为什么PostgreSQL会中止这个可序列化的计划?

T1: r1x   w1y  c1
T2:    w2x   c2

(i.e., T1 reads x, T2 writes x, T1 writes y, T2 commits, and finally, T1 commits)

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