pgrowlocks
pgrowlocks
The pgrowlocks module provides a function to show row
locking information for a specified table.
Overview
pgrowlocks(text) RETURNS pgrowlocks_type
The parameter is a name of table. And pgrowlocks_type is
defined as:
CREATE TYPE pgrowlocks_type AS (
locked_row TID, -- row TID
lock_type TEXT, -- lock type
locker XID, -- locking XID
multi bool, -- multi XID?
xids xid[], -- multi XIDs
pids INTEGER[] -- locker's process id
);
pgrowlocks_type
locked_row
tuple ID(TID) of each locked rows
lock_type
"Shared" for shared lock, "Exclusive" for exclusive lock
locker
transaction ID of locker (Note 1)
multi
"t" if locker is a multi transaction, otherwise "f"
xids
XIDs of lockers (Note 2)
pids
process ids of locking backends
Note1: If the locker is multi transaction, it represents the multi ID.
Note2: If the locker is multi, multiple data are shown.
The calling sequence for pgrowlocks is as follows:
pgrowlocks grabs AccessShareLock for the target table and
reads each row one by one to get the row locking information. You should
notice that:
if the table is exclusive locked by someone else,
pgrowlocks will be blocked.
pgrowlocks may show incorrect information if there's a
new lock or a lock is freeed while its execution.
pgrowlocks does not show the contents of locked rows. If
you want to take a look at the row contents at the same time, you could do
something like this:
SELECT * FROM accounts AS a, pgrowlocks('accounts') AS p WHERE p.locked_ row = a.ctid;
Example
pgrowlocks returns the following data type:
Here is a sample execution of pgrowlocks:
test=# SELECT * FROM pgrowlocks('t1');
locked_row | lock_type | locker | multi | xids | pids
------------+-----------+--------+-------+-----------+---------------
(0,1) | Shared | 19 | t | {804,805} | {29066,29068}
(0,2) | Shared | 19 | t | {804,805} | {29066,29068}
(0,3) | Exclusive | 804 | f | {804} | {29066}
(0,4) | Exclusive | 804 | f | {804} | {29066}
(4 rows)