What happens if (for example) DateStyle changes between the two parses?
From my original email:
This is the common case of a more general problem: a query plan depends on various parts of the environment at plan-creation time. That environment includes the definitions of database objects, but also GUC variables (most importantly search_path, but also optimizer-tuning variables for example), the state of database statistics, and so on. I'll leave resolution of the more general problem to someone else -- I think if we can manage to invalidate plans automatically when dependent objects change, that's better than nothing.
I'm don't think recreating the plan from the query string changes this fundamentally -- the interaction between (for example) GUC variables and prepared plans will likely not be well-defined (which will be no worse than it is today).
(It might be plausible to work something out where a backend-local SET of some subset of the GUC variables flushes all the cached query plans, but I'm don't have any plans to look at it myself.)
-Neil
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