9a5f369adc
In commit 1345cc67bbb014209714af32b5681b1e11eaf964, I introduced caching of expressions representing type-cast operations into plpgsql. However, I supposed that I could cache both the expression trees and the evaluation state trees derived from them for the life of the session. This doesn't work, because we execute the expressions in plpgsql's simple_eval_estate, which has an ecxt_per_query_memory that is only transaction-lifespan. Therefore we can end up putting pointers into the evaluation state tree that point to transaction-lifespan memory; in particular this happens if the cast expression calls a SQL-language function, as reported by Geoff Winkless. The minimum-risk fix seems to be to treat the state trees the same way we do for "simple expression" trees in plpgsql, ie create them in the simple_eval_estate's ecxt_per_query_memory, which means recreating them once per transaction. Since I had to introduce bookkeeping overhead for that anyway, I bought back some of the added cost by sharing the read-only expression trees across all functions in the session, instead of using a per-function table as originally. The simple-expression bookkeeping takes care of the recursive-usage risk that I was concerned about avoiding before. At some point we should take a harder look at how all this works, and see if we can't reduce the amount of tree reinitialization needed. But that won't happen for 9.5.
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