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drop index does not work for unique keys

Posted on 2010-08-19
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Last Modified: 2012-05-10
what is the reason DROP index syntax is disabled for deleting unique keys (non PKs)

thanks
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Question by:anushahanna
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8 Comments
 
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by:Guy Hengel [angelIII / a3]
Guy Hengel [angelIII / a3] earned 1700 total points
ID: 33474819
this is because the PK is relying on the underlying index for it's function.

imagine you had a 2GB table with a primary key, but no index on that column.
the primary key constraint would have to do a full table scan for each insert & update for that field value !!!
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LVL 18

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by:Cluskitt
Cluskitt earned 300 total points
ID: 33474894
If you want to drop the index (possibly for rebuilding it), then drop the constraint first.
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by:anushahanna
ID: 33475442
>>this is because the PK is relying on the underlying index for it's function.

even if PK and UK are both different? (example PK is on Col1 and UK is on Col3..)
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LVL 143

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Guy Hengel [angelIII / a3] earned 1700 total points
ID: 33475652
if you have a PK and a UK, you will also have 2 indexes, 1 index per UK or PK constraint.
for the rest, it's the same situation: the database engine will not allow to drop the index.
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by:anushahanna
ID: 33476646
are you saying PK will be relying on the unique index, regardless if they are the same or 2 diff indexes?
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by:anushahanna
ID: 33476973
will DROP INDEX syntax work sometimes and othertimes not, for UNIQUE Indexes?
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LVL 143

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by:Guy Hengel [angelIII / a3]
Guy Hengel [angelIII / a3] earned 1700 total points
ID: 33479687
>are you saying PK will be relying on the unique index, regardless if they are the same or 2 diff indexes?

no. let me clarify:
when you create a PK, sql server will create automatically create a index, for that PK constraint. and it will not let you drop that index.

when you create a UC (unique constraint), sql server will create automatically create a index, for that UC constraint. and it will not let you drop that index.

when you create whatever index, they will NOT be used for any PK or UC, and sql will let your drop that index without issues
this includes also unique indexes, as they are not created by a constraint as such

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by:anushahanna
ID: 33484609
thanks angelIII, for the helpful explanation.
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