Creating an index, why is it insert into itself?

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Creating an index, why is it insert into itself?

Maddave

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Points: 6828
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May 22, 2014 at 3:56 am

#289145

Hi, I am creating an non clustered index on a table which has over 30 million rows in it as follows:
CREATE NONCLUSTERED INDEX IX_ParentCode
ON [dbo].[Codes] ([ParentCode])
INCLUDE ([Id], [ExternalOrderNumber])
When using sp_whoisActive to monitor the process I am seeing the command being run as:
INSERT INTO Codes SELECT * FROM Codes
I am a bit confused as I don't see any increase in usage of the database log file (using dbcc sqlperf(logspace)). I can see the CREATE INDEX command under Sp_who2 though.
Can anyone explain what it is actually going on for this index creation and why it selects into itself?
I realise nothing may be wrong, but more curious as I know this command is going to take some time to complete.

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Gail Shaw SSC Guru Points: 1004504 More actions · Answer 1

Read from the table, insert into the index. It's just how an index creation shows up.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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ScottPletcher SSC Guru Points: 100947 More actions · Answer 2

If you're on Enterprise Edition you can add "ONLINE = ON" and the table can still be used while the index is being built.

I prefer to always explicitly specify the "FILLFACTOR = nnn" you want; a good "default FILLFACTOR" is not really possible, because each index's needs are specific.

If you have tempdb on faster drives, you might want to specify "SORT_IN_TEMP = ON":

CREATE NONCLUSTERED INDEX IX_ParentCode

ON [dbo].[Codes] ( [ParentCode] )

INCLUDE ( [ExternalOrderNumber], [Id] )

WITH ( FILLFACTOR = 99, ONLINE = ON, SORT_IN_TEMPDB = ON )

SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.

tim_harkin SSCrazy Points: 2399 More actions · Answer 3

Maddave (5/22/2014)
Hi, I am creating an non clustered index on a table which has over 30 million rows in it as follows:
CREATE NONCLUSTERED INDEX IX_ParentCode
ON [dbo].[Codes] ([ParentCode])
INCLUDE ([Id], [ExternalOrderNumber])
When using sp_whoisActive to monitor the process I am seeing the command being run as:
INSERT INTO Codes SELECT * FROM Codes
I am a bit confused as I don't see any increase in usage of the database log file (using dbcc sqlperf(logspace)). I can see the CREATE INDEX command under Sp_who2 though.
Can anyone explain what it is actually going on for this index creation and why it selects into itself?
I realise nothing may be wrong, but more curious as I know this command is going to take some time to complete.

Try using this parameter

@get_outer_command = 1