Consider a relaxation of two-phase locks in which read only transactions can release read locks early. Would a read only transaction have consistent retrievals? Would the objects become inconsistent? Illustrate your answer with the following transactions T and U at the server

Question

T: x = read (i); y= read(j);

U: write(i, 55);write(j, 66);

in which initial values of ai and aj are 10 and 20.

Answer 1

There is no guarantee of consistent retrievals because overlapping transactions can alter the objects after they are unlocked.

The database does not become inconsistent.





In the above example T is read only and conflicts with U in access to ai and aj. ai is accessed by T before U and aj by U before T. The interleavings are not serially equivalent. The values observed by T are x=10, y= 66, and the values of the objects at the end are ai=55, aj= 66.



Serial executions give either (T before U) x=10, y=20, ai=55, aj=66; or (U before T) x=55, y=66, ai=55, aj=66). This confirms that retrievals are inconsistent but that the database does not become inconsistent.