What are you doing?

const Sequelize = require('sequelize');

async function run() {

    const s = new Sequelize('test', 'user', 'password', {
        host: 'mysql',
        port: '3606',
        dialect: 'mysql',
        pool: {
            max: 1,
            min: 1,
            acquire: 30000,
            idle: 10000,
            log: true
        }
    });

    return s.transaction({autocommit: false}).then((txn) => {
        return s.query('SELECT @@SESSION.autocommit', {transaction: txn}).spread((results, metadata) => {
            console.log(results);
        }).then(() => {
            return txn.commit();
        });
    }).then(() => {
        return s.query('SELECT @@SESSION.autocommit').spread((results, meta) => { console.log(results); });
    }).catch((err) => {
        console.log(err);
    });
}
run();

What do you expect to happen?

I expect the second SELECT statement to show autocommit=1.

What is actually happening?

autocommit=0. The 'SET AUTOCOMMIT = 0' is run inside of the transaction, but affects the session. So when the connection goes back into the pool it's in an unexpected state. Unless the application does all reads within an explicit transaction, it can lead to inconsistent reads.

Executing (b74d6416-e1ed-4e88-9a70-47f076e9bfc8): START TRANSACTION;
Executing (b74d6416-e1ed-4e88-9a70-47f076e9bfc8): SET autocommit = 0;
Executing (b74d6416-e1ed-4e88-9a70-47f076e9bfc8): SELECT @@SESSION.autocommit
[ TextRow { '@@SESSION.autocommit': 0 } ]
Executing (b74d6416-e1ed-4e88-9a70-47f076e9bfc8): COMMIT;
Executing (default): SELECT @@SESSION.autocommit
[ TextRow { '@@SESSION.autocommit': 0 } ]

Dialect: mysql
Dialect version: XXX
Database version: 5.6.34
Sequelize version: 4.38.0
Tested with latest release: Yes

Note : Your issue may be ignored OR closed by maintainers if it's not tested against latest version OR does not follow issue template.