Does Index Slows Down INSERT Statements in SQL Server?

If you want to see the impact of indexes on INSERT statements, you can repeat the same insert script on the table "fyi_links" of the same structure with two indexes: one non-clustered index on column "url" and one non-clustered index on column "counts". See the tutorial exercise below:

USE FyiCenterData
GO

-- Drop the old table, if needed
DROP TABLE fyi_links_indexed;
GO

-- Create a table
CREATE TABLE fyi_links_indexed (
  id INT,
  url VARCHAR(80) NOT NULL,
  notes VARCHAR(1024),
  counts INT,
  created DATETIME NOT NULL DEFAULT(getdate())
);
GO

-- Create two indexes
CREATE INDEX fyi_links_url ON fyi_links_indexed (url);
CREATE INDEX fyi_links_counts ON fyi_links_indexed (counts);
GO

-- Empty the table if needed
DELETE FROM fyi_links_indexed;
GO

-- Performance test of INSERT
DECLARE @start_time DATETIME, @end_time DATETIME;
SET @start_time = GETDATE();
INSERT INTO fyi_links_indexed 
   SELECT id, rand_string, REVERSE(rand_string), 
      rand_integer, rand_datetime
   FROM fyi_random
SET @end_time = GETDATE();
PRINT 'Milliseconds used: '+CONVERT(VARCHAR(20),
   DATEDIFF(MILLISECOND,@start_time,@end_time));
GO

-- First time
(100000 row(s) affected)
Milliseconds used: 15516

-- Second time
(100000 row(s) affected)
Milliseconds used: 2766

-- Third time
(100000 row(s) affected)
Milliseconds used: 3186

Comparing the result between this tutorial and the previous tutorial, two indexes make 100000 insert statements about 4 times slower. Note that the first measurement of both tests seems to be affected by the database engine caching.

⇒ Index Speeding Up SELECT Statements in SQL Server

⇐ Measuring Performance of INSERT Statements in SQL Server

⇑ Understanding and Managing Indexes in SQL Server

⇑⇑ SQL Server Database Tutorials