Oracle DB Transactions — Read Skew
Nov 15, 2020
Transaction T1 reads x, and then a second transaction T2 updates x and y to new values and commits . If now T1 reads y, it may see an inconsistent state, and therefore produce an inconsistent state as output.
r1[x]…w2[x]…w2[y]…c2…r1[y]…(c1 or a1)
💡 How to read the shorthand notations? w means write, r means read, number(1 or 2) indicates transaction id, c means commit and a means abort/rollback
Example
Read skew is avoided in Oracle DB, provided there is a parent-child relationship between the tables which were read at different point-in-time.
💡Serializable isolation level in Oracle is actually a “Snapshot Isolation”
Transaction 1
Transaction 2
