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How does the row level lock work in InnoDB?
InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. A next-key lock on an index record also affects the “gap” before that index record.
How does InnoDB support multiple granularity locking?
Instead, transaction T2 has to wait for transaction T1 to release its lock on row r . InnoDB supports multiple granularity locking which permits coexistence of row locks and table locks.
What does InnoDB DO FOR UPDATE statement in MySQL?
For UPDATE statements, InnoDB does a “semi-consistent” read, such that it returns the latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE . A next-key lock is a combination of a record lock on the index record and a gap lock on the gap before the index record.
Why are conflicting gap locks allowed in InnoDB?
The reason conflicting gap locks are allowed is that if a record is purged from an index, the gap locks held on the record by different transactions must be merged. Gap locks in InnoDB are “purely inhibitive”, which means that their only purpose is to prevent other transactions from inserting to the gap. Gap locks can co-exist.
InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. A next-key lock on an index record also affects the “gap” before that index record.
Which is the default isolation level for InnoDB?
InnoDB offers all four transaction isolation levels described by the SQL:1992 standard: READ UNCOMMITTED , READ COMMITTED , REPEATABLE READ, and SERIALIZABLE. The default isolation level for InnoDB is REPEATABLE READ .
Here’s an example session from the Sakila database which demonstrates some of the behaviors of FOR UPDATE queries. First, just so we’re crystal clear, set the transaction isolation level to REPEATABLE READ. This is normally unnecessary, as it is the default isolation level for InnoDB: In the other session, update this row.
Why is it important to understand InnoDB transaction model?
To implement a large-scale, busy, or highly reliable database application, to port substantial code from a different database system, or to tune MySQL performance, it is important to understand InnoDB locking and the InnoDB transaction model.
Why do we need row locking in MySQL?
In conjunction with multi-versioning, this results in good query concurrency because a given table can be read and modified by different clients at the same time. Row-level concurrency properties are as follows: Different clients can read the same rows simultaneously. Different clients can modify different rows simultaneously.