Bucky Jordan wrote:
Is this along the lines of "I'm loading a big table and touching every row of data, so I may as well collect some stats along the way" and "I know my data contains these statistical properties, but the analyzer wasn't able to figure that out (or maybe can't figure it out efficiently enough)"? While it seems like this would require more knowledge from the user (e.g. more about their data, how the planner works, and how it uses statistics) this would actually be helpful/required for those who really care about performance. ...
The user would have to know his data, but he wouldn't need to know how the planner works. While with hints like "use index X", he *does* need to know how the planner works.
Being able to give hints about statistical properties of relations and their relationships seems like a good idea to me. And we can later figure out ways to calculate them automatically.
BTW, in DB2 you can declare a table as volatile, which means that the cardinality of the table varies greatly. The planner favors index scans on volatile tables.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com
