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SQL Anywhere 12.0.0 (Français) » SQL Anywhere Server - SQL Usage » Using transactions and isolation levels » Isolation levels and consistency


Typical types of inconsistency

There are three typical types of inconsistency that can occur during the execution of concurrent transactions. This list is not exhaustive as other types of inconsistencies can also occur. These three types are mentioned in the ISO SQL/2008 standard and are important because behavior at lower isolation levels is used to define them.

  • Dirty read   Transaction A modifies a row, but does not commit or roll back the change. Transaction B reads the modified row. Transaction A then either further changes the row before performing a COMMIT, or rolls back its modification. In either case, transaction B has seen the row in a state which was never committed.

    For more information about dirty reads, see Tutorial: Dirty reads.

  • Non-repeatable read   Transaction A reads a row. Transaction B then modifies or deletes the row and performs a COMMIT. If transaction A then attempts to read the same row again, the row will have been changed or deleted.

    For more information about non-repeatable reads, see Tutorial: Non-repeatable reads.

  • Phantom row   Transaction A reads a set of rows that satisfy some condition. Transaction B then executes an INSERT or an UPDATE on a row which did not previously meet A's condition. Transaction B commits these changes. These newly committed rows now satisfy Transaction A's condition. If Transaction A then repeats the read, it obtains the updated set of rows.

    For more information about phantom rows, see Tutorial: Phantom rows.

 Isolation levels and dirty reads, non-repeatable reads, and phantom rows

Cursor instability