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.
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.19.
Table F.19. 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 |
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 EXCLUSIVE lock is taken on the table, pgrowlocks will be blocked.
pgrowlocks is 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.
=# 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)
Tatsuo Ishii
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