“Introduction: Concurrency”

Allows multiple queries from multiple users to be executed in an efficient order.

• Not concurrent if queries are running sequentially

Concurency Problems

We will need to tackle the following problems when allowing concurrency.

• Inconsistent Read User reads in the middle of a transaction

• Lost Update

  • A conflict with update where multiple users update the record. Any update may have been rejected.

• Dirty Read

  • Reading transactions to be rejected

Confusion: Even with concurrency control, in using SQL interactively what happens when User A reads before and after User B changes the database