F.29. pgrowlocks — show a table's row locking information
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## F.29. pgrowlocks — show a table's row locking information #

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.

### F.29.1. 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; see Section 66.1
`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 `For Key Share`, `For 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.29.2. 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.29.3. Author #

Tatsuo Ishii

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