AUTO_INCREMENTid for this table -- The PK given is the 'natural' PK; there is no good reason for a surrogate.
MEDIUMINT-- This is a reminder that all
INTsshould be made as small as is safe (smaller ⇒ faster). Of course the declaration here must match the definition in the table being linked to.
UNSIGNED-- Nearly all INTs may as well be declared non-negative
NOT NULL-- Well, that's true, isn't it?
InnoDB-- More effecient than MyISAM because of the way the
PRIMARY KEYis clustered with the data in InnoDB.
INDEX(y_id, x_id)-- The
PRIMARY KEYmakes it efficient to go one direction; the makes the other direction efficient. No need to say
UNIQUE; that would be extra effort on
INDEX(y_id)would work because it would implicit include
x_id. But I would rather make it more obvious that I am hoping for a 'covering' index.
You may want to add more columns to the table; this is rare. The extra columns could provide information about the relationship that the table represents.
You may want to add
FOREIGN KEY constraints.