Demand locking with serial execution

Figure 10-4 illustrates how the demand lock scheme works for serial query execution. It shows four tasks with shared locks in the active lock position, meaning that all four tasks are currently reading the page. These tasks can access the same page simultaneously because they hold compatible locks. Two other tasks are in the queue waiting for locks on the page. Here is a series of events that could lead to the situation shown in Figure 10-4: