Shaun Thomas <stho...@optionshouse.com> wrote: > >> these issues tend to get solved through optimization fences. >>> Reorganize a query into a CTE, or use the (gross) OFFSET 0 trick. >>> How are these nothing other than unofficial hints? >>> >> Yeah, the cognitive dissonance levels get pretty high around this >> issue. Some of the same people who argue strenuously against >> adding hints about what plan should be chosen also argue against >> having clearly equivalent queries optimize to the same plan because >> they find the fact that they don't useful for coercing a decent >> plan sometimes. That amounts to a hint, but obscure and >> undocumented. (The OP may be wondering what this "OFFSET 0 trick" >> is, and how he can use it.) >> > +1. I've said this or something like it at least a half-dozen times. Postgres DOES have hints, they're just obscure, undocumented and hard to use. If a developer chooses to use them, they become embedded in the app and forgotten. They're hard to find because there's nothing explicit in the SQL to look for. You have to know to look for things like "OFFSET" or "SET ...". Five years down the road when the developer is long gone, who's going to know why "... OFFSET 0" was put in the code unless the developer made careful comments?
> With explicit, documented hints, one could search for hints of a >> particular type should the optimizer improve to the point where >> they are no longer needed. It is harder to do that with subtle >> differences in syntax choice. Figuring out which CTEs or LIMITs >> were chosen because they caused optimization barriers rather than >> for their semantic merit takes some effort. > > Exactly. I'll make a bet here. I'll bet that the majority of large Postgres installations have at least one, probably several, SQL statements that have been "hinted" in some way, either with CTEs or LIMITs, or by using SET to disable a particular query type, and that these "hints" are critical to the system's performance. The question is not whether to have hints. The question is how to expose hints to users. Craig
