PostgreSQL provides you with highly advanced transaction machinery that offers countless features to developers and administrators alike. In this section, we will look at the basic concept of transactions.
The first important thing to know is that, 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
-------------------------------+-------------------------------
2020-08-13 11:03:17.741316+02 | 2020-08-13 11:03:17.741316+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 a part of the same transaction, the BEGIN statement must be used, as follows:
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
URL: https://www.postgresql.org/docs/13/sql-begin.html
The BEGIN statement will ensure that more than one command is packed into a transaction. Here is how it works:
test=# BEGIN;
BEGIN
test=*# SELECT now();
now
-------------------------------
2020-08-13 11:04:15.379104+02
(1 row)
test=*# SELECT now();
now
-------------------------------
2020-08-13 11:04:15.379104+02
(1 row)
test=*# COMMIT;
COMMIT
The important point here is that both timestamps will be identical. As we mentioned earlier, we are talking about transaction time.
To end the transaction, COMMIT can be used:
test=# \h COMMIT
Command: COMMIT
Description: commit the current transaction
Syntax:
COMMIT [ WORK | TRANSACTION ] [ AND [ NO ] CHAIN ]
URL: https://www.postgresql.org/docs/13/sql-commit.html
There are a few syntax elements here. You can just use COMMIT, COMMIT WORK, or COMMIT TRANSACTION. All three commands have the same meaning. If this is not enough, there's more:
test=# \h END
Command: END
Description: commit the current transaction
Syntax:
END [ WORK | TRANSACTION ] [ AND [ NO ] CHAIN ]
URL: https://www.postgresql.org/docs/13/sql-end.html
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, as shown in the following code:
test=# \h ROLLBACK
Command: ROLLBACK
Description: abort the current transaction
Syntax:
ROLLBACK [ WORK | TRANSACTION ] [ AND [ NO ] CHAIN ]
URL: https://www.postgresql.org/docs/13/sql-rollback.html
Some applications use ABORT instead of ROLLBACK. The meaning is the same. What was new in PostgreSQL 12 was the concept of a chained transaction. What is the point of all this? The following listing shows an example:
test=# SHOW transaction_read_only;
transaction_read_only
-----------------------
off
(1 row)
test=# BEGIN TRANSACTION READ ONLY ;
BEGIN
test=*# SELECT 1;
?column?
----------
1
(1 row)
test=*# COMMIT AND CHAIN;
COMMIT
test=*# SHOW transaction_read_only;
transaction_read_only
-----------------------
on
(1 row)
test=*# SELECT 1;
?column?
----------
1
(1 row)
test=*# COMMIT AND NO CHAIN;
COMMIT
test=# SHOW transaction_read_only;
transaction_read_only
-----------------------
off
(1 row)
test=# COMMIT;
WARNING: there is no transaction in progress
COMMIT
Let's go through this example step by step:
- Display the content of the transaction_read_only setting. It is off because, by default, we are in read/write mode.
- Start a read-only transaction using BEGIN. This will automatically adjust the transaction_read_only variable.
- Commit the transaction using AND CHAIN, then PostgreSQL will automatically start a new transaction featuring the same properties as the previous transaction.
In our example, we will also be in read-only mode, just like the transaction before. There is no need to explicitly open a new transaction and set whatever values again, which can dramatically reduce the number of roundtrips between application and server. If a transaction is committed normally (= NO CHAIN) the read-only attribute of the transaction will be gone.
Handling errors inside a transaction
It is not always the case that transactions are correct from beginning to end. Things might just go wrong for whatever reason. However, in PostgreSQL, only error-free transactions can be committed. The following listing shows a failing transaction, which errors out due to a division-by-zero error:
test=# BEGIN;
BEGIN
test=*# SELECT 1;
?column?
----------
1
(1 row)
test=*# SELECT 1 / 0;
ERROR: division by zero
test=!# SELECT 1;
ERROR: current transaction is aborted, commands ignored until end of transaction block
test=!# SELECT 1;
ERROR: current transaction is aborted, commands ignored until end of transaction block
test=!# COMMIT;
ROLLBACK
Note that division by zero did not work out.
It is important to point out that PostgreSQL will error out. After an error has occurred, no more instructions will be accepted, even if those instructions are semantically and syntactically correct. It is still possible to issue COMMIT. However, PostgreSQL will roll back the transaction because it is the only correct thing to be done at that point.
Making use of SAVEPOINT
In professional applications, it can be pretty hard to write reasonably long transactions without ever encountering a single error. To solve this problem, users can utilize something called SAVEPOINT. As the name indicates, a savepoint is a safe place inside a transaction that the application can return to if things go terribly wrong. Here is an example:
test=# BEGIN;
BEGIN
test=*# SELECT 1;
?column?
----------
1
(1 row)
test=*# SAVEPOINT a;
SAVEPOINT
test=*# SELECT 2 / 0;
ERROR: division by zero
test=!# SELECT 2;
ERROR: current transaction is aborted, commands ignored until end of transaction block
test=!# ROLLBACK TO SAVEPOINT a;
ROLLBACK
test=*# SELECT 3;
?column?
----------
3
(1 row)
test=*# COMMIT;
COMMIT
After the first SELECT clause, I decided to create a savepoint to make sure that the application can always return to this point inside the transaction. As you can see, the savepoint has a name, which is referred to later.
After returning to the savepoint called a, the transaction can proceed normally. The code has jumped back to before the error, so everything is fine.
The number of savepoints inside a transaction is practically unlimited. We have seen customers with over 250,000 savepoints in a single operation. PostgreSQL can easily handle this.
If you want to remove a savepoint from inside a transaction, there's the RELEASE SAVEPOINT command:
test=# \h RELEASE
Command: RELEASE SAVEPOINT
Description: destroy a previously defined savepoint
Syntax:
RELEASE [ SAVEPOINT ] savepoint_name
URL: https://www.postgresql.org/docs/13/sql-release-savepoint.html
Many people ask what will happen if you try to reach a savepoint after a transaction has ended. The answer is that the life of a savepoint ends as soon as the transaction ends. In other words, there is no way to return to a certain point in time after the transactions have been completed.
Transactional DDLs
PostgreSQL has a very nice feature that is unfortunately not present in many commercial database systems. In PostgreSQL, it is possible to run DDLs (commands that change the data's structure) inside a transaction block. In a typical commercial system, a DDL will implicitly commit the current transaction. This does not occur in PostgreSQL.
Apart from some minor exceptions (DROP DATABASE, CREATE TABLESPACE, DROP TABLESPACE, and so on), all DDLs in PostgreSQL are transactional, which is a huge advantage and a real benefit to end users.
Here is an example:
test=# BEGIN;
BEGIN
test=*# CREATE TABLE t_test (id int);
CREATE TABLE
test=*# ALTER TABLE t_test ALTER COLUMN id TYPE int8;
ALTER TABLE
test=*# \d t_test
Table "public.t_test"
Column | Type | Collation | Nullable | Default
--------+--------+-----------+----------+---------
id | bigint | | |
test=*# ROLLBACK;
ROLLBACK
test=# \d t_test
Did not find any relation named "t_test".
In this example, a table has been created and modified, and the entire transaction has been aborted. As you can see, there is no implicit COMMIT command or any other strange behavior. PostgreSQL simply acts as expected.
Transactional DDLs are especially important if you want to deploy software. Just imagine running a content management system (CMS). If a new version is released, you'll want to upgrade. Running the old version would still be OK; running the new version would also be OK, but you really don't want a mixture of old and new. Therefore, deploying an upgrade in a single transaction is highly beneficial as it upgrades an atomic operation.
