The pgrowlocks
module provides a function to show row
locking information for a specified table.
By default use is restricted to superusers, members of the
pg_stat_scan_tables
role, and users with
SELECT
permissions on the 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 shown in Table F.21.
Table F.21. 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 |
multi | boolean | True if locker is a multitransaction |
xids | xid[] | Transaction IDs of lockers (more than one if multitransaction) |
lock_type | text[] | Lock mode of lockers (more than one if multitransaction),
an array of Key Share , 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:
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 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