On Tue, Nov 22, 2022 at 3:44 PM Richard Guo <guofengli...@gmail.com> wrote:
> On Wed, Nov 16, 2022 at 7:38 AM Sergey Shinderuk < > s.shinde...@postgrespro.ru> wrote: > >> The failing query is: >> SELECT * FROM >> (SELECT *, >> count(salary) OVER (PARTITION BY depname || '') c1, -- w1 >> row_number() OVER (PARTITION BY depname) rn, -- w2 >> count(*) OVER (PARTITION BY depname) c2, -- w2 >> count(*) OVER (PARTITION BY '' || depname) c3 -- w3 >> FROM empsalary >> ) e WHERE rn <= 1 AND c1 <= 3; >> As far as I understand, ExecWindowAgg for the intermediate WindowAgg >> node switches into pass-through mode, stops evaluating row_number(), and >> returns the previous value instead. But if int8 is passed by reference, >> the previous value stored in econtext->ecxt_aggvalues becomes a dangling >> pointer when the per-output-tuple memory context is reset. > > > Yeah, you're right. In this example the window function row_number() > goes into pass-through mode after the second evaluation because its > run condition does not hold true any more. The remaining run would just > return the result from the second evaluation, which is stored in > econtext->ecxt_aggvalues[wfuncno]. > > If int8 is configured as pass-by-ref, the precomputed value from the > second evaluation is actually located in a memory area from context > ecxt_per_tuple_memory, with its pointer stored in ecxt_aggvalues. As > this memory context is reset once per tuple, we would be prone to wrong > results. > Regarding how to fix this problem, firstly I believe we need to evaluate window functions in the per-tuple memory context, as the HEAD does. When we decide we need to go into pass-through mode, I'm thinking that we can just copy out the results of the last evaluation to the per-query memory context, while still storing their pointers in ecxt_aggvalues. Does this idea work? Thanks Richard
