MDEV-14111 Inplace update assert after instant adding column

row_upd_changes_field_size_or_external(): Always return TRUE
if an instantly added column is being updated such that the
column value is absent from the record.

Also, avoid some unnecessary computations.

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

@@ -58,8 +58,12 @@ SELECT * FROM t1;
 ALTER TABLE t1 ADD COLUMN (
   d1 INT, d2 INT UNSIGNED DEFAULT 10, d3 VARCHAR(20) NOT NULL DEFAULT 'abcde',
   d4 TIMESTAMP NOT NULL DEFAULT current_timestamp());
+ALTER TABLE t1 ADD INDEX(d3);
 --disable_info
 
+BEGIN;
+UPDATE t1 SET d3='';
+ROLLBACK;
 SELECT * FROM t1;
 INSERT INTO t1 (id) VALUES(2),(3),(4),(5),(6);
 
@@ -69,8 +73,11 @@ CHANGE d3 d3 VARCHAR(20) NOT NULL DEFAULT 'fghij',
 CHANGE d4 dfour TIMESTAMP NOT NULL DEFAULT now();
 --disable_info
 
+UPDATE t1 SET d3='foo' WHERE id = 2;
+UPDATE t1 SET d3=DEFAULT WHERE id = 4;
 INSERT INTO t1 SET id = 7;
 SELECT * FROM t1;
+CHECK TABLE t1;
 
 # add virtual columns
 --enable_info
@@ -80,7 +87,8 @@ ALTER TABLE t1 ADD COLUMN e3 INT AS (id * 2);
 
 # instant alter
 ALTER TABLE t1 CHANGE d3 d3 VARCHAR(20) NOT NULL DEFAULT 'foobar',
-ADD COLUMN (d5 CHAR(20) DEFAULT 'hijkl', d6 INT DEFAULT -12345, d7 INT);
+ADD COLUMN (d5 CHAR(20) DEFAULT 'hijkl', d6 INT DEFAULT -12345, d7 INT),
+DROP INDEX d3;
 --disable_info
 
 INSERT INTO t1 SET id = 8;

storage/innobase/row/row0upd.cc

@@ -594,6 +594,9 @@ row_upd_changes_field_size_or_external(
 		}
 
 		new_val = &(upd_field->new_val);
+		if (dfield_is_ext(new_val)) {
+			return(TRUE);
+		}
 		new_len = dfield_get_len(new_val);
 		ut_ad(new_len != UNIV_SQL_DEFAULT);
 
@@ -609,11 +612,14 @@ row_upd_changes_field_size_or_external(
 				0);
 		}
 
-		old_len = rec_offs_nth_size(offsets, upd_field->field_no);
+		if (rec_offs_nth_default(offsets, upd_field->field_no)) {
+			/* This is an instantly added column that is
+			at the initial default value. */
+			return(TRUE);
+		}
 
 		if (rec_offs_comp(offsets)
-		    && rec_offs_nth_sql_null(offsets,
-					     upd_field->field_no)) {
+		    && rec_offs_nth_sql_null(offsets, upd_field->field_no)) {
 			/* Note that in the compact table format, for a
 			variable length field, an SQL NULL will use zero
 			bytes in the offset array at the start of the physical
@@ -622,9 +628,12 @@ row_upd_changes_field_size_or_external(
 			if we update an SQL NULL varchar to an empty string! */
 
 			old_len = UNIV_SQL_NULL;
+		} else {
+			old_len = rec_offs_nth_size(offsets,
+						    upd_field->field_no);
 		}
 
-		if (dfield_is_ext(new_val) || old_len != new_len
+		if (old_len != new_len
 		    || rec_offs_nth_extern(offsets, upd_field->field_no)) {
 
 			return(TRUE);