I get that the (nolock)
optimizer hint allows for "dirty reads", but under what very specific scenarios is this a bad idea? I've never seen such widespread use of (nolock)
in an organization, and it makes me nervous. I'd like an explanation in terms of user stories. "Paul does A, Peter does B, X happens instead of Y".
The NOLOCK hint allows SQL to read data from tables by ignoring any locks and therefore not get blocked by other processes. This can improve query performance by removing the blocks, but introduces the possibility of dirty reads.
Please note that in some cases Optimizer might think to behave differently even when query hints are used. You cannot always say optimizer would only make decision based on Query hint. Although its highly unlikely for NOLOCK to cause deadlock but it can.
The WITH (NOLOCK) table hint is a good idea when the system uses explicit transactions heavily, which blocks the data reading very frequently. The WITH (NOLOCK) table hint is used when working with systems that accept out of sync data, such as the reporting systems.
A delete statement acquires an exclusive intent lock on the reference table; therefore, during that time, no other transactions can modify the data. You can use NOLOCK hint to read the data.
Reposting this answer:
NOLOCK
means placing no locks at all.
Your query may returns portions of data as of before UPDATE
and portions as of after UPDATE
in a single query.
Like, a debit without a credit and these kinds of stuff.
For instance, I just ran this query on a large table:
SELECT SUM(LEN(name))
FROM master WITH (NOLOCK)
OPTION (MAXDOP 1)
---
18874367
All name
's have length of 1
.
Then I reran it and in the middle of the query updated the table:
UPDATE master
SET name = 'tt'
WHERE id <= 10000
SELECT SUM(LEN(name))
FROM master WITH (NOLOCK)
OPTION (MAXDOP 1)
---
18874944
As we can see, this query noticed 577
rows as updated (length 2
), all other rows as not updated (length 1
).
SELECT SUM(LEN(name))
FROM master WITH (NOLOCK)
OPTION (MAXDOP 1)
---
18884367
And this query, run right after the previous one finished, sees all updates.
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