Postgresql – isolation level of autocommit

auto-commitisolation-levelpostgresqltransaction

I'm trying to make sense from "autocommit" concept.

Originally I've heart about it in the context of Java JDBC API Connection.setAutoCommit(boolean autoCommit)

Quick search revealed that the term is not specific to JDBC, PostgreSQL refer to "autocommit" too:

https://www.postgresql.org/docs/current/sql-start-transaction.html

In the standard, it is not necessary to issue START TRANSACTION to start a transaction block: any SQL command implicitly begins a block. PostgreSQL's behavior can be seen as implicitly issuing a COMMIT after each command that does not follow START TRANSACTION (or BEGIN), and it is therefore often called “autocommit”. Other relational database systems might offer an autocommit feature as a convenience.

If it is widespread what is isolation level of autocommit mode?

Conceptually as we have only one SQL statement before implicit COMMIT isolation levels almost make no sense for autocommit. Only READ UNCOMMITTED might influence the result, other anomalies are defined in term of a sequence of SQL statements.

Can I alter autocommit isolation level to / away from READ UNCOMMITTED?

Best Answer

The default isolation level is controlled through default_transaction_isolation and is read committed in a standard installation.

You can check your current setting using:

show default_transaction_isolation;

As Postgres does not support (or implement) read uncommitted there is no need to change this "away from read uncommitted".

If you want to change the isolation level to "higher" one, even in autocommit mode, you can use:

set default_transaction_isolation = serializable;

The above will change the default for the current session (=connection).

Related Solutions

Sql-server – SQL Server – what isolation level for non-blocking select statements

In an ideal world you would have two choices, SNAPSHOT and READ COMMITTED SNAPSHOT (RCSI). Make sure you understand the basics of transaction isolation levels before you decide which is appropriate for your workload. Specifically be aware of the different results you may see as a result of moving to RCSI.

This sounds like it's not an ideal world as you don't have any control over the application that is generating the select statements. In that case, your only option is to enable RCSI for the database in question such that the selects will automatically use RCSI instead of READ COMMITTED.

Postgresql – Read Committed Isolation Level

The difference lies between a query and a transaction. A transaction can contain any number of queries. To illustrate the difference, I set up a small example:

CREATE TABLE table_to_be_updated (
    id serial PRIMARY KEY,
    other_column text,
    column_changing text
);

INSERT INTO table_to_be_updated (other_column, column_changing)
VALUES
('value', 'old_value'),
('value', 'other_value'),
('nonvalue', 'doesnt matter');

Then run two transactions concurrently (issuing the commands one by one, the middle line wants to depict the timeline):

                                | <-- BEGIN;
                                |
                                |
                                |     UPDATE table_to_be_updated
BEGIN; -----------------------> |     SET column_changing = 'new_value'
                                |     WHERE
                                |         other_column = 'value' AND
                                |         column_changing = 'old_value';
                                |
                                |
SELECT column_changing -------> |     -- update not yet committed
FROM table_to_be_updated        |
WHERE other_column = 'value';   | <-- COMMIT;
                                |
                                |
SELECT column_changing -------> |
FROM table_to_be_updated        |
WHERE other_column = 'value';   |
                                |
                                |
COMMIT; ----------------------> |

Running these in READ COMMITTED isolation level, the first query returns a row with 'old_value', while the second one shows a row with 'new_value'. On an other run, I change the left-hand-side transaction isolation level:

SET transaction ISOLATION LEVEL REPEATABLE READ;

(The command must be the first statement in a transaction.) Now both SELECTs return the same rowset, while a third one after committing both transactions will show the new row.

Best Answer

Related Solutions

Sql-server – SQL Server – what isolation level for non-blocking select statements

Postgresql – Read Committed Isolation Level

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