An index can be added when you create a new table. New rows will be indexed as they are inserted into the table. But you can also add a new index to an existing table with the same CREATE INDEX statement. The existing rows will be indexed as part of the CREATE INDEX statement.

If you add a new index to an existing table with a large number of rows. The CREATE INDEX statement could take some time to finish. See the tutorial exercise below:

USE GlobalGuideLineDatabase

GO

-- Drop indexes if needed

DROP INDEX ggl_links_indexed.ggl_links_url;

DROP INDEX ggl_links_indexed.ggl_links_counts;

GO

SELECT COUNT(*) FROM ggl_links_indexed;

GO

100000

-- Create two indexes

DECLARE @start_time DATETIME, @end_time DATETIME;

SET @start_time = GETDATE();

CREATE INDEX ggl_links_url ON ggl_links_indexed (url);

CREATE INDEX ggl_links_counts ON ggl_links_indexed (counts);

SET @end_time = GETDATE();

PRINT 'Milliseconds used: '+CONVERT(VARCHAR(20),

DATEDIFF(MILLISECOND,@start_time,@end_time));

GO

-- First time

Milliseconds used: 12626

-- Second time

Milliseconds used: 11763

-- Third time

Milliseconds used: 13890

You can see creating two indexes on a table of 100000 rows costs about 12 seconds.