Mysql – Why does START TRANSACTION WITH CONSISTENT SNAPSHOT not work with SERIALIZABLE isolation level

Every connection will work with its own transaction isolation level. Setting SERIALIZABLE level for one connection doesn't change isolation level of other connections (new or existing).

However interactions between transactions performed on different isolation level are quite complicated. Everything depends on many factors:

• If different transactions operate on the same table/tables?
• How your database engine implements transaction isolation levels (pessimistics versus optimistics locking)?
• What is characteristics of access from your app to particular tables?

If you have to work with SERIALIZABLE level for some connections in your app you should know answers for above questions.

For example. If you have database with pessimistics locking (e.g. IBM DB2) and you have long running transactions with SERIALIZABLE level it is very probable that your system would hang (or slow down).

From the MySQL Documentation

The type of read varies for selects in clauses like INSERT INTO ... SELECT, UPDATE ... (SELECT), and CREATE TABLE ... SELECT that do not specify FOR UPDATE or LOCK IN SHARE MODE:

By default, InnoDB uses stronger locks and the SELECT part acts like READ COMMITTED, where each consistent read, even within the same transaction, sets and reads its own fresh snapshot.

To use a consistent read in such cases, enable the innodb_locks_unsafe_for_binlog option and set the isolation level of the transaction to READ UNCOMMITTED, READ COMMITTED, or REPEATABLE READ (that is, anything other than SERIALIZABLE). In this case, no locks are set on rows read from the selected table.

Evidently, you do not need stronger locks when just doing reads. Picture copying data into a table using INSERT ... SELECT or CREATE TABLE ... SELECT. The SELECT needs to be a frozen snapshot for loading the table. If it were a moving target, that would require a transaction within a transaction. If you wanted transaction within a transaction behavior, you would need to script that using SAVEPOINT. Otherwise, stronger locks allow for consistent SELECTs for an atomic INSERT.

Doing the simple SELECT allows your DB Session to trust other transactions not to play smoke-and-mirrors with your view of the data. If it has to violate that trust, it will politely lock every row it can to do its work, rather than deceive DB Sessions that are only doing SELECTs into thinking the data is stable when it is, in fact, changing.

No matter which isolation level you pick, none of them is so granular that a simple SELECT gets held up. That's why SELECT ... FOR UPDATE and SELECT ... FROM ... LOCK IN SHARE MODE were invented, so you can be that granular if politeless is not an option.

I wrote about this before : How can I make a select statement get blocked?