When it comes to optimizing queries in SQL Server, statistics play a crucial role. Just like we rely on our past experiences and observations to make decisions, SQL Server uses statistics to make informed choices about how to execute queries efficiently.
Statistics in SQL Server are a collection of data samples that provide information about the distribution and selectivity of values within a table. By analyzing these statistics, the query optimizer can determine the best execution plan for a query.
For example, let’s say we have a table called SalesInvoiceDetail. By examining a small sample of the data, we can make some initial observations. The InvoiceDetailID field appears to have unique values, making it highly selective. On the other hand, the Quantity field has many repeated values, indicating lower selectivity.
Based on these statistics, the query optimizer can make decisions on whether to use an index seek or scan for a particular field. If a field is highly selective and has a covering index, a seek operation can be performed, resulting in faster query execution.
SQL Server gathers statistics by either scanning the entire table or sampling a portion of the data. The choice depends on the size of the table and the urgency of updating the statistics. For larger tables, sampling is preferred to save time.
It’s important to note that statistics need to be constantly updated to reflect the most recent selectivity information. As new data is added to a table, the histogram record counts can become outdated. SQL Server automatically updates statistics, but you also have the option to manually update them using the UPDATE STATISTICS command.
By leveraging statistics, SQL Server can optimize query execution plans and improve overall performance. It allows the query optimizer to have a better understanding of the selectivity level of values before running a query, leading to more efficient and accurate results.
So, the next time you’re working with SQL Server, remember the importance of statistics in optimizing your queries. By keeping them up to date, you can ensure that your database performs at its best.