Statistics play an important role for the query optmizer to draw a optimizd execution plan , statistics are created against on indexed columns and and non indexed
column as default behaviour of sql server, but the point is how does sql server manage to update statistics when underlying data is updated are new data is inserted
lets see simple example when does sql server updates the statistics for indexed columns
SQL Server also updates the statistics
- When a table has fewer than 500 rows and is increased by 500 or more rows
- When a table has more than 500 rows and is increased by 500 rows + 20 percent of the number of rows
As a developer I was always curious to find when does sql server decides to for go Index seek or Index scan for a given table
DBCC SHOW_STATISTICS
Sqljunkieshare 
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