On Thu, Apr 19, 2012 at 11:39 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> A variant idea would be to replace the exact cost comparison with a > second round of fuzzy cost comparison, but with a much tighter fuzz > factor, maybe 1e-6 instead of 0.01. The fuzz factor is a better idea, IMHO. I would like to see that as a user set parameter. Jim is right that plan stability is a wide problem, which could be addressed by setting a higher fuzz factor when plan instability is observed. Instability normally occurs for queries near a decision point in the cost models, which can fluctuate as exacts stats change. For those queries, it would be much better to address the instability directly via a fuzz factor than to attempt to fiddle with the enable_* parameters as is now common practice. Doc entry for the parameter... plan_choice_factor (float) - plans that vary in cost by less than this factor are considered equal by the planner, leading to a consistent choice of the first derived plans even across different types of hardware. This has a tendency to favour indexed plans when they are available. This is designed to address problems related to plan stability and may not produce desired results if used as a hinting/tweaking mechanism. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
