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I am trying to run some analysis on missing indexes on a server (I haven't ever done it on this server before). I was running sp_blitzindex and it would hang up on "Inserting data into #MissingIndexes." I dug into the query and found where it hits sys.dm_db_missing_index_details.

Trying to select from sys.dm_db_missing_index_details will return about 300 records, and then it'll sit and spin. I didn't want to let it keep going to see how long it would take to finish, but I let it run for over two minutes and it didn't complete.

Why would this take a long time to return? What would cause this?

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This sounds like other sessions are blocking the dynamic management view.

CREATE TABLE and DROP TABLE force an update to the internal tables maintained for the sys.dm_db_missing_index_details.

I was able to recreate a blocking scenario where an open transaction prevents reads against sys.dm_db_missing_index_details. Run this in an SSMS window on your development machine:

BEGIN TRANSACTION
IF OBJECT_ID(N'dbo.mid_test_2', N'U') IS NOT NULL
DROP TABLE dbo.mid_test_2;
IF OBJECT_ID(N'dbo.mid_test', N'U') IS NOT NULL
DROP TABLE dbo.mid_test;
CREATE TABLE dbo.mid_test
(
    ID int NOT NULL PRIMARY KEY CLUSTERED
    , SomeVal varbinary(2000) NOT NULL DEFAULT (CRYPT_GEN_RANDOM(2000))
);

CREATE TABLE dbo.mid_test_2
(
    ID int NOT NULL FOREIGN KEY REFERENCES dbo.mid_test(ID)
    , SomeVal varbinary(2000) NOT NULL DEFAULT (CRYPT_GEN_RANDOM(2000))
);
GO

;WITH src AS 
(
    SELECT v.num
    FROM (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9))v(num)
)
INSERT INTO dbo.mid_test (ID)
SELECT s1.num 
    + (s2.num * 10) 
    + (s3.num * 100) 
    + (s4.num * 1000) 
    + (s5.num * 10000) 
FROM src s1
    CROSS JOIN src s2
    CROSS JOIN src s3
    CROSS JOIN src s4
    CROSS JOIN src s5
ORDER BY 1;

;WITH src AS 
(
    SELECT v.num
    FROM (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9))v(num)
)
INSERT INTO dbo.mid_test_2 (ID)
SELECT s1.num 
    + (s2.num * 10) 
    + (s3.num * 100) 
    + (s4.num * 1000) 
    + (s5.num * 10000) 
FROM src s1
    CROSS JOIN src s2
    CROSS JOIN src s3
    CROSS JOIN src s4
    CROSS JOIN src s5
ORDER BY 1;

SELECT *
FROM dbo.mid_test m1
    INNER JOIN dbo.mid_test_2 m2 ON m1.ID = m2.ID
WHERE m2.ID = 66666

Now, in another SSMS window, run the following (you may need to run it several times depending on how quick you are!)

SELECT *
FROM sys.dm_db_missing_index_details;

For me, the above query blocks until I either commit or rollback the transaction started in the first query window, with:

COMMIT TRANSACTION

or

ROLLBACK TRANSACTION

Unfortunately, running SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; prior to querying sys.dm_db_missing_index_details does not help; it still blocks until the open transaction commits or rolls back.

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