Why is an IX-lock compatible with another IX-lock in InnoDB?

Question

According to innodb lock mode lock type compatibility matrix

    X           IX          S           IS
X   Conflict    Conflict    Conflict    Conflict         


        

Answer 1

IS and IX locks allow access by multiple clients. They won't necessarily conflict until they try to get real locks on the same rows.

But a table lock (ALTER TABLE, DROP TABLE, LOCK TABLES) blocks both IS and IX, and vice-versa.

Therefore the IX-lock compatible with another IX-lock (they both are table level) or IX-lock conflict with another X-lock(table level not row level)

Reference: http://www.slideshare.net/billkarwin/innodb-locking-explained-with-stick-figures