Performance of SQL Server indexing

Question

I want to see the performance differences between an indexed vs non-indexed column when running the same query.

The table I've created (FileCollection) is very simple and has the following 3 columns

1. Id (int) - Primary key which auto generates the number.
2. FileName (nvarchar(MAX)) - Contains a file name.
3. Description (nvarchar(MAX)) - Contains a description of the file.

The query I'm using is:

SELECT * FROM FileCollection WHERE FileName='readme.txt'

I've populated the database with more than 100k records. Using the SQL Server Profiler tool, I initially ran the query with no index on the FileName column and again when adding the index to FileName column. However, I cannot see any difference in performance.

Both tests average around the same stats: CPU: 500 Reads: 13,000 Writes: 0 Duration: 200

Can anyone help me figure out why? I would assume 100k records is enough to show some difference but do I need more records?

TIA

Edit: I've now changed the FileName column to nvarchar(255) but still the problem exists. BTW, I'm using SQL Server 2008 R2.

Answer 1

Depending on the amount of RAM your server has, it might just read the whole table into memory and keep it there - so your first query will load it, and any subsequent queries will just operate on those pages loaded in memory.

To get any meaningful data, you need to flush the cache before every run of your query! (run DBCC FREEPROCCACHE and DBCC DROPCLEANBUFFERS before any query execution!)

I tried your scenario with 100'000 dummy file names and almost the same table structure (I changed FileName to be VARCHAR(260)).

---

Without index:

Table 'TestFiles'. Scan count 1, logical reads 2137, physical reads 5, read-ahead reads 2136, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Query Subtree Cost 1.68884

---

With non-clustered index on filename:

Table 'TestFiles'. Scan count 1, logical reads 6, physical reads 4, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Query Subtree Cost 0.0065704

---

This was done with this statement:

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS 

SET STATISTICS IO ON

SELECT * 
FROM testfiles
WHERE FileName = 'File-D8584B44-518F-428A-86A1-7836E0B60502'

Answer 2

I'm pretty sure you're leaving something out:

(n)varchar(max) fields CANNOT BE INDEXED

From MSDN:

Columns that are of the large object (LOB) data types ntext, text, varchar(max), nvarchar(max), varbinary(max), xml, or image cannot be specified as key columns for an index. Also, a view definition cannot include ntext, text, or image columns, even if they are not referenced in the CREATE INDEX statement.

Did you get an error when you tried to create the index?

Both your queries are the same because they are the same - since you can't index that field, they are both creating table scans.