pgrowlocks

PostgreSQL 9.2.24 Documentation
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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-21 .

Table F-21. 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:

If the table as a whole is exclusive-locked by someone else, 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 (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

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