On Mon, Mar 17, 2014 at 9:45 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
David Johnston <polobo@yahoo.com> writes: > Need to discuss the general "why" before any meaningful help on the "how" is > going to be considered by hackers.
Possibly worth noting is that in past discussions, we've concluded that the most sensible type of hint would not be "use this plan" at all, but "here's what to assume about the selectivity of this WHERE clause". That seems considerably less likely to break than any attempt to directly specify plan details.
Isnt using a user given value for selectivity a pretty risky situation as it can horribly screw up the plan selection?
Why not allow the user to specify an alternate plan and have the planner assign a higher preference to it during plan evaluation? This shall allow us to still have a fair evaluation of all possible plans as we do right now and yet have a higher preference for the user given plan during evaluation?
