-

Indexing in SQL

Let us understand how normal (without indexing) query runs. It basically traverse each rows one by one and when it finds the data it returns.
Without Index:
Without indexingSo suppose query is to find 50 , it will have to read 49 records as a linear search.
With Index:
enter image description here
When we apply indexing, the query will quickly find out the data without reading each one of them just by eliminating half of the data in each traversal like a binary search. 

Comments

Post a Comment

Popular posts from this blog

-
Image
CLUSTERED INDEX VS NON CLUSTERED INDEX Clustered Index Only one clustered index can be there in a table because of the sorted order Sort the records and store them physically according to the order Data retrieval is faster than non-clustered indexes Do not need extra space to store logical structure A primary key by default creates a clustered index on it. Non Clustered Index There can be any number of non-clustered indexes in a table because of the number of unique keys we can create. Do not affect the physical order. Create a logical order for data rows and use pointers to physical data files Data insertion/update is faster than clustered index Use extra space to store logical structure A unique key/keys by dafault created an unclustered index on it. In Depth Explanation: Clustered Technically speaking, a clustered index is a B-Tree data structure where all the rows in a table are stored at the leaf level of the index. In other words, a clustered ind...
Read more
-
Image
SQL JOINS Tutorial: INNER, OUTER, LEFT, RIGHT, CROSS What are JOINS? Joins help retrieving data from two or more database tables. The tables are mutually related using primary and foreign keys. id first_name last_name movie_id 1 Adam Smith 1 2 Ravi Kumar 2 3 Susan Davidson 5 4 Jenny Adrianna 8 6 Lee Pong 10 id title category 1 ASSASSIN'S CREED: EMBERS Animations 2 Real Steel(2012) Animations 3 Alvin and the Chipmunks Animations 4 The Adventures of Tin Tin Animations 5 Safe (2012) Action 6 Safe House(2012) Action 7 GIA 18+ 8 Deadline 2009 18+ 9 The Dirty Picture 18+ 10 Marley and me Romance   Types of joins Cross JOIN Cross JOIN is a simplest form of JOINs which matches each row from one database table to all rows of another. In other words it gives us combinations of each row of first table with all records in second table. Suppose we want to get all member records against all the movie records, we can use the script shown below to ge...
Read more
-
Image
COMMON TABLE EXPRESSIONS(cte): It is used best used to extract the hierarchical data  and it is best explained with an example: Problem Many organizations have some type of hierarchy for business processes. When it comes to large organizations, the hierarchy can get very complex and large, so building a hierarchy in a RDBMS is a tedious task. We have to create views, cursors and so on, but using a CTE in SQL Server is a better solution to retrieve hierarchy-based data. Solution Common Table Expressions (CTE) have two types, recursive and non-recursive. We will see how the recursive CTE works with examples. A recursive CTE can be explained in three parts: Anchor Query: This is the first statement which is executed. This query will give the base data for the CTE. Separator: This is the middle part where in we generally use a UNION ALL and few more operators. Recursive Query: This is the main part, this is the CTE query which refers to the same CTE by recursion. ...
Read more