问题
According to the official docs from here:
the lock compatibility matrix:
X IX S IS
X Conflict Conflict Conflict Conflict
IX Conflict Compatible Conflict Compatible
S Conflict Conflict Compatible Compatible
IS Conflict Compatible Compatible Compatible
The docs also say:
Thus, intention locks do not block anything except full table requests (for example, LOCK TABLES ... WRITE). The main purpose of IX and IS locks is to show that someone is locking a row, or going to lock a row in the table.
If the intention locks only block full table requests, then how to explain the IX conflicts with S lock in the above lock compatibility matrix? To my understanding, the S and X in the lock compatibility matrix are both record locks, it's that right?
回答1:
To my understanding, the S and X in the lock compatibility matrix are both record locks, it's that right?
That is correct. Incorrect is the assumption that you can directly compare table and record locks, which the documentation probably does not make completely clear, and the part "These rules can be conveniently summarized by means of the following lock type compatibility matrix" might be a bit misleading, as it does not cover everything (namely any conflict information about S/X-table locks with record locks).
Technically, that matrix defines the result when checking locks on some object, e.g. whenever MySQL tries to add a lock to something. If you try to get an S lock on a table, it would conflict with an IX lock on that table.
If a record could have an intention lock, it would conflict there too. Just because a lockable object doesn't use intention locks doesn't change the (general) compatibility matrix.
Technically, the internal datatype for locks is the same for records and tables, the intention locks are just never set for records. A record lock will actually never be compared to a table lock (as these are two different objects), and the only reason a record lock interferes with a table lock is the locking protocol (which requires a lock on both the table and the record to lock a record).
So to lock a record, you will typically require a different table lock. The compatibity matrix is the same, but the value for the table can and usually will differ from the value of the record.
So
Thus, intention locks do not block anything except full table requests (for example, LOCK TABLES ... WRITE).
is correct because only full table requests require a lock that conflicts with an existing intention lock, and records do not use intention locks. But to repeat it: you still have to compare tow different locks. An S-lock on a record cannot conflict with the lock on a table, as these two object will never be compared.
The manual is mixing table and record locks a bit. It actually defines IS and IX as:
- Intention shared (IS): Transaction T intends to set S locks on individual rows in table t.
- Intention exclusive (IX): Transaction T intends to set X locks on those rows.
so if you want, IS and IX in the matrix can somewhat be interpreted as properties of rows (which they technically are not), while you read them as the lock on the table (as it can only be set for a table, but which is a different lock). But the matrix still only describes the situation to compare records (which the manual probably does not make clear enough), and it does not include any compatibility information with an S or X table lock.
So to summarize: you do not need "to explain the IX conflicts with S lock in the above lock compatibility matrix", as it simply does not cover that situation.
来源：https://stackoverflow.com/questions/48317009/mysql-lock-compatibility