Qualcuno può spiegare il motivo per cui selezionare con nolock interroga una pozione di dati aggiornati?

Question

Stavo leggendo la risposta da qui (da StackOverflow, credo che dovrebbe chiedere qui)

mezzi NOLOCK immissione serrature a tutti.

La vostra query potrebbe restituisce porzioni di dati come di prima dell'aggiornamento e porzioni di come dopo UPDATE in una singola query.

Mi capita che nolock non sarà posto di blocco al tavolo, in modo che altre persone possano interrogare contemporaneamente.

Dalla risposta e l'esempio è mostrare, è recuperare i dati, mentre i dati vengono aggiornamento.

Perché succede questo?

Io parto dal presupposto per il normale selezionarlo cercherà posto di blocco sul tavolo, in modo che quando si esegue un'istruzione di aggiornamento, è mettere un lucchetto alla riga o pagina. Poi, quando provo a fare funzionare dichiarazione prescelta, non può mettere il blocco fino a quando il blocco istruzione di aggiornamento viene rilasciato.

Ma in questo caso perché l'istruzione select non cerca di mettere blocco sul tavolo, in modo che possa funzionare senza attendere la dichiarazione di aggiornamento rilasciare il blocco?

Answer 1

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

Answer 2

The hint NOLOCK is equivalent to the transaction isolation level READ UNCOMMITTED, just restricted to the scope of one table access method.

What RU does that makes the non-committed appear on your resultset? Hmm, actually is a question of "what it not does. I'll explain below.

Well (this is a gross simplification, I know) MSSQL (in its' default behavior) is a lock engine - which means that it uses lock to read/write data on a consistent manner. In this oversimplified explanation, MSSQL uses two kinds of locks: shared lock and exclusive lock.

An shared(S) lock is a lock that allow an resource (which can be a row, page of rows or even an entire table) to be read - but not allows an write to it. So if transaction T1 puts a S lock on R1 row, all transactions that tries to read R1 will get that read, but while the S lock is alive nobody can write to R1.

An exclusive(X) lock is the counterpart of the shared lock. It allows exclusive access to a resource - no other transaction can read or write except the one that got the X lock. In the above example, if T1 got not an S lock but an X lock on R1, no one except T1 can read or write it.

That's the teory. The isolation levels honor the locks, and respect their prevalence and characteristics. All, except READ UNCOMMITTED. It simple gives a * (put your bad mouth word of your preference here) to the locks regarding the reading - you still cannot update the row another transaction got an X lock. It simply says: "I'll read everything that's relevant to the query plan - disregard what locks are on it." And do it.