Why is this an Index Scan and not a Index Seek?

It is using an Index Scan primarily because it is also using a Merge Join. The Merge Join operator requires two input streams that are both sorted in an order that is compatible with the Join conditions.

And it is using the Merge Join operator to realize your INNER JOIN because it believes that that will be faster than the more typical Nested Loop Join operator. And it is probably right (it usually is), by using the two indexes it has chosen, it has input streams that are both pre-sorted according your join condition (LocationID). When the input streams are pre-sorted lie this, then Merge Joins are almost always faster than the other two (Loop and Hash Joins).

The downside is what you have noticed: it appears to be scanning the whole index in, so how can that be faster if it is reading so many records that may never be used? The answer is that Scans (because of their sequential nature) can read anywhere from 10 to 100 times as many records/second as seeks.

Now Seeks usually win because they are selective: they only get the rows that you ask for, whereas Scans are non-selective: they must return every row in the range. But because Scans have a muchhigher read rate, they can frequently beat Seeks as long as the ratio of Discarded Rows to Matching Rows is lower than the ratio of Scan rows/sec VS. Seek rows/sec.

Questions?

OK, I have been asked to explain the last sentence more:

A "Discarded Row" is one that the the Scan reads (because it has to read everything in the index), but that will be rejected by the Merge Join operator, because it does not have a match on the other side, possibly because the WHERE clause condition has already excluded it.

"Matching Rows" are the ones that it read that are actually matched to something in the Merge Join. These are the same rows that would have been read by a Seek if the Scan were replaced by a Seek.

You can figure out what there are by looking at the statistics in the Query Plan. See that huge fat arrow to the right of the Index Scan? That represents how many rows the optimizer thinks that it will read with the Scan. The statistics box of the Index Scan that you posted shows the Actual Rows returned is about 5.4M (5,394,402). This is equal to:

TotalScanRows = (MatchingRows + RejectedRows)

(In my terms, anyway). To get the Matching Rows, look at the "Actual Rows" reported by the Merge Join operator (you may have to take off the TOP 100 to get this accurately). Once you know this, you can get the Discarded rows by:

RejectedRows = (TotalScanRows - MatchingRows)

And now you can calculate the ratio.

Above Resourse Link

Below List Article Reference

Merge join Vs Hash join Vs Nested loop join

Const vs. Readonly

const vs. readonly

const and readonly perform a similar function on data members, but they have a few important differences.

const

A constant member is defined at compile time and cannot be changed at runtime. Constants are declared as a field, using the const keyword and must be initialized as they are declared. For example;

public class MyClass {   public const double PI = 3.14159; }

PI cannot be changed in the application anywhere else in the code as this will cause a compiler error.

Constants must be a value type (sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, or bool), an enumeration, a string literal, or a reference to null.

Since classes or structures are initialized at run time with the new keyword, and not at compile time, you can't set a constant to a class or structure.

Constants can be marked as public, private, protected, internal, or protected internal.

Constants are accessed as if they were static fields, although they cannot use the static keyword.

To use a constant outside of the class that it is declared in, you must fully qualify it using the class name.

A read only member is like a constant in that it represents an unchanging value. The difference is that a readonly member can be initialized at runtime, in a constructor as well being able to be initialized as they are declared. For example:

readonly

public class MyClass {   public readonly double PI = 3.14159; }

or

public class MyClass {   public readonly double PI;     public MyClass()   {     PI = 3.14159;   } }

Because a readonly field can be initialized either at the declaration or in a constructor, readonly fields can have different values depending on the constructor used. A readonly field can also be used for runtime constants as in the following example:

public static readonly uint l1 = (uint)DateTime.Now.Ticks;

Notes

• readonly members are not implicitly static, and therefore the static keyword can be applied to a readonly field explicitly if required.

• A readonly member can hold a complex object by using the new keyword at initialization.

static

Use of the static modifier to declare a static member, means that the member is no longer tied to a specific object. This means that the member can be accessed without creating an instance of the class. Only one copy of static fields and events exists, andstatic methods and properties can only access static fields and static events. For example:

public class Car {   public static int NumberOfWheels = 4; }

The static modifier can be used with classes, fields, methods, properties, operators, events and constructors, but cannot be used with indexers, destructors, or types other than classes.

static members are initialized before the static member is accessed for the first time, and before the static constructor, if any is called. To access a static class member, use the name of the class instead of a variable name to specify the location of the member. For example:

int i = Car.NumberOfWheels;

Reference URL : http://en.csharp-online.net/const,_static_and_readonly

SQL SERVER – 2005 – Find Index Fragmentation Details – Slow Index Performance

Sample script :

SELECT ps.database_id, ps.OBJECT_ID,
ps.index_id, b.name,
ps.avg_fragmentation_in_percent
FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL) AS ps
INNER JOIN sys.indexes AS b ON ps.OBJECT_ID = b.OBJECT_ID
AND ps.index_id = b.index_id
WHERE ps.database_id = DB_ID()
ORDER BY ps.OBJECT_ID
GO


Ref Link