Week 09 (72)

Example of repeatable read vs serializable

table R(class,value) containing (1,10) (1,20) (2,100) (2,200)
T1: X = sum(value) where class=1; insert R(2,X); commit
T2: X = sum(value) where class=2; insert R(1,X); commit
with repeatable read, both transactions commit, giving
- updated table: (1,10) (1,20) (2,100) (2,200) (1,300) (2,30)
with serial transactions, only one transaction commits
- T1;T2 gives (1,10) (1,20) (2,100) (2,200) (2,30) (1,330)
- T2;T1 gives (1,10) (1,20) (2,100) (2,200) (1,300) (2,330)
PG recognises that committing both gives serialization violation