On Thu, Jan 17, 2019 at 10:48 AM Andreas Karlsson <andr...@proxel.se> wrote: > On 1/11/19 8:10 PM, Robert Haas wrote: > > WITH cte_name [[NOT] MATERIALIZED] AS (query) main_query... > > Hm, when would one want "NOT MATERIALIZED"? I am not sure I see the > usefulness of forcing inlining other than if we by default do not inline > when a CTE is referenced multiple times.
When the planner materializes it, but the performance of the resulting plan therefore sucks, I suppose. I don't feel super-strongly about this, and Tom is right that there may be cases where materialization is just not practical due to implementation restrictions. But it's not crazy to imagine that inlining a multiply-referenced CTE might create opportunities for optimization at each of those places, perhaps not the same ones in each case, whereas materializing it results in doing extra work. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
