Shaun Thomas <sthomas@xxxxxxxxxxxxxxxx> 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.) > Well... they're worse, really. Hints can be deprecated, disabled > in configs, or ignored in extreme cases. Optimization fences are > truly forever. +1 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. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
