Quote from docs:
Read Committed is the default isolation level in PostgreSQL. When a transaction uses this isolation level, a SELECT query (without a FOR UPDATE/SHARE clause) sees only data committed before the query began; it never sees either uncommitted data or changes committed during query execution by concurrent transactions. In effect, a SELECT query sees a snapshot of the database as of the instant the query begins to run. However, SELECT does see the effects of previous updates executed within its own transaction, even though they are not yet committed. Also note that two successive SELECT commands can see different data, even though they are within a single transaction, if other transactions commit changes during execution of the first SELECT.
So does PostgreSQL see changes committed by another transactions or not?
Best Answer
The difference lies between a query and a transaction. A transaction can contain any number of queries. To illustrate the difference, I set up a small example:
Then run two transactions concurrently (issuing the commands one by one, the middle line wants to depict the timeline):
Running these in
READ COMMITTED
isolation level, the first query returns a row with 'old_value', while the second one shows a row with 'new_value'. On an other run, I change the left-hand-side transaction isolation level:(The command must be the first statement in a transaction.) Now both SELECTs return the same rowset, while a third one after committing both transactions will show the new row.