Recently, for example, I watched a query go from taking hours to taking seconds just because we got a good set of statistics on the data.
The topic of statistics and their maintenance is not straightforward.
Column statistics can help the query optimizer choose a better plan in specific cases, such as for queries involving multiple tables or tables that already have indexes.
SQL Server 2005 generates and automatically maintains statistics for every index you create, a feature that can help you in your query performance-tuning efforts.
Often statistics are created on a single column but it’s not uncommon to create statistics on multiple columns.
How do you find out if statistics are correct, and what can you do if the automatic update of statistics isn't right for the way a table is used?
Statistics are critical metadata used by SQL Server’s query optimizer, which influence the selected execution plan for a query.
The optimizer obtains its knowledge of the data, its distribution, and the number of rows a given query is likely to return from the available statistics.
Based on this knowledge, it decides the optimal access path, making choices such as whether to scan a table or perform an index seek, use a nested loop join or a hash join, and so on.