MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0
The server_locks
table is similar in
structure to the cluster_locks
table, and
provides a subset of the information found in the latter table,
but which is specific to the SQL node (MySQL server) where it
resides. (The cluster_locks
table provides
information about all locks in the cluster.) More precisely,
server_locks
contains information about locks
requested by threads belonging to the current
mysqld instance, and serves as a companion
table to server_operations
.
This may be useful for correlating locking patterns with
specific MySQL user sessions, queries, or use cases.
The server_locks
table contains the following
columns:
mysql_connection_id
MySQL connection ID
node_id
ID of reporting node
block_instance
ID of reporting LDM instance
tableid
ID of table containing this row
fragmentid
ID of fragment containing locked row
rowid
ID of locked row
transid
Transaction ID
mode
Lock request mode
state
Lock state
detail
Whether this is first holding lock in row lock queue
op
Operation type
duration_millis
Milliseconds spent waiting or holding lock
lock_num
ID of lock object
waiting_for
Waiting for lock with this ID
The mysql_connection_id
column shows the
MySQL connection or thread ID as shown by
SHOW PROCESSLIST
.
block_instance
refers to an instance of a
kernel block. Together with the block name, this number can be
used to look up a given instance in the
threadblocks
table.
The tableid
is assigned to the table by
NDB
; the same ID is used for this table in
other ndbinfo
tables, as well as in the
output of ndb_show_tables.
The transaction ID shown in the transid
column is the identifier generated by the NDB API for the
transaction requesting or holding the current lock.
The mode
column shows the lock mode, which is
always one of S
(shared lock) or
X
(exclusive lock). If a transaction has an
exclusive lock on a given row, all other locks on that row have
the same transaction ID.
The state
column shows the lock state. Its
value is always one of H
(holding) or
W
(waiting). A waiting lock request waits for
a lock held by a different transaction.
The detail
column indicates whether this lock
is the first holding lock in the affected row's lock queue,
in which case it contains a *
(asterisk
character); otherwise, this column is empty. This information
can be used to help identify the unique entries in a list of
lock requests.
The op
column shows the type of operation
requesting the lock. This is always one of the values
READ
, INSERT
,
UPDATE
, DELETE
,
SCAN
, or REFRESH
.
The duration_millis
column shows the number
of milliseconds for which this lock request has been waiting or
holding the lock. This is reset to 0 when a lock is granted for
a waiting request.
The lock ID (lockid
column) is unique to this
node and block instance.
If the lock_state
column's value is
W
, this lock is waiting to be granted, and
the waiting_for
column shows the lock ID of
the lock object this request is waiting for. Otherwise,
waiting_for
is empty.
waiting_for
can refer only to locks on the
same row (as identified by node_id
,
block_instance
, tableid
,
fragmentid
, and rowid
).