An index will have an entry in the sys. The below T-SQL script can be used to query the sys. The script will be as shown below:. The previous result provides us with more detailed information about the number of data inserting and modification operations occurred on each index. This result, in conjunction with the sys. SQL Server provides us with two built-in reports that help us in monitoring the database indexes fragmentation and usage statistics, the Index Usage Statistics and the Index Physical Statistics.
These standard reports use the previously described DMOs, and the reports data will be refreshed when the SQL Server service is restarted. The first report is the Index Usage Statistics report, that consists of two parts; the Index Usage Statistics report that shows statistics about the number of Scans, Seeks, Updates and Lookups with the latest date for each operation, that is retrieved by querying the sys. Note that the result in the report differ from the previous DMV result, as the SQL Server service is restarted on my machine and a different workload performed on that database.
The statistics in that report are retrieved by querying the sys. The second report is the Index Physical Statistics report, that returns statistics about the index partitions, fragmentation percentage and the number of pages on each index partition.
This report also gives the recommendation to rebuild or reorganize the index depending on the fragmentation percentage of the index.
The recommendation provided by that report does not take into consideration the size of the table. If you try to rebuild an index on a small table, you may still receive the same recommendation from that report. The report in our case will be as shown below:.
In this article, we described the different methods that can be used to gather statistical information about the SQL Server indexes structure and usage. FROM sys. Should there be only one column used, a single-column index should be the choice. Should there be two or more columns that are frequently used in the WHERE clause as filters, the composite index would be the best choice. Implicit indexes are indexes that are automatically created by the database server when an object is created.
Indexes are automatically created for primary key constraints and unique constraints. Care should be taken when dropping an index because the performance may either slow down or improve. When performing a table scan, the query optimizer reads all the rows in the table, and extracts the rows that meet the criteria of the query.
However, a table scan could be the most efficient method if, for example, the result set of the query is a high percentage of rows from the table. When the query optimizer uses an index, it searches the index key columns, finds the storage location of the rows needed by the query and extracts the matching rows from that location.
Generally, searching the index is much faster than searching the table because unlike a table, an index frequently contains very few columns per row and the rows are in sorted order.
The query optimizer typically selects the most efficient method when executing queries. However, if no indexes are available, the query optimizer must use a table scan. Your task is to design and create indexes that are best suited to your environment so that the query optimizer has a selection of efficient indexes from which to select. SQL Server provides the Database Engine Tuning Advisor to help with the analysis of your database environment and in the selection of appropriate indexes.
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Privacy policy. There is not much for users to do on SQL Server when it comes to statistics because leaving the defaults is generally the best practice which ultimately auto-creates and updates statistics. Also worth mentioning are selectivity and density when creating SQL indexes. These are just measurements used to measure index weight and quality:. These two are proportional one to another and are used to measure both index weight and quality. Essentially how this works in the real world can be explained in an artificial example.
If there is a query that hits that column often coming either from us or application and retrieves no more than 5 rows that means that our selectivity is 0. That is what we should aim for when creating an index. In the best-case scenario, we should have indexes that are highly selective which basically means that queries coming at them should return a low number of rows. So the first thing we can do is to enable IO statistics. This is a great way to see how much work SQL Server has to do under the hood to retrieve the data.
This tool will show us the execution plan that was used to retrieve the data so we can see what SQL indexes, if any, are used. The following query will be used as an example:. They are basically creating a clean system state without shutting down and restarting the SQL Server. If expanded, the folder is empty:.
This will prompt the Database connection dialog first time in which we have to choose the SQL Server, authentication method and the appropriate database to connect to:. To be more specific, the query returns only 1. If we hover the mouse over the red exclamation mark, an additional tooltip will show the IO cost as well.
In this case,
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