There are certain operations where dropping an index, loading data and then again creating the index can speed up data loading. SQL server 2005 introduced a way to disable an index.
Let’s take a look, first create this table
Create table TestIndex (id int, somecol varchar(20))
Insert a little bit of data
insert into TestIndex
select number,CONVERT(varchar(20),getdate(),100)
from master..spt_values
where type = 'p'
Create a nonclustered index
create index ix_TestIndex on TestIndex(id,somecol)
Now let’s disable this index
ALTER INDEX ix_TestIndex
ON TestIndex
DISABLE
Now when we run our query against the table and look at the plan we get a table scan
set showplan_text on
go
select * from TestIndex
go
set showplan_text off
go
|--Table Scan(OBJECT:([master].[dbo].[TestIndex]))
Now let’s rebuild the index again
ALTER INDEX ix_TestIndex
ON TestIndex
REBUILD
Now we will run the same query again
set showplan_text on
go
select * from TestIndex
go
set showplan_text off
go
|--Index Scan(OBJECT:([master].[dbo].[TestIndex].[ix_TestIndex]))
As you can see, it uses the index again
Now let’s drop this index
drop index TestIndex.ix_TestIndex
Is there a difference how disable works between nonclustered and clustered indexes?
Let’s take a look, first create this clustered index
create clustered index ix_TestIndexClustered on TestIndex(id,somecol)
Now let’s disable this clustered index
ALTER INDEX ix_TestIndexClustered
ON TestIndex
DISABLE
And now when we run the query from before
set showplan_text on
go
select * from TestIndex
go
set showplan_text off
go
We get this error
_Msg 8655, Level 16, State 1, Line 1
The query processor is unable to produce a plan because the index ‘ixTestIndexClustered’ on table or view ‘TestIndex’ is disabled.
As you can see while a clustered index is disabled the data is unavailable. Not only that, you can also not insert anything into the table,
So this query
insert into TestIndex
select 2,'Bla'
Fails with the same error from before
_Msg 8655, Level 16, State 1, Line 1
The query processor is unable to produce a plan because the index ‘ixTestIndexClustered’ on table or view ‘TestIndex’ is disabled.
If we rebuild the clustered index again
ALTER INDEX ix_TestIndexClustered
ON TestIndex
REBUILD
And if we run this query again
set showplan_text on
go
select * from TestIndex
go
set showplan_text off
go
|--Clustered Index Scan(OBJECT:([master].[dbo].[TestIndex].[ix_TestIndexClustered]))
We can see that it does use the clustered index
My question to you.
So my question to you people is, do any of you use this instead of drop and create index? One advantage I see is that you don’t need to update the code that drops and recreates the non clustered index if your index definition changes when using disable index in your ETL process. If you disable a clustered index you can also not insert into the table.
*** Remember, if you have a SQL related question, try our Microsoft SQL Server Programming forum or our Microsoft SQL Server Admin forum