MySQL 5.7: Scope of SET TRANSACTION ISOLATION LEVEL for SELECT

isolation-levelMySQLoptimizationtransaction

The documentation states that without the GLOBAL or SESSION keyword, the isolation level will be valid for the next transaction and will be reverted to the session default after the transaction was commited.

With autocommit enabled, what is the scope of an transaction isolation level for a SELECT statement?

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SELECT * FROM mytable WHERE id = 1;

Is a SELECT statement a complete transaction, meaning the isolation level will be reverted to session default after the SELECT was executed, or does a SELECT somehow not represent its own transaction, meaning that the isolation level will not be restored after a SELECT statement?

Best Answer

(To answer question in comment)

There are three ways to build a transaction:

(1)

BEGIN;
any number of statements  -- all in one transaction
COMMIT;

(2)

SET AUTOCOMMIT = ON;
...
statement  -- This is a transaction
...
statement  -- This is a different transaction

(3)

SET AUTOCOMMIT = OFF;
...
statement
statement
COMMIT;  -- 2 statements in a transaction

statement
statement
COMMIT;  -- another 2 statements in a different transaction

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

Related Question