Hi, On 2023-11-28 20:52:31 +0800, Jingxian Li wrote: > postgres=*# lock table test in exclusive mode ; > > > T4 > > Case 1: > > postgres=*# lock table test in share row exclusive mode nowait; > > ERROR: could not obtain lock on relation > "test" > > -------------------------------------------- > > Case 2: > > postgres=*# lock table test in share row exclusive mode; > > LOCK TABLE > > > > > At T4 moment in session A, (case 1) when executing SQL “lock table test in > share row exclusive mode nowait;”, an error occurs with message “could not > obtain lock on relation test";However, (case 2) when executing the SQL above > without nowait, lock can be obtained successfully. > > Digging into the source code, I find that in case 2 the lock was obtained in > the function ProcSleep instead of LockAcquireExtended. Due to nowait logic > processed before WaitOnLock->ProcSleep, acquiring lock failed in case > 1. Can any changes be made so that the act of such lock granted occurs > before WaitOnLock?
I don't think that'd make sense - lock reordering is done to prevent deadlocks and is quite expensive. Why should NOWAIT incur that cost? > > > Providing a more universal case: > > Transaction A already holds an n-mode lock on table test. If then transaction > A requests an m-mode lock on table Test, m and n have the following > constraints: > > (lockMethodTable->conflictTab[n] & lockMethodTable->conflictTab[m]) > == lockMethodTable->conflictTab[m] > > Obviously, in this case, m<=n. > > Should the m-mode lock be granted before WaitOnLock? > > > > In the case of m=n (i.e. we already hold the lock), the m-mode lock is > immediately granted in the LocalLock path, without the need of lock conflict > check. Sure - it'd not help anybody to wait for a lock we already hold - in fact it'd create a lot of deadlocks. > Based on the facts above, can we obtain a weaker lock (m<n) on the same > object within the same transaction without doing lock conflict check? Perhaps. There's no inherent "lock strength" ordering for all locks though. Greetings, Andres Freund
