SET @saved_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
#
# MDEV-12288 Reset DB_TRX_ID when the history is removed,
# to speed up MVCC
#
SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
CREATE TABLE t1(a INT PRIMARY KEY, b INT NOT NULL)
ROW_FORMAT=REDUNDANT ENGINE=InnoDB;
connect  prevent_purge,localhost,root;
START TRANSACTION WITH CONSISTENT SNAPSHOT;
connection default;
INSERT INTO t1 VALUES(1,2),(3,4);
ALTER TABLE t1 ADD COLUMN c INT;
UPDATE t1 SET b=-3 WHERE a=3;
connect  con1,localhost,root;
BEGIN;
UPDATE t1 SET b=4 WHERE a=3;
disconnect prevent_purge;
connection default;
InnoDB		0 transactions not purged
disconnect con1;
FLUSH TABLE t1 FOR EXPORT;
Clustered index root page contents:
N_RECS=3; LEVEL=0
header=0x0100000300c6 (a=0x696e66696d756d00)
header=0x1000200b0087 (a=0x80000000,
 DB_TRX_ID=0x000000000000,
 DB_ROLL_PTR=0x80000000000000,
 b=0x80000000,
 c=NULL(4 bytes))
header=0x0000100900a6 (a=0x80000001,
 DB_TRX_ID=0x000000000000,
 DB_ROLL_PTR=0x80000000000000,
 b=0x80000002)
header=0x000018090074 (a=0x80000003,
 DB_TRX_ID=0x000000000000,
 DB_ROLL_PTR=0x80000000000000,
 b=0x7ffffffd)
header=0x040008030000 (a=0x73757072656d756d00)
UNLOCK TABLES;
SELECT * FROM t1;
a	b	c
1	2	NULL
3	-3	NULL
DROP TABLE t1;
SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;