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...