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Transaction Isolation Levels (cont)

A PostgreSQL tx consists of a sequence of SQL statements: 

BEGIN S1; S2; ... Sn; COMMIT;

Isolation levels affect view of DB provided to each Si:

  • in read committed ...
    • each Si sees snapshot of DB at start of Si
  • in repeatable read and serializable ...
    • each Si sees snapshot of DB at start of tx
    • serializable checks for extra conditions
Transactions fail if the system detects violation of isolation level.