Transaction isolation limitations with in-memory tables
The SERIALIZABLE isolation level is not supported with M-tables
You cannot use in-memory tables in transactions where the transaction isolation level is SERIALIZABLE. The levels of transaction isolation that are supported for in-memory tables are REPEATABLE READ and READ COMMITTED.
For in-memory tables in the HotStandby secondary server, the transaction isolation level is always READ COMMITTED
If you are using HotStandby and you connected to the HotStandby secondary server, when you read data from in-memory tables, the transaction isolation level is automatically set to READ COMMITTED, even if you specified REPEATABLE READ.
REPEATABLE READ related differences between M-tables and D-tables
If you use in-memory tables and have set the transaction isolation level to REPEATABLE READ (default is READ COMMITTED), read operations block write operations for the duration of the read transaction. Moreover, with READ REPEATABLE isolation level, deadlocks are possible with in-memory tables whereas they cannot occur on versioning disk-based tables. On the other hand, concurrency conflicts can occur when optimistic concurrency control is in use.