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F.28. pgrowlocks

F.28. pgrowlocks
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F.28.1. Overview
F.28.2. Sample Output
F.28.3. Author

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

By default use is restricted to superusers, members of the pg_stat_scan_tables role, and users with SELECT permissions on the 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.20 .

Table F.20. 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:

  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.

F.28.2. 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.28.3. Author

Tatsuo Ishii

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