F.28. pgrowlocks
The pgrowlocks module provides a function to show row locking information for a specified table.
F.28.1. 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-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) |
| lock_type | 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 the table as a whole is exclusive-locked by someone else,
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.28.2. Sample Output
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)
F.28.3. Author
Tatsuo Ishii