When working with SQL Server, it’s important to understand the concepts of index scan and index seek. These two operations play a crucial role in retrieving data efficiently from a table.
Index Scan
An index scan retrieves all the rows from a table. It scans through every row, regardless of whether it qualifies for the query’s predicate. The cost of an index scan is proportional to the total number of rows in the table. Therefore, it is an efficient strategy for small tables or when most of the rows qualify for the query.
For example, if a query is retrieving more than 50% or 90% of the data from a table, the optimizer may choose to perform an index scan. This means that it will scan all the data pages from the first page to the last page to retrieve the required data rows. If there is no index on the table, a table scan (index scan) may be seen in the execution plan.
Index Seek
An index seek, on the other hand, retrieves selective rows from a table. It only touches the rows and pages that qualify for the query’s predicate. The cost of an index seek is proportional to the number of qualifying rows and pages, rather than the total number of rows in the table.
Index seeks are generally preferred for highly selective queries. These are queries that request a smaller number of rows or retrieve a specific percentage (e.g., 10% or 15%) of the rows from the table. The query optimizer tries to use an index seek when it finds a useful index to retrieve the desired recordset.
However, if there is no index or no useful indexes on the table, SQL Server has to scan all the records that satisfy the query condition.
Conclusion
Understanding the difference between index scan and index seek is crucial for optimizing query performance in SQL Server. Index scans are efficient for retrieving a large amount of data or when most of the rows qualify for the query. On the other hand, index seeks are preferred for highly selective queries that retrieve a smaller subset of rows.
By analyzing the execution plans and considering the selectivity of your queries, you can make informed decisions on when to use index scan or index seek to improve the performance of your SQL Server database.