[DERBY-1907] Improve optimizer to prune query plans more aggressively.

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 10.1.3.2, 10.2.1.6, 10.3.1.4
• Component/s: SQL
• Labels: derby_triage10_11
• Urgency: Normal
• Bug behavior facts: Performance
• Assignee: Unassigned
• Reporter: A B
• Created: 29/Sep/06 21:23
• Updated: 25/Jun/13 05:05

Description

For large queries with numerous and deeply-nested subqueries like those in DERBY-1777, the number of different index/joinOrder/joinStrategy combinations that the optimizer can process is extremely high. In the absence of an efficient timeout mechanism (see DERBY-1906) this can lead to situations where the optimizer spends far too much time trying out different access paths before picking a plan. In the case of the DERBY-1777 queries, the optimizer can literally spend hours trying out all of the possible combinations before deciding on the best one.

Even if/when the timeout mechanism is improved, it would still be great if the Derby optimizer could somehow figure out how to aggressively prune out access paths that are known (or suspected) to be sub-optimal. This way the optimizer would spend less time optimizing "bad" access paths and could therefore try out more "good" plans before timeout occurs.

I have no idea how this pruning would occur, but I do think it would be extremely beneficial to the Derby optimization process when dealing with deeply nested queries with large FROM lists.

Issue Links

• relates to: DERBY-4327 Implement optimizer support for bushy trees.