concurrent read level

Assume we have 2 transactions execute concurrently.

possible problem

dirty read

Transaction 1 reads uncommitted data from transaction 2. Uncommitted means these data could be removed or changed somehow, that results into read inconsistency.

-	A	B
1	begin	begin
2	read
3		update
4	commit
5		rollback

Session A unnecessarily read data from B.

Non-repeatable reads

Read operation within one single transaction must stay same even if another transaction is updating data.
Transaction 1 reads different data from rows as transaction 2 updates and commits those rows before.

-	A	B
1	begin
2	read
3		update
4	read
5	commit

Session A read 2 times, each read different data within same transaction.

Phantom reads

Transaction 1 reads different number of row because transaction 2 inserts or deletes on this table

-	A	B
1	begin
2	read
3		insert
4	read
5	commit

Session A read 2 times in a transaction, but get different number of row in result

method to deal

Isolation Level	Dirty Read	Nonrepeatable Read	Phantom Read	Serialization Anomaly
Read uncommitted	Allowed, but not in PG	Possible	Possible	Possible
Read committed	Not possible	Possible	Possible	Possible
Repeatable read	Not possible	Not possible	Allowed, but not in PG	Possible
Serializable	Not possible	Not possible	Not possible	Not possible