Sql isolation levels, Read and Write locks

Question

A bit lame question but I got confused... Difference between isolation levels as far as I understood is how they managed their locks (http://en.wikipedia.org/wiki/Isolation_(database_systems)). So as mentioned in the article there are Read, Write and Range locks but there is no definition what they are itself. What are you allowed to do and what not. When I googled for it there was nothing concrete and instead I got confused with new terms like Pessimistic Lock an Optimistic Lock, Exclusive lock, Gap lock and so on. I'd be pleased if someone give me a short overview and maybe point me a good bunch materials to enlighten myself.

My initial question which started the research of isolation levels was:

What happens when I have concurrent inserts (different users of web app) into one table when my transactions isolation level is READ_COMMITED. Is the whole table locked or not? Or generally what happens down there :) ?

Thanks in advance !

Answer 1

What happens when I have concurrent inserts (different users of web app) into one table when my transactions isolation level is READ_COMMITED.

"Read committed" means that other sessions cannot see the newly inserted row until its transaction is committed. A SQL statement that runs without an explicit transaction is wrapped in an implicit one, so "read committed" affects all inserts.

Some databases implement "read committed" with locks. For example, a read lock can be placed on the inserted row, preventing other tractions from reading it. Other databases, like Oracle, use multiversion concurrency control. That means they can represent a version of the database before the insert. This allows them to implement "read committed" without locks.