F.27. pgrowlocks

The pgrowlocks module provides a function to show row locking information for a specified table.

F.27.1. 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
lock_type text Shared for shared lock, or Exclusive for exclusive lock
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)
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:

  1. If the table as a whole is exclusive-locked by someone else, pgrowlocks will be blocked.

  2. 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 (as of PostgreSQL 8.3) such a query will be very inefficient.

F.27.2. 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.27.3. Author

Tatsuo Ishii

copyright  ©  April 19 2014 sean dreilinger url: http://durak.org/sean/pubs/software/postgresql/pgrowlocks.html