Working with PostgreSQL transactions

PostgreSQL provides you with a highly advanced transaction machinery that offers countless features to developers and administrators alike. In this section, it is time to look at the basic concept.

The first important thing to know is this: in PostgreSQL, everything is a transaction. If you send a simple query to the server, it is already a transaction. Here is an example:

test=# SELECT now(), now();
             now              |             now               
------------------------------+------------------------------
 2016-08-30 12:03:27.84596+02 | 2016-08-30 12:03:27.84596+02
(1 row)

In this case, the SELECT statement will be a separate transaction. If the same command is executed again, different timestamps will be returned.

If more than one statement has to be part of the same transactions, the BEGIN clause must be used:

test=# h BEGIN 
Command:     BEGIN 
Description: start a transaction block 
Syntax: 
BEGIN [ WORK | TRANSACTION ] [ transaction_mode [, ...] ] 

where transaction_mode is one of: 

    ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ  
   | READ COMMITTED | READ UNCOMMITTED } 
    READ WRITE | READ ONLY 
    [ NOT ] DEFERRABLE 

The BEGIN clause will ensure that more than one command will be packed into a transaction. Here is how it works:

test=# BEGIN;
BEGIN 
test=# SELECT now(); 
              now               
------------------------------- 
 2016-08-30 12:13:54.839277+02 
(1 row) 
test=# SELECT now(); 
              now               
------------------------------- 
 2016-08-30 12:13:54.839277+02 
(1 row) 
test=# COMMIT; 
COMMIT 

The important point here is that both timestamps will be identical. As mentioned earlier, we are talking about transaction time here.

To end the transaction, COMMIT can be used:

test=# h COMMIT 
Command:     COMMIT 
Description: commit the current transaction 
Syntax: 
COMMIT [ WORK | TRANSACTION ] 

There are a couple of syntax elements here. You can just use COMMIT, COMMIT WORK, or COMMIT TRANSACTION. All three options have the same meaning. If this is not enough, there is more:

test=# h END 
Command:     END 
Description: commit the current transaction 
Syntax: 
END [ WORK | TRANSACTION ] 

The END clause is the same as the COMMIT clause.

ROLLBACK is the counterpart of COMMIT. Instead of successfully ending a transaction, it will simply stop the transaction without ever making things visible to other transactions:

test=# h ROLLBACK 
Command:     ROLLBACK 
Description: abort the current transaction 
Syntax: 
ROLLBACK [ WORK | TRANSACTION ] 

Some applications use ABORT instead of ROLLBACK. The meaning is the same.