pgrowlocks

PostgreSQL 9.1.2 Documentation
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# F.30. pgrowlocks

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

## F.30.1. 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 multi-transaction
`multi` | `boolean` | True if locker is a multi-transaction
`xids` | `xid[]` | Transaction IDs of lockers (more than one if multi-transaction)
`pids` | `integer[]` | Process IDs of locking backends (more than one if multi-transaction)

`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.30.2. 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.30.3. Author

Tatsuo Ishii

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