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This Video contains In-depth coverage of Difference Between Table Scan, Index Scan, And Index Seek in Index. Covers all of the important queries and SQL commands. In this videos series, you'll learn how to read and write complex queries. Handle tricky SQL interview questions. Implement complex SQL queries and answer popular interview questions on SQL. These videos series will help to crack interviews and get jobs in SQL Developer, Database Developer, Data Engineer, Reporting Analyst, Data Analyst, Data Scientist, System Analyst, Power BI Developer, Tableau Developer, BI Developer, etc.
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Table Scan, Index Scan, And Index Seek
Difference between Scan and Seek Operation
Table Scan
It is a very simple process. While performing table scan, the query engine starts from the physical beginning of the table and it goes through every row into the table. If a row matches with the criteria then it includes that into the result set.
It is the fastest way to retrieve the data especially when there is quite a small table.
For a small table, a query engine can load all the data in a one-shot but from a large table it is not possible i.e. more IO and more time will be required to process those large data.
Generally, a full table scan is used when a query doesn’t have a WHERE clause i.e. all data.
Index Scan
When you have a clustered index and your query needs all or most of the records (i.e. query without where or having clause) then it uses an index scan.
Index scan works similar to the table scan during the query optimization process. The query optimizer takes a look at the available index and chooses one of the best, based on JOINs and WHERE clauses.
As the right index is being chosen, the SQL query processing engine will navigate the tree structure to the pointer of the data which matches the criteria and further extracts only the needed/required records.
The key difference between Table Scan and Index Scan is that data is stored in the index tree, the query processor knows it when it reaches the end of the current it is looking for. Then it can send the query or move on to the next range of data.
An index scan is slightly faster than the Table scan but significantly slower than an Index.
Index Seek
When the search criterion matches the index well enough which can navigate directly to particular points into the data, this is known as the Index seek.
The index seeks the fastest way to retrieve the data in the database.
For example, the following query will use the Index seek which can be confirmed by checking the execution plan of the query
The query optimizer can use an index directly to go to the 3rd employee id and fetch the data.
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13 мар 2022
