTO t1(pk, c) VALUES (1, repeat('a', 255));
+ALTER TABLE t1 FORCE;
+CHECK TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+SELECT LENGTH(c) FROM t1;
+LENGTH(c)
+1020
+DROP TABLE t1;
+set @@sql_mode = @old_sql_mode;
 # End of 10.4 tests
 SET GLOBAL innodb_default_row_format = @row_format;

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

@@ -150,6 +150,23 @@ ALTER TABLE t1 DROP b;
 SELECT ROW_FORMAT FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='t1';
 DROP TABLE t1;
 
+--echo #
+--echo #  MDEV-31025 Redundant table alter fails when fixed column
+--echo #		stored externally
+--echo #
+set @old_sql_mode = @@sql_mode;
+SET @@sql_mode='';
+CREATE TABLE t1(pk INT,c CHAR(255),c2 CHAR(255),c3 CHAR(255),
+		c4 char(255), c5 char(255), c6 char(255),
+		c7 char(255), c8 char(255), primary key(pk)
+		)Engine=InnoDB character set utf32 ROW_FORMAT=REDUNDANT;
+INSERT INTO t1(pk, c) VALUES (1, repeat('a', 255));
+ALTER TABLE t1 FORCE;
+CHECK TABLE t1;
+SELECT LENGTH(c) FROM t1;
+DROP TABLE t1;
+set @@sql_mode = @old_sql_mode;
+
 --echo # End of 10.4 tests
 
 SET GLOBAL innodb_default_row_format = @row_format;

storage/innobase/btr/btr0btr.cc

@@ -4713,7 +4713,7 @@ btr_index_rec_validate(
 				len -= BTR_EXTERN_FIELD_REF_SIZE;
 				ulint extern_len = mach_read_from_4(
 					data + len + BTR_EXTERN_LEN + 4);
-				if (fixed_size == extern_len) {
+				if (fixed_size == extern_len + len) {
 					goto next_field;
 				}
 			}

storage/innobase/row/row0merge.cc

@@ -681,11 +681,6 @@ row_merge_buf_add(
 						row_field, field, col->len,
 						old_table->space->zip_size(),
 						conv_heap);
-				} else {
-					/* Field length mismatch should not
-					happen when rebuilding redundant row
-					format table. */
-					ut_ad(index->table->not_redundant());
 				}
 			}
 		}