MYSQL：分区表保持id唯一

Question

We are using a table which has schema like following:-

CREATE TABLE `user_subscription` (
`ID` varchar(40) NOT NULL,
`COL1` varchar(40) NOT NULL,
`COL2` varchar(30) NOT NULL,
`COL3` datetime NOT NULL,
`COL4` datetime NOT NULL,
`ARCHIVE` tinyint(1) NOT NULL DEFAULT '0',
PRIMARY KEY (`ID`)
)

Now we wanted to do partition on column ARCHIVE. ARCHIVE can have only 2 values 0 or 1 and so 2 partitions.

Actually in our case, we are using partitioning as a Archival process. To do partition, we need to make ARCHIVE column as a part of primary key. But the problem here is that 2 rows can have same ID with different ARCHIVE column value. Actually thats not the main problem for us as 2 rows will be in different partitions. Problem is when we will update the archive column value of one of them to other to move one of the row to archive partition, then it will not allow us to update the entry giving "Duplicate Error".

Can somebody help in this regard?

Answer 1

Unfortunately,

A UNIQUE INDEX (or a PRIMARY KEY ) must include all columns in the table's partitioning function

and since MySQL does not support check constraints either, the only ugly workaround I can think of is enforcing the uniqueness manually though triggers:

CREATE TABLE t (
  id INT NOT NULL,
  archived TINYINT(1) NOT NULL DEFAULT 0,
  PRIMARY KEY (id, archived),  -- required by MySQL limitation on partitioning
)
PARTITION BY LIST(archived) (
  PARTITION pActive VALUES IN (0),
  PARTITION pArchived VALUES IN (1)
);

CREATE TRIGGER tInsert
BEFORE INSERT ON t FOR EACH ROW
CALL checkUnique(NEW.id);

CREATE TRIGGER tUpdate
BEFORE UPDATE ON t FOR EACH ROW
CALL checkUnique(NEW.id);

DELIMITER //
CREATE PROCEDURE checkUnique(pId INT)
BEGIN
  DECLARE flag INT;
  DECLARE message VARCHAR(50);
  SELECT id INTO flag FROM t WHERE id = pId;
  IF flag IS NOT NULL THEN
    -- the below tries to mimic the error raised
    -- by a regular UNIQUE constraint violation
    SET message = CONCAT("Duplicate entry '", pId, "'");
    SIGNAL SQLSTATE "23000" SET
      MYSQL_ERRNO = 1062,
      MESSAGE_TEXT = message,
      COLUMN_NAME = "id";
  END IF;
END //

( fiddle )

MySQL's limitations on partitioning being such a downer (in particular its lack of support for foreign keys), I would advise against using it altogether until the table grows so large that it becomes an actual concern.