SQL 2008+ NOLOCK vs READPAST Considerations for Reporting Accuracy

Question

Understanding the final decision is business decision, what are the accuracy considerations between NOLOCK & READPAST running in SQL 2008 R2? I would like to have a better understanding before discussing changes with the business area.

I have inherited a number of queries, used to create data views for management reporting. ‘WITH (NOLOCK)’ is used liberally but inconsistently. The data being read is from the production server of a widely used application that is constantly being updated. We are migrating from a SQL 2005 server to a SQL 2008 R2 server. These reports want data fresher than the 24 hour old data on the archive server. The use of NOLOCK suggests a past decision; potential for conflict exists and it a bit of accuracy loss is acceptable. Data is used to populate dashboards for human awareness/decision making.

All the Queries are SELECT, with Read Only access for the data view login. The majority of the queries are single table with a few 2 and 3 table joins. Given the low level of joins WITH () seems a better choice than SET TRANSACTION ISOLATION LEVEL {}

Table Hints (Transact-SQL) http://msdn.microsoft.com/en-us/library/ms187373.aspx (as well as multiple questions on SO) says that NOLOCK and/or READUNCOMMITTED are likely to have duplicate read issues, in addition to missing locked records.

READPAST looks like the more accurate, as it will only miss locked records without a chance of duplicates. But I am not sure the level of missing locked records is consistent between it and NOLOCK.

There is good article by Tim Chapman comparing the two but it was written in 2007, most of the comments revolve around 2000 & 2005, with one comment indicating READPAST is problematic in 2008 R2

References

Effect of NOLOCK hint in SELECT statements

When should you use "with (nolock)"

Using NOLOCK and READPAST table hints in SQL Server (By Tim Chapman)

Edit:

Snapshot isolation is suggested in two answers below. Snapshot isolation is dependent setting of the DB, this Q/A https://serverfault.com/questions/117104/how-can-i-tell-if-snapshot-isolation-is-turned-on describes how to see what setting are in place on the database. I now know it is disabled, I am reading for reports from a major applications database. Changing the setting is not an option. +- a couple of percent accuracy is acceptable, application (OLTP) impact is not acceptable. Most simple queries do not need lock considerations but in some extreme cases, lock consideration is required. With the advent of Snapshot isolation for SQL 2005, little information is available on NOLOCK & READPAST behavior in SQL 2008 or higher. Yet they remain my only choices.

Answer 1

A better option worth consideration is enabling READ COMMITTED SNAPSHOT for the database itself. This uses versioning in the tempdb to capture the state of a table at the beginning of the transaction.

There is a very good read on various aspects of NOLOCK, READPAST etc, at http://www.brentozar.com/archive/2013/01/implementing-snapshot-or-read-committed-snapshot-isolation-in-sql-server-a-guide/

WITH (NOLOCK) can provide incorrect results if someone is updating the table when you are selecting from it. If a page-split happens as a result of an insert while you are reading the table, and the new page happens to be beyond the point you've read, WITH (NOLOCK) will have already returned rows from the old page, and will then return duplicate rows from the new page. This is just a single example of why (NOLOCK) is bad.

WITH (READPAST) will skip any records that are being updated or inserted while you are reading from the table. Neither option is good in a busy database.

In light of the recent edit to your question where you state you cannot change the database setting for READ COMMITTED SNAPSHOT, perhaps you should consider using a stored procedure to gather data for you reports, and setting the transaction isolation level at the beginning of the stored proc using SET TRANSACTION ISOLATION LEVEL SNAPSHOT;. In order to do this, you would need to change the database option 'allow snapshot isolation'.

From SQL Server Books Online:

SNAPSHOT

Specifies that data read by any statement in a transaction will be the transactionally consistent version of the data that existed at the start of the transaction. The transaction can only recognize data modifications that were committed before the start of the transaction. Data modifications made by other transactions after the start of the current transaction are not visible to statements executing in the current transaction. The effect is as if the statements in a transaction get a snapshot of the committed data as it existed at the start of the transaction.

Except when a database is being recovered, SNAPSHOT transactions do not request locks when reading data. SNAPSHOT transactions reading data do not block other transactions from writing data. Transactions writing data do not block SNAPSHOT transactions from reading data.

During the roll-back phase of a database recovery, SNAPSHOT transactions will request a lock if an attempt is made to read data that is locked by another transaction that is being rolled back. The SNAPSHOT transaction is blocked until that transaction has been rolled back. The lock is released immediately after it has been granted.

The ALLOW_SNAPSHOT_ISOLATION database option must be set to ON before you can start a transaction that uses the SNAPSHOT isolation level. If a transaction using the SNAPSHOT isolation level accesses data in multiple databases, ALLOW_SNAPSHOT_ISOLATION must be set to ON in each database.

A transaction cannot be set to SNAPSHOT isolation level that started with another isolation level; doing so will cause the transaction to abort. If a transaction starts in the SNAPSHOT isolation level, you can change it to another isolation level and then back to SNAPSHOT. A transaction starts the first time it accesses data.

