pgrowlocks — show a table's row locking information

pgrowlocks show a table's row locking information#

pgrowlocks

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.

Overview#

pgrowlocks

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 Output Columns.

pgrowlocks Output Columns {#pgrowlocks-columns}

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 an ACCESS EXCLUSIVE lock is taken on the table, 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 such a query will be very inefficient.

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)

Author#

Tatsuo Ishii