What's blocking “Select top 1 * from TableName with (nolock)” from returning a result?

问题

I'm currently running the following statement

select * into adhoc..san_savedi from dps_san..savedi_record

It's taking a painfully long time and I'd like to see how far along it is so I ran this:

select count(*) from adhoc..san_savedi with (nolock)

That didn't return anything in a timely manner so for the heck of it I did this:

select top 1 * from adhoc..san_savedi with (nolock)

Even that seems to run indefinitely. I could understand if there are millions of records that the count(*) could take a long time, but I don't understand why selecting the top 1 record wouldn't come back pretty much immediately considering I specified nolock.

In the name of full disclosure, dps_san is a view that pulls from an odbc connection via linked server. I don't think that'd be affecting why I can't return the top row but just throwing it out there in case I'm wrong.

So I want to know what is keeping that statement from running?

EDIT:

As I mentioned above, yes dps_san..savedi_record is a view. Here's what it does:

select * from DPS_SAN..root.SAVEDI_RECORD

It's nothing more than an alias and does no grouping/sorting/etc so I don't think the problem lies here, but please enlighten me if I'm wrong about that.

回答1:

SELECT queries with NOLOCK don't actually take no locks, they still need a SCH-S (schema stability) lock on the table (and as it is a heap it will also take a hobt lock).

Additionally before the SELECT can even begin SQL Server must compile a plan for the statement, which also requires it to take a SCH-S lock out on the table.

As your long running transaction creates the table via SELECT ... INTO it holds an incompatible SCH-M lock on it until the statement completes.

You can verify this by looking in sys.dm_os_waiting_tasks ~~whilst~~ ~~while~~ during the period of blocking.

When I tried the following in one connection

BEGIN TRAN

SELECT *
INTO NewT
FROM master..spt_values

/*Remember to rollback/commit this later*/

And then executing (or just simply trying to view the estimated execution plan)

SELECT *
FROM NewT
WITH (NOLOCK)

in a second the reading query was blocked.

SELECT wait_type,
       resource_description
FROM sys.dm_os_waiting_tasks
WHERE session_id = <spid_of_waiting_task>

Shows the wait type is indeed SCH_S and the blocking resource SCH-M

wait_type        resource_description
---------------- -------------------------------------------------------------------------------------------------------------------------------
LCK_M_SCH_S      objectlock lockPartition=0 objid=461960722 subresource=FULL dbid=1 id=lock4a8a540 mode=Sch-M associatedObjectId=461960722

回答2:

It very well may be that there are no locks... If dps_san..savedi_record is a view, then it may be taking a long time to execute, because it may be accessing tables without using an index, or it may be sorting millions of records, or whatever reason. Then your query, even a simple top or count, will be only as fast as that view can be executed.

回答3:

A few issues to consider here. Is dps_san..savedi_record a view? If so, it could just be taking a really long time to get your data. The other thing I can think of is that you're trying to create a temp table by using the select into syntax, which is a bad idea. select * into ... syntax will lock the tempdb for duration of the select.

If you are creating the table using that syntax, then there is a workaround. First, create the table by throwing where 1=0 at the end of your initial statement:

select * into ... from ... where 1=0

This will create the table first (which is quick) which allows you to insert into because the table exists now (without penalty of locking tempdb for duration of query).

回答4:

Find the session_id that is performing the select into:

SELECT r.session_id, r.blocking_session_id, r.wait_type, r.wait_time
  FROM sys.dm_exec_requests AS r
  CROSS APPLY sys.dm_exec_sql_text(r.plan_handle) AS t
  WHERE t.[text] LIKE '%select%into%adhoc..san_savedi%';

This should let you know if another session is blocking the select into or if it has a wait type that is causing a problem.

You can repeat the process in another window for the session that is trying to do the select. I suspect Martin is right and that my earlier comment about schema lock is relevant.

来源：https://stackoverflow.com/questions/10955752/whats-blocking-select-top-1-from-tablename-with-nolock-from-returning-a-r

标签

sql

sql-server-2008

locking