F.31. pgrowlocks - show a table's row locking information
The
pgrowlocks
module provides a function to show row
locking information for a specified table.
By default use is restricted to superusers, roles with privileges of the
pg_stat_scan_tables
role, and users with
SELECT
permissions on the table.
F.31.1. Overview #
pgrowlocks(text) returns setof record
The parameter is the name of a table. The result is a set of records, with one row for each locked row within the table. The output columns are shown in Table F.21 .
Table F.21.
pgrowlocks
Output Columns
| Name | Type | Description |
|---|---|---|
locked_row
|
tid
|
Tuple ID (TID) of locked row |
locker
|
xid
|
Transaction ID of locker, or multixact ID if multitransaction; see Section 74.1 |
multi
|
boolean
|
True if locker is a multitransaction |
xids
|
xid[]
|
Transaction IDs of lockers (more than one if multitransaction) |
modes
|
text[]
|
Lock mode of lockers (more than one if multitransaction),
an array of
Key Share
,
Share
,
For No Key Update
,
No Key Update
,
For Update
,
Update
.
|
pids
|
integer[]
|
Process IDs of locking backends (more than one if multitransaction) |
pgrowlocks
takes
AccessShareLock
for the
target table and reads each row one by one to collect the row locking
information. This is not very speedy for a large table. Note that:
-
If an
ACCESS EXCLUSIVElock is taken on the table,pgrowlockswill be blocked. -
pgrowlocksis not guaranteed to produce a self-consistent snapshot. It is possible that a new row lock is taken, or an old lock is freed, during 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;
Be aware however that such a query will be very inefficient.
F.31.2. Sample Output #
=# SELECT * FROM pgrowlocks('t1');
locked_row | locker | multi | xids | modes | pids
------------+--------+-------+-------+----------------+--------
(0,1) | 609 | f | {609} | {"For Share"} | {3161}
(0,2) | 609 | f | {609} | {"For Share"} | {3161}
(0,3) | 607 | f | {607} | {"For Update"} | {3107}
(0,4) | 607 | f | {607} | {"For Update"} | {3107}
(4 rows)
F.31.3. Author #
Tatsuo Ishii