Re: pg_locks "at-a-glance" view

[David Fetter <david@xxxxxxxxxx>]

On Wed, Jun 18, 2008 at 05:39:59PM -0700, Jeff Davis wrote:
> I was trying to create a more "at-a-glance" view of the pg_locks table.
> I included the SQL I came up with (after talking to Merlin) at the
> bottom of this message.
> 
> The idea is to show any queries that are waiting on a lock, and the
> query that currently holds the lock on which those queries are waiting.
> 
> Is my logic correct?

I'm not exactly sure, but it appears to match, at first blush, what's
in src/backend/storage/lmgr/lock.c:
    static const LOCKMASK LockConflicts[] = {

> Does anyone have any suggestions?

The function could be in SQL, with one minor bit in PL/PgSQL.  File
attached.

> I couldn't find a generally accepted way to do this, although I'm sure
> someone must have done something like this before.

There's stuff in the aforementioned lock.c, but I don't see anything
visible to SQL.

Cheers,
David.
-- 
David Fetter <david@xxxxxxxxxx> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david.fetter@xxxxxxxxx

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CREATE OR REPLACE FUNCTION raise_exception(in_message TEXT)
RETURNS BOOLEAN
LANGUAGE plpgsql
AS $$
BEGIN
    RAISE EXCEPTION '%', in_message;
    RETURN true;
END;
$$;

CREATE OR REPLACE FUNCTION lock_conflict(TEXT, TEXT)
RETURNS BOOLEAN
STRICT
LANGUAGE SQL
AS $$
SELECT
  CASE
  WHEN $1 NOT IN (
    'AccessShareLock', 'RowShareLock', 'RowExclusiveLock',
    'ShareUpdateExclusiveLock', 'ShareLock', 'ShareRowExclusiveLock',
    'ExclusiveLock', 'AccessExclusiveLock'
    )
  OR $2 NOT IN (
    'AccessShareLock', 'RowShareLock', 'RowExclusiveLock',
    'ShareUpdateExclusiveLock', 'ShareLock', 'ShareRowExclusiveLock',
    'ExclusiveLock', 'AccessExclusiveLock'
    ) THEN
    raise_exception('Both arguments must be valid lock names.')
  WHEN $1 = 'AccessShareLock' THEN
    CASE WHEN $2 = 'AccessExclusiveLock' THEN
         TRUE
    ELSE
         FALSE
    END
  WHEN $1 = 'RowShareLock' THEN
    CASE WHEN $2 = 'ExclusiveLock' OR
       $2 = 'AccessExclusiveLock' THEN
         TRUE
    ELSE
         FALSE
    END
  WHEN $1 = 'RowExclusiveLock' THEN
    CASE WHEN $2 = 'ShareLock' OR
       $2 = 'ShareRowExclusiveLock' OR
       $2 = 'ExclusiveLock' OR
       $2 = 'AccessExclusiveLock' THEN
         TRUE
    ELSE
         FALSE
    END
  WHEN $1 = 'ShareUpdateExclusiveLock' THEN
    CASE WHEN $2 = 'ShareUpdateExclusiveLock' OR
       $2 = 'ShareLock' OR
       $2 = 'ShareRowExclusiveLock' OR
       $2 = 'ExclusiveLock' OR
       $2 = 'AccessExclusiveLock' THEN
         TRUE
    ELSE
         FALSE
    END
  WHEN $1 = 'ShareLock' THEN
    CASE WHEN $2 = 'RowExclusiveLock' OR
       $2 = 'ShareUpdateExclusiveLock' OR
       $2 = 'ShareRowExclusiveLock' OR
       $2 = 'ExclusiveLock' OR
       $2 = 'AccessExclusiveLock' THEN
         TRUE
    ELSE
         FALSE
    END
  WHEN $1 = 'ShareRowExclusiveLock' THEN
    CASE WHEN $2 = 'RowExclusiveLock' OR
       $2 = 'ShareUpdateExclusiveLock' OR
       $2 = 'ShareLock' OR
       $2 = 'ShareRowExclusiveLock' OR
       $2 = 'ExclusiveLock' OR
       $2 = 'AccessExclusiveLock' THEN
         TRUE
    ELSE
         FALSE
    END
  WHEN $1 = 'ExclusiveLock' THEN
    CASE WHEN $2 = 'RowShareLock' OR 
       $2 = 'RowExclusiveLock' OR
       $2 = 'ShareUpdateExclusiveLock' OR
       $2 = 'ShareLock' OR
       $2 = 'ShareRowExclusiveLock' OR
       $2 = 'ExclusiveLock' OR
       $2 = 'AccessExclusiveLock' THEN
         TRUE
    ELSE
         FALSE
    END
  WHEN $1 = 'AccessExclusiveLock' THEN
    TRUE
  END
$$;