| PostgreSQL 8.3.0 Documentation | ||||
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The pgrowlocks module provides a function to show row locking information for a specified table.
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:
Table F-25. 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:
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.
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)
Tatsuo Ishii