MariaDB primary key vs unique key

Question

In some ways this question follows on from this one. One of the injunctions in the MariaDB documnetation suggests that having tables on a clustered database without a primary key is not desirable. In my application I have as a rule been using one UNIQUE key (VARCHAR(8)) per table with no PRIMARY key. My understanding is that a PRIMARY key is merely a special kind of UNIQUE key. Question - is the current UNIQUE key usage adequate for keeping MariaDB Galera happy or do I explciitly need to convert my UNIQUEs to PRIMARYs? On the face of it this does not make much sense to me but perhaps there are reasons for doing so?

Answer 1

In the absence of a PRIMARY key, InnoDB/XtraDB will first try to use a UNIQUE index. If neither exist it will make up an internal primary key that is not reliable between galera nodes.

You are correct that a PRIMARY key is basically a UNIQUE index with the only difference being that there can only be one PRIMARY key. It is also used for the physical layout of the data but that isn't as important here.

As long as there is only one UNIQUE index, you should be fine. However, I don't think it would be reliable if you add another UNIQUE index. Because of that and for good practice, you should probably make that UNIQUE index the PRIMARY key.

Answer 2

a PRIMARY KEY require that the column is NOT NULL

in the table CREATE TABLE p (a INT, b INT, UNIQUE (a)); you can have 2 rows where a IS NULL.

in the table CREATE TABLE p2 (a INT, b INT, PRIMARY KEY (a)); the a column automaticly becomes a NOT NULL column.

SHOW CREATE TABLE p2;
CREATE TABLE `p2` (
  `a` int(11) NOT NULL DEFAULT '0',
  `b` int(11) DEFAULT NULL,
  PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1