Sql-server – SQL Server – NOLOCK vs. READONLY connections

It is not quite true that NOLOCK means placing no locks at all. Queries under this hint will still take Sch-S locks and (possibly HOBT locks).

Under read committed isolation level SQL Server will (usually) take row level S locks and release them as soon as the data is read. These are incompatible with the X locks held on uncommited updates and thus prevent dirty reads.

In the example in the linked answer the SELECT query is not blocked when it encounters a modified row so reading partial updates is quite likely.

It can also happen at default read committed isolation level too though that a SELECT reads some rows with the "before" value and others with the "after" value. It is just needed to engineer a situation where

1. Select query reads value of row R1 and releases its S lock
2. Update query updates R2 and takes an X lock
3. Select query tries to read R2 and is blocked.
4. Update query updates R1 and takes an X lock.
5. Update transaction commits thus releasing its locks and allowing the Select to read R2

This type of situation might arise for example if the SELECT and UPDATE are using different indexes to locate the rows of interest.

Example

CREATE TABLE T
(
X INT IDENTITY PRIMARY KEY,
Y AS -X UNIQUE,
Name varchar(10),
Filler char(4000) DEFAULT 'X'
)


INSERT INTO T (Name)
SELECT TOP 2500 'A'
FROM master..spt_values

Now in one query window run

DECLARE @Sum int

SELECT 'SET @@ROWCOUNT' WHERE 1=0

WHILE (@@ROWCOUNT = 0)
SELECT @Sum = SUM(LEN(Name))
FROM T 
WHERE Y IN (-1, -2500)
HAVING SUM(LEN(Name)) = 3

This will run in an infinite loop. In another run

UPDATE T 
SET Name=CASE WHEN Name = 'A' THEN 'AA' ELSE 'A' END

This will likely stop the loop in the other query (try again if not) meaning that it must have read either A,AA or AA,A

The table hint in the query takes precedence, as I would have expected.

Here's my test script:

CREATE TABLE t1 (a int);

INSERT INTO t1(a) SELECT TOP 1000 message_id FROM sys.messages;


SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

BEGIN TRANSACTION;

    SELECT * FROM t1 WITH(NOLOCK); /* Try it with and without the hint */

    EXEC sp_lock;

ROLLBACK;

With the NOLOCK hint included, no locks are taken by the SELECT statement.

In other words, setting the isolation level on the transaction makes that isolation level the default within the transaction, but that default can still be overridden by table hints, as I demonstrated.