Tell me about transaction isolation levels. How many are there and what is the default in PostgreSQL?

There are four main levels of isolation in transactions defined by the SQL standard:

1. Read Uncommitted — allows reading data that has not yet been committed (dirty read).
2. Read Committed — only allows reading committed data, preventing dirty reads.
3. Repeatable Read — guarantees that repeated reads of the same data within a transaction will be identical, preventing non-repeatable reads.
4. Serializable — the strictest level, providing full transaction isolation as if transactions were executed sequentially.

In PostgreSQL, the default level is Read Committed. This means that a transaction sees only data that was committed before the current read operation started, but it does not guarantee repeatability of reads within the entire transaction.