postgres/contrib/pgrowlocks/README.pgrowlocks

$PostgreSQL: pgsql/contrib/pgrowlocks/README.pgrowlocks,v 1.1 2006/04/23 01:12:58 ishii Exp $

pgrowlocks README			Tatsuo Ishii

1. What is pgrowlocks?

   pgrowlocks shows row locking information for specified table.

   pgrowlocks returns following data type:

CREATE TYPE pgrowlocks_type AS (
	locked_row TID,		-- row TID
	lock_type TEXT,		-- lock type
	locker XID,		-- locking XID
	multi bool,		-- multi XID?
	xids xid[],		-- multi XIDs
	pids INTEGER[]		-- locker's process id
);

  Here is a sample execution of pgrowlocks:

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)

  locked_row		-- tuple ID(TID) of each locked rows
  lock_type		-- "Shared" for shared lock, "Exclusive" for exclusive lock
  locker		-- transaction ID of locker (note 1)
  multi			-- "t" if locker is a multi transaction, otherwise "f"
  xids			-- XIDs of lockers (note 2)
  pids			-- process ids of locking backends

  note1: if the locker is multi transaction, it represents the multi ID

  note2: if the locker is multi, multiple data are shown

2. Installing pgrowlocks

   Installing pgrowlocks requires PostgreSQL 8.0 or later source tree.

      $ cd /usr/local/src/postgresql-8.1/contrib
      $ tar xfz /tmp/pgrowlocks-1.0.tar.gz

   If you are using PostgreSQL 8.0, you need to modify pgrowlocks source code.
   Around line 61, you will see:

      #undef MAKERANGEVARFROMNAMELIST_HAS_TWO_ARGS

   change this to:

      #define MAKERANGEVARFROMNAMELIST_HAS_TWO_ARGS

      $ make
      $ make install

      $ psql -e -f pgrowlocks.sql test

3. How to use pgrowlocks

   The calling sequence for pgrowlocks is as follows:

   CREATE OR REPLACE FUNCTION pgrowlocks(text) RETURNS pgrowlocks_type
     AS 'MODULE_PATHNAME', 'pgrowlocks'
     LANGUAGE 'c' WITH (isstrict);

   The parameter is a name of table. pgrowlocks returns type pgrowlocks_type.

   pgrowlocks grab AccessShareLock for the target table and read each
   row one by one to get the row locking information. You should
   notice that:

   1) if the table is exclusive locked by someone else, pgrowlocks
      will be blocked.

   2) pgrowlocks may show incorrect information if there's a new
      lock or a lock is freeed while 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;


4. License

   pgrowlocks is distribute under (modified) BSD license described in
   the source file.

5. History

   2006/03/21 pgrowlocks version 1.1 released (tested on 8.2 current)
   2005/08/22 pgrowlocks version 1.0 released