On 2019/04/21 15:25, David Rowley wrote: > On Fri, 19 Apr 2019 at 20:01, Amit Langote > <langote_amit...@lab.ntt.co.jp> wrote: >> Another approach, as I mentioned above, is to extend the hack that begins >> in nodeAppend.c (and nodeMergeAppend.c) into explain.c, as in the >> attached. Then: >> >> explain verbose select * from t1 where dt = current_date + 400 order by id; >> QUERY PLAN >> ─────────────────────────────────────────────────── >> Sort (cost=199.62..199.73 rows=44 width=8) >> Output: t1_1.id, t1_1.dt >> Sort Key: t1_1.id >> -> Append (cost=0.00..198.42 rows=44 width=8) >> Subplans Removed: 4 >> (5 rows) > > We could do that, but I feel that's making EXPLAIN tell lies, which is > probably a path we should avoid. The lies might be fairly innocent > today, but maintaining them over time, like any lie, might become more > difficult. We did perform init on a subnode, the subnode might be an > index scan, which we'd have obtained a lock on the index. It could be > fairly difficult to explain why that is given the lack of mention of > it in the explain output.
I had overlooked the fact that ExecInitAppend and ExecInitMergeAppend actually perform ExecInitNode on the subplan, so on second thought, I agree we've got to show it. Should this have been documented? The chance that users may query for values that they've not defined partitions for might well be be non-zero. > The fix I was working on before heading away for Easter was around > changing ruleutils.c to look at Plan nodes rather than PlanState > nodes. I'm afraid that this would still suffer from showing the alias > of the first subnode but not show it as in the explain output you show > above, but it does allow us to get rid of the code the initialises the > first subnode. I think that's a much cleaner way to do it. I agree. > I agree with Tom about the v13 part. If we were having this discussion > this time last year, then I'd have likely pushed for a v11 fix, but > since it's already shipped like this in one release then there's not > much more additional harm in two releases working this way. I'll try > and finished off the patch I was working on soon and submit to v13's > first commitfest. OK, I'll try to review it. Thanks, Amit
