SQL Server SSIS package: August 2013

Monday, August 26, 2013

a. What is unique index?
b. What are the advantages and disadvantages of index?

Unique index

By default, Primary key constraint creates a unique clustered index on a table, whereas unique constraint creates unique non clustered index.

When the index is dropped, primary key constraint is also dropped.

Unique is a property of an index (for both clustered and non clustered index.

There is no difference between unique constraint and unique index. When unique constraint is added, then index also gets created behind the scene.

If a UNIQUE constraint is added to a column that has duplicated values, the Database Engine returns an error and does not add the constraint.

Advantages of indexes

Indexes provide faster data access during specific data fetch and retrieval form huge data tables. Below are the scenarios in which it works well.

Update and delete commands also work well when specific records or range of records are searched for modification or deletion. For example, if an index is created on salary column of employee table, and one has to modify salary for the employees who are having salary 10000, so it will be easier and faster to look for particular records and apply delete and update operations.

In case of sorting also, when we ask for a sorted dataset, the database will try to find an index and avoid sorting the results during execution of the query.

In case of grouping also, say one has to count the number of employee as per annual compensation (salary column), and the index is created on salary. Since matching salary appears in consecutive index entries, then the database will be able to count the number of employee at each salary quickly. While if index was not there on salary then it will first sort the results.

Disadvantage

Additional disk space is required in case of non-clustered index.

Insert, update and delete can become slow. As it has to locate the records then it takes time in case of huge data and too many indexes are there, because on each DML operation indexes will also be updated.

If we update a record and change the value of an indexed column in a clustered index, the database might need to move the entire row into a new position to keep the rows in sorted order. This behavior essentially turns an update query into a DELETE followed by an INSERT, with an obvious decrease in performance.

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Understanding on Indexes

a. What is non clustered index?

b. How many non clustered indexes can be created on one table?

c. Can you tell me some example on non clustered index that relates to real life experience?

d. How non-clustered is different from clustered indexes?

Non-Clustered Index

non-clustered index can be created as shown below:

create nonclustered index indexname on [TableName](ColumnName)

Some points regarding non-clustered indexex

It’s similar to index in textbook, index at one place and data at other place. These indexes will have the pointers to the storage location of the actual data.

A table can have more than 1 non-clustered index, just like in a book, one index in starting of the book, another at the end of the book.

A table can only have one Clustered index and up to 249 Non-Clustered Indexes. If a table does not have a clustered index it is referred to as a Heap.

Difference between Clustered and non clustered index

Max one clustered index and max 249 non clustered can be created on each table.

Clustered indexing is faster than non-clustered index. As clustered index will always cover a query ,since it contains all of the data in the leaf node itself.

Non-clustered indexes requires extra disk storage space.

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Understanding on Indexes

a.    What is a clustered index?
b.    How many clustered indexes can be there for one table?
c.    Can you tell some example on clustered index that relates to real life experience?
d.    When you declare primary key constraint, then which index is created on that column?

Clustered Index

Clustered index is created on Primary key automatically, if there is no clustered index already exists on table. This index determines physical order of data in a table. As there can be only one way, in which data can be arranged on disk or memory, therefore there can be only one clustered index on each table.

One can create clustered index explicitly as shown below

create clustered index indexname on [TableName](ColumnName)

Some points on clustered indexes

Even when the data is inserted in random order, it will be arranged as per the clustered index created on a particular column. For example, a clustered index is created on empid of employee table, then even though the order of records(empid) inserted is for e.g. 1,4,5,3,2, But when the data will be fetched from the table through a select query, you will see the order of records as 1,2,3,4,5. Data is automatically arranged on sequential ascending order after each insert.

It’s similar to a telephone directory in which data is arranged by the last name.

Clustered an index that includes more than one column are called Composite Clustered Index.

Only one clustered index can be created on a table, before creating another, one has to drop the existing clustered index.

A clustered index can be created on a maximum of 16 columns.

Clustered the index will always cover a query, since it contains all of the data in the leaf the node itself, or should say table itself

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Understanding on Indexes

Sql Server: index part 1

a.    What is index? Why do we need indexes?
b.    What are the various types of indexes in sql server?
c.    How will you check if indexes already exist on a table
d.    What will happen in case of no indexes?

Indexes are created to provide faster data access during specific data fetch and retrieval form huge data tables.

There are various types of indexes in sql server

a. Clustered Index

b. Non clustered index

c. Unique

d. Filtered

e. Spatial

f. Xml

g. Full text

h. Index with computed column

i. Index with non computed column

Using below procedure we can identify, if indexes exist on a table

Execute sp_helpindex table_name

For example:

In case of No index

If no index exists on a table, a table scan must be performed for each table referenced in a database query. The larger the table, the longer a table scan takes because a table scan requires each table row to be accessed sequentially. Although a table scan might be more efficient for a complex query that requires most of the rows in a table, for a query that returns only some table rows an index scan can access table rows more efficiently.

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Understanding on Indexes