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Transaction Isolation

Updated at: Mar 13, 2020 GMT+08:00

DWS manages transactions based on MVCC and two-phase locks, avoiding conflicts between read and write operations. SELECT is a read-only operation, whereas UPDATE and DELETE are read/write operations.

  • There is no conflict between read/write and read-only operations, or between read/write operations. Each concurrent transaction creates a snapshot when it starts. Concurrent transactions cannot detect updates made by each other.
    • At the READ COMMITTED isolation level, if transaction T1 is committed, transaction T2 can see changes made by T1.
    • At the REPEATABLE READ level, if T2 starts before T1 is committed, T2 will not see changes made by T1 even after T1 is committed. The query results in a transaction are consistent and unaffected by other transactions.
  • Read/Write operations use row-level locks. Different transactions can concurrently update the same table but not the same row. A row update transaction will start only after the previous one is committed.
    • READ COMMITTED: At this level, a transaction can access only committed data. This is the default level.
    • REPEATABLE READ: Only the data committed before transaction start is read. Uncommitted data or data committed in other concurrent transactions cannot be read.

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