pgrowlocks

Postgres-XC 1.1 Documentation
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# F.27. pgrowlocks

> Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.

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

> Note: The following description applies only to Postgres-XC

Functions of this module returns information about connecting Coordinators locally. To get information of a Datanode, you can connect to the Datanode using psql directly. In this case, statements will be handled by local transaction control without GTM, you will have warnings and the visibility could be somewhat inconsistent.

## F.27.1. Overview

    pgrowlocks(text) returns setof record

> Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.

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 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:

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.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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