从常量插入的表中选择

Question

How would I go about grabbing data from a table that is CONSTANTLY being inserted into (and needs to be) without causing any locking so that the inserts will continue unheeded.

I've looked around and found select with nolock option but, if I'm understanding correctly, this does not stop the lock creation, rather goes around current locks and grabs everything?

Thanks.

EDIT: This table will never be UPDATED, only INSERTS and SELECTS

Answer 1

You can use the NOLOCK hint when selecting from the table. There are some side effects like this (you can basically get a dirty read.)

NOLOCK issues no row locks in the query you add it to, and has no impact on the locks issued by other running queries. NOLOCK does issue aa Sch-S lock, Schema Stability lock, which isn't going to cause you a problem.

Answer 2

I believe you have misunderstood. select ... with (nolock) will not acquire any locks. That is to say, it will not block any other writes.

The downside seems to be that it will include uncommitted reads, so the result may not hold it the writing transaction rolls back.

Answer 3

As long as you don't mind getting dirty reads from your table this shouldn't be a problem for you. Make sure that the translation isolation level is set appropriately and that your calling code (if applicable) isn't using implicit transactions and you should be fine.

Microsoft's Transaction Isolation Docs: http://msdn.microsoft.com/en-us/library/ms173763.aspx

NOLOCK is a common, and in my opinion, abused option when running into situations like this. Although it can help you overcome problems in high contention situations it can also cause difficult to track down bugs. Although this is something of an ongoing argument check out http://blogs.msdn.com/b/davidlean/archive/2009/04/06/sql-server-nolock-hint-other-poor-ideas.aspx for an idea of some of the risks with using hints like this.

Answer 4

You can use NOLOCK, but I would only recommend that in cases where you know that "dirty data" is acceptable (for example, a syslog database where you know data will never be altered or deleted once it's been inserted). The best way to do it is to SELECT from data that is NOT being locked; can you identify rows that aren't being affected by your insert? For example, if your data is being inserted with a CreateDate column defaulting to GETDATE(), make sure your queries pull data from BEFORE that point.

Of course, it all depends on how much data is being written and whether or not the insert statement is generating row or page or table locks...

Answer 5

One option not discussed here is to use replication. If you replicate the table in question and run your queries on the replicated database, you will not block inserts/updates. (In your case, I would use transactional replication - https://msdn.microsoft.com/en-us/library/ms151176.aspx ).