Question

Does MVCC database isolation mode allow in-progress transactions to see rows inserted (and committed) by other transactions?

For example, given:

• Table names[id BIGINT NOT NULL, name VARCHAR(30), PRIMARY KEY(id), UNIQUE(name)]
• Transactions T1 and T2,

T1: open transaction
T2: open transaction
T1: select * from names;
    insert into names(name) values("John");
    // do something
    commit;
T2: select * from names;
    insert into names values("John");
    // do something
    commit;

When does T2 first become aware of the new row? At select time? At insert time? Or at commit time?

Answer 1

Answer really depends on server implementation and whether unique constraint is marked deferrable or not.

I have not tested it for other databases, but in PostgreSQL (as one of most prominent open-source MVCC databases) in my test replicating your setup T2 fails on INSERT. However, T2 cannot see any changes made by T1 by using SELECT.

I have executed following statements almost at the same time in 2 separate SQL connections:

BEGIN;
SELECT * FROM names;
SELECT pg_sleep(10);
INSERT INTO names values('john');
SELECT pg_sleep(10);
COMMIT;

One succeeded, but another failed after 10 seconds with:

ERROR:  duplicate key value violates unique constraint "names_pkey"
DETAIL:  Key (name)=(john) already exists.

This makes sense, because documentation says:

If a conflicting row has been inserted by an as-yet-uncommitted
transaction, the would-be inserter must wait to see if that
transaction commits. If it rolls back then there is no conflict. If it
commits without deleting the conflicting row again, there is a
uniqueness violation.

If, however, unique constraint was marked deferrable, uniqueness will be checked at COMMIT time:

If the unique constraint is deferrable, there is additional
complexity: we need to be able to insert an index entry for a new row,
but defer any uniqueness-violation error until end of statement or
even later.