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Discuss content posted by Alok Dwivedi
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An Un-indexed Foreign Key Gotcha
23 posts, Page 2 of 3
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An Un-indexed Foreign Key Gotcha
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TheSQLGuru
TheSQLGuru
Posted Monday, November 09, 2009 3:52 PM
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sqlpro (11/9/2009)
thanks for the reply. probably i misunderstood but following is the link i got my understanding from.its here http://www.informit.com/library/content.aspx?b=STY_Sql_Server_7&seqNum=145
In general, nonclustered indexes (and the associated bookmark lookup to the base table) will be used only if the query is estimated to hit between 0 and perhaps as much as 2 percent of the total rows. This is a surprisingly low percentage to most people. Often, and certainly the case in my example where I explicitely stated there was exactly one FK child row per parent in a billion row table, FK children tables have VERY few rows per parent key and an index WILL be beneficial for lookups/joins.
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru at GMail
Post #816209
mcvilbar
mcvilbar
Posted Monday, November 09, 2009 3:56 PM
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Last Login: Wednesday, November 07, 2012 12:32 AM
Points: 110,
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I've been realizing that creating a non-clustered index on the foreign key is quite a very good idea. Nice article, very informative and interesting.
Post #816215
sqlpro
sqlpro
Posted Monday, November 09, 2009 3:57 PM
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that makes sense. thanks for correcting that TheSQLGuru
Post #816216
denise.crabtree
denise.crabtree
Posted Monday, November 09, 2009 4:05 PM
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Last Login: Thursday, May 16, 2013 4:30 PM
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I couldn't resist -- I wanted to query to find them. Didn't read all replies first. So here is my simple query to find a table without an index on a foreign key.
If FK exists and column is in NO index -- then it will list out. Could be refined more. (exact order for multiple column FK match exact order for indexes )
-----------------------------------------------------------------------
-- Find a list of Foreign Keys that are not indexed
--
-- Created 11-09-2009 Denise Crabtree
-----------------------------------------------------------------------
-----------------------------------------------------------------------
SELECT
object_name(parent_object_id) [FK table]
,object_name(referenced_object_id) [References this Table]
,object_name(constraint_object_id) [Foreign Key Name]
,sys.columns.name [ChildTable Column]
FROM sys.foreign_key_columns
INNER JOIN sys.columns
ON sys.foreign_key_columns.parent_object_id = sys.columns.object_id
AND sys.foreign_key_columns.Parent_column_id = sys.columns.column_id
LEFT OUTER JOIN sys.index_columns
ON sys.foreign_key_columns.parent_object_id = sys.index_columns.object_id
AND sys.foreign_key_columns.Parent_column_id = sys.index_columns.column_id
WHERE
sys.index_columns.object_id is null
Post #816226
TheSQLGuru
TheSQLGuru
Posted Monday, November 09, 2009 4:18 PM
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See the earlier reply by richardd. It has a link to a script by Paul Nielson that not only identifies FKs missing indexes but will actually generate CREATE INDEX scripts for them (IIRC).
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru at GMail
Post #816235
SQLRNNR
SQLRNNR
Posted Monday, November 09, 2009 4:52 PM
SSCoach
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Last Login: 2 days ago @ 1:07 PM
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Visits: 12,332
Good Article with useful info. Thanks
Jason
AKA CirqueDeSQLeil
I have given a name to my pain...
MCM SQL Server 2008
SQL RNNR
Posting Performance Based Questions - Gail Shaw
Posting Data Etiquette - Jeff Moden
Hidden RBAR - Jeff Moden
VLFs and the Tran Log - Kimberly Tripp
Post #816242
mister_zed
mister_zed
Posted Tuesday, November 10, 2009 7:36 AM
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Last Login: Monday, January 11, 2010 9:47 AM
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The indexed FKs have nothing to do with performance; increased performance MIGHT be a side-effect, but - as you point out - it might as well not be such an effect. The idea of indexing FK columns has to do with the locking mechanisms.
In order to be sure that you can delete a parent record you must also make sure that there are no children belonging to that parent. This can only be achieved by actually reading the child table(s). When you don't have any usable index on the child table, then you must read all records - and they get locked at the same time, just because they are part of an updating (read-write) transaction.
This is very common truth för many relational databases, including SQL Server, Oracle, Firebird and others, and it has so been since "ancient" times.
P.S. I thought it was common knowledge...
Post #816523
TheSQLGuru
TheSQLGuru
Posted Tuesday, November 10, 2009 8:15 AM
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The indexed FKs have nothing to do with performance;
I think that is a bit of a stretch. There is almost always at least some performance benefit (often dramatic), both in efficient join/seek and in improved concurrency from reduced locking.
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru at GMail
Post #816574
jswong05
jswong05
Posted Wednesday, November 11, 2009 7:58 AM
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Last Login: Monday, March 18, 2013 7:54 AM
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Please read Thomas Kyte's books. He wrote for Oracle but the principles about "transaction processing" and "locking" and "index" are the same.
When FK records can be fastly located by index, the transaction processing will be fast, that reduces required lock-time, hence improves performance. This applies to any records.
Jason
http://dbace.us
Post #817172
mister_zed
mister_zed
Posted Wednesday, November 11, 2009 8:06 AM
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Group: General Forum Members
Last Login: Monday, January 11, 2010 9:47 AM
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I agree it's a bit of a stretch.
My point was that indexing might have different purposes, optimizing search operations being just ONE of them.
Post #817186
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