Explicit locking

In addition to MVCC locking, one can control locking explicitly when MVCC does not provide a desirable behavior. Generally speaking, PostgreSQL provides three locking mechanisms, which are:

• Table-level locks
• Row-level locks 
• Advisory locks

Table-level locks 

Tables can be locked in several locking modes; the syntax of the LOCK statement is as follows:

LOCK [ TABLE ] [ ONLY ] name [ * ] [, ...] [ IN lockmode MODE ] [ NOWAIT ]

where lockmode is one of:

    ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
    | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE 

PostgreSQL locks the table implicitly when invoking an SQL command. It locks the table using the least restrictive mode to increase concurrency. When the developer desires a more restrictive lock, then the LOCK statement can be used. 

Table locking modes

Table locks are often acquired automatically, but they can also be acquired explicitly with the LOCK command. The following is the list of locking...