Theory states that a group of concurrent transactions is serializable if, and only if, their concurrent execution is equivalent to one of their possible serial executions.
Now the following concurrent execution of transactions T1 and T2 is serializable, because it is equivalent to the serial execution "T1 then T2"
T1: r1x w1y c1 T2: w2x c2 (i.e., T1 reads x, T2 writes x, T1 writes y, T2 commits, and finally, T1 commits)
However, when tried in PostgreSQL 10.4, like this:
T1: begin T1: set transaction isolation level serializable; T2: begin T2: set transaction isolation level serializable; T2: update variables set value = value + 1 where name = 'x' T1: update variables set value = value + 1 where name = 'y' T2: commit T1: commit
the database aborts T1 when this transaction tries to commit. Why?