A transaction running under SNAPSHOT isolation level can view changes made by that transaction. For example, if the transaction performs an UPDATE on a table and then issues a SELECT statement against the same table, the modified data will be included in the result set.

Answer 2

NOLOCK can cause duplicate data to be read, data to be missed and the query to actually fail with an error messaged (something with "data movement").

On the other hand, a non-NONLOCK query can also read duplicate data and mis data! It is by no means a consistent snapshot of the database. The difference is that it will not read uncommitted data and will never fail.

The problem with NOLOCK is mostly that it can fail randomly, so you need retry. Also, the probability of wrong data being read is slightly higher.

NOLOCK has a big advantage when you're doing table scans: SQL Server can use allocation order scanning instead of index-order scans. TABLOCK has the same effect. This can be a significant speedup in the presence of fragmentation.

Consider just using the snapshot isolation level as it gets rid of all of these concerns. It comes with some other trade-offs and you don't get allocation-order scans. But it permanently and comprehensively removes locking problems.

Answer 3

Answering my own question after stress testing with SQLQueryStress http://www.datamanipulation.net/sqlquerystress/ (this is wonderful tool that is extremely easy to use). Results from SQLQueryStress are tested against SQL Server Profiler; the accuracy is the same as SQL Server Profiler, though the precession is two decimal places of a second less (but is sufficient for this test).

As mentioned in the question, the primary concern is application performance impact, with report accuracy and performance the secondary consideration. All testing occurred on the test server with where the test application is active and has some minor activity.

After downloading and becoming familiar with SQLQueryStress I set up a simple ‘ReportQuery’ to act as resource hog. It set to run 15 Iterations with 15 threads (225 total queries). Total run time is around 28 seconds, with an average iteration time of 1.49 seconds.

Created an Add/Delete ‘ApplicationQuery’ to represent ongoing application activity. It is set to run 2000 iterations with 1 thread. There are two versions, with a select statement (runs 31 seconds) and without a select statement (runs 28 seconds). These represent normal peak time application activity.

10 test runs of each of three versions of “ReportQuery” are run, this is to identify if there is any performance benefit between ‘with(nolock)’, ‘with(readpast), and without hints . Results indicate no significant difference the ReportQuery runs constantly in about 28 seconds with an average 1.5 second iteration time.

There are no big outliers so, decision to drop to 5 test runs for the following tests.

5 test runs of ApplicationQuery with a select statement; with one of each of three versions of “ReportQuery” also running. In each of 15 totals test, the ApplicationQuery is manually started, with the ReportQuery manually started immediately after. This scenario represents a resource heavy report query struggling with the applications ongoing activity for resources.

Repeated the test runs but this time used ApplicationQuery without a select statement.

Results: In every case the ApplicationQuery was throttled back to almost no forward progress, while the ReportQuery was running.

The ReportQuery had no significant loss of performance when struggling for resources with multiple ApplicationQuery’s against the database.

The ApplicationQuery was able to run queries parallel to the ReportQuery, but progress was very slow while competing for resources. In essence the total time to run 2000 Application Add/Delete Queries was extended by the time used by the ReportQuery.

The initial question was about which was more accurate, becomes pointless. There is essentially no report or application performance difference between using or not using the hints NOLOCK or READPAST, so don’t use either in a busy database and get the highest accuracy possible.

‘ReportQuery’

select 
ID
, [TABLE_NAME]
, NUMBER
, FIELD
, OLD_VALUE
, NEW_VALUE
, SYSMODUSER
, SYSMODTIME
, SYSMODCOUNT

from dbo.UPMCINCIDENTMGMTAUDITRECORDSM1 

where Number like '%'
or NUMBER like '2010-01-01'

‘ApplicationQuery’ (with Select Statement)

select * 

from dbo.UPMCINCIDENTMGMTAUDITRECORDSM1 
where FIELD = 'JJTestingPerformance'


insert into dbo.UPMCINCIDENTMGMTAUDITRECORDSM1 (ID
, [TABLE_NAME]
, NUMBER
, FIELD
, OLD_VALUE
, NEW_VALUE
)

values ('Test+Time'
, 'none'
, 'tst01'
, 'JJTestingPerformance'
, 'No Value'
, 'Test'
)

delete from dbo.UPMCINCIDENTMGMTAUDITRECORDSM1

where FIELD = 'JJTestingPerformance'

‘ApplicationQuery’ (without Select Statement)

insert into dbo.UPMCINCIDENTMGMTAUDITRECORDSM1 (ID
, [TABLE_NAME]
, NUMBER
, FIELD
, OLD_VALUE
, NEW_VALUE
 )

values ('Test+Time'
, 'none'
, 'tst01'
, 'JJTestingPerformance'
, 'No Value'
, 'Test'
)

delete from dbo.UPMCINCIDENTMGMTAUDITRECORDSM1

where FIELD = 'JJTestingPerformance'