InnoDB has several types of record-level
locks including record locks, gap locks, and next-key locks. For
information about shared locks, exclusive locks, and intention
locks, see Section 126.96.36.199, “InnoDB Lock Modes”.
Record lock: This is a lock on an index record.
Gap lock: This is a lock on a gap between index records, or a lock on the gap before the first or after the last index record.
Next-key lock: This is a combination of a record lock on the index record and a gap lock on the gap before the index record.
Record locks always lock index records, even if a table is
defined with no indexes. For such cases,
InnoDB creates a hidden clustered index and
uses this index for record locking. See
Section 188.8.131.52, “Clustered and Secondary Indexes”.
InnoDB operates in
REPEATABLE READ transaction
isolation level and with the
system variable disabled. In this case,
InnoDB uses next-key locks for searches and
index scans, which prevents phantom rows (see
Section 184.108.40.206, “Avoiding the Phantom Problem Using Next-Key Locking”).
Next-key locking combines index-row locking with gap locking.
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. In addition, a
next-key lock on an index record also affects the
“gap” before that index record. That is, a next-key
lock is an index-record lock plus a gap lock on the gap
preceding the index record. If one session has a shared or
exclusive lock on record
R in an index,
another session cannot insert a new index record in the gap
R in the index order.
Suppose that an index contains the values 10, 11, 13, and 20.
The possible next-key locks for this index cover the following
denote exclusion of the interval endpoint and
] denote inclusion of
(negative infinity, 10] (10, 11] (11, 13] (13, 20] (20, positive infinity)
For the last interval, the next-key lock locks the gap above the largest value in the index and the “supremum” pseudo-record having a value higher than any value actually in the index. The supremum is not a real index record, so, in effect, this next-key lock locks only the gap following the largest index value.
The next-key locking example in the previous section shows that a gap might span a single index value, multiple index values, or even be empty.
Gap locking is not needed for statements that lock rows using a
unique index to search for a unique row. (This does not include
the case that the search condition includes only some columns of
a multiple-column unique index; in that case, gap locking does
occur.) For example, if the
id column has a
unique index, the following statement uses only an index-record
lock for the row having
id value 100 and it
does not matter whether other sessions insert rows in the
SELECT * FROM child WHERE id = 100;
id is not indexed or has a nonunique
index, the statement does lock the preceding gap.
A type of gap lock called an insert intention gap lock is set by
INSERT operations prior to row
insertion. This lock signals the intent to insert in such a way
that multiple transactions inserting into the same index gap
need not wait for each other if they are not inserting at the
same position within the gap. Suppose that there are index
records with values of 4 and 7. Separate transactions that
attempt to insert values of 5 and 6 each lock the gap between 4
and 7 with insert intention locks prior to obtaining the
exclusive lock on the inserted row, but do not block each other
because the rows are nonconflicting.
It is also worth noting here that conflicting locks can be held on a gap by different transactions. For example, transaction A can hold a shared gap lock (gap S-lock) on a gap while transaction B holds an exclusive gap lock (gap X-lock) on the same gap. 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 they only stop other
transactions from inserting to the gap. Thus, a gap X-lock has
the same effect as a gap S-lock.
Gap locking can be disabled explicitly. This occurs if you
change the transaction isolation level to
READ COMMITTED or enable the
system variable. Under these circumstances, gap locking is
disabled for searches and index scans and is used only for
foreign-key constraint checking and duplicate-key checking.
There is also another effect of using the
READ COMMITTED isolation
level or enabling
Record locks for nonmatching rows are released after MySQL has