On Tue, 16 Apr 2019 at 23:55, Yuzuko Hosoya <hosoya.yuz...@lab.ntt.co.jp> wrote: > postgres=# explain analyze select * from t1 where dt = current_date + 400; > QUERY PLAN > --------------------------------------------------------------------------------------- > Append (cost=0.00..198.42 rows=44 width=8) (actual time=0.000..0.001 rows=0 > loops=1) > Subplans Removed: 3 > -> Seq Scan on t1_1 (cost=0.00..49.55 rows=11 width=8) (never executed) > Filter: (dt = (CURRENT_DATE + 400)) > Planning Time: 0.400 ms > Execution Time: 0.070 ms > (6 rows) > ---- > > I realized t1_1 was not scanned actually since "never executed" > was displayed in the plan using EXPLAIN ANALYZE. But I think > "One-Time Filter: false" and "Subplans Removed: ALL" or something > like that should be displayed instead. > > What do you think?
This is intended behaviour explained by the following comment in nodeAppend.c /* * The case where no subplans survive pruning must be handled * specially. The problem here is that code in explain.c requires * an Append to have at least one subplan in order for it to * properly determine the Vars in that subplan's targetlist. We * sidestep this issue by just initializing the first subplan and * setting as_whichplan to NO_MATCHING_SUBPLANS to indicate that * we don't really need to scan any subnodes. */ It's true that there is a small overhead in this case of having to initialise a useless subplan, but the code never tries to pull any tuples from it, so it should be fairly minimal. I expected that using a value that matches no partitions would be unusual enough not to go contorting explain.c into working for this case. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
