Lock conflict

begin;
select * from tb;

alter table tb add c int;

exec_simple_query
    PortalRun | PortalRunMulti | PortalRunUtility | ProcessUtility
        standard_ProcessUtility | ProcessUtilitySlow
            AlterTableLookupRelation | RangeVarGetRelidExtended
                LockRelationOid | LockAcquireExtended | WaitOnLock
                    ProcSleep
                        WaitLatch | WaitEventSetWait /* src/backend/storage/ipc/latch.c */
                            WaitEventSetWaitBlock

进程A（持锁）        进程B（请求锁）

LockAcquire
                     LockAcquire
                     → 冲突
                     → 加入 wait queue
                     → ProcSleep（睡眠）

LockRelease
→ ProcLockWakeup
→ SetLatch(B)

                     被唤醒
                     → 重新检查
                     → 获取锁成功

PROC A (持有锁)	PROC B(请求锁)
LockAcquire
	LockAcquire -> 冲突 -> 加入 wait queue -> ProcSleep
LockRelease -> ProcLockWakeup -> SetLatch(B)
	被唤醒 -> 重新检查 -> 获取锁