On Thu, Jan 17, 2019 at 10:48 AM Andreas Karlsson <andreas@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
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