Hello,I'm trying to understand concurrency in PostgreSQL so I'm slowly reading through chapter 13 of the fine manual and I believe I've found a contradiction in section 13.2.1.My understanding of the second sentence of the first paragraph is that read committed mode never sees "changes committed during query execution by concurrent transactions". For example let's assume two transactions, A & B, and the following:
- A started before B
- B starts before A commits
My understanding of the second sentence means that if A commits before B then any updates made by A will continue to be invisible to B because B's snapshot was before A committed. Now if I'm wrong about this then there is no contradiction forthcoming.
The final sentence of the first paragraph is where I find the contradiction. It says: "Also note that two successive SELECT commands can see different data, even though they are within a single transaction, if other transactions commit changes after the first SELECT starts and before the second SELECT starts".So the mental model I've built based on the first four sentences of the first paragraph is that when a transaction starts in read committed mode a snapshot is taken of the (database) universe as it exists at the moment of its creation and that it's only updated by changes made by the transaction that created the snapshot. So for successive SELECTs to see different data because of updates outside of the transaction that created the snapshot is a contradiction.Now my guess is that I'm thinking about it all wrong so if someone in the know could shed some light on where/how my mental model breaks down I would appreciate it.Regards,
Dane
The main thing to remember is that "query != transaction".
A1 - BEGIN;
A1 - SELECT FROM a
B1 - BEGIN;
B2 - UPDATE a
B3 - COMMIT;
A2 - SELECT FROM a - again
A3 - COMMIT;
Since the commit in B3 occurs before the second select A2 in READ COMMITTED the query A2 will see the update made in B2. But B3 must complete in its entirety for A2 to see it otherwise "it never sees [...] changes committed during query execution by concurrent transactions". The concurrency is with the individual statement A2 and not the entire A transaction. This is why it is called "READ COMMITTED" because within transaction A externally committed data is able to be read.
David J.
