| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Richard Guo <guofenglinux(at)gmail(dot)com> |
| Cc: | Andrey Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Optimization issue of branching UNION ALL |
| Date: | 2022-12-22 04:14:26 |
| Message-ID: | 1803395.1671682466@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
I wrote:
> Richard Guo <guofenglinux(at)gmail(dot)com> writes:
>> I noticed we also check 'parse->hasSubLinks' when we fix PHVs and
>> AppendRelInfos in pull_up_simple_subquery. I'm not sure why we have
>> this check. It seems not necessary.
> Yeah, I was wondering about that too ... maybe it was important
> in some previous state of the code? I didn't do any archeology
> though.
After a bit of "git blame"-ing, it appears that that hasSubLinks
check was introduced in e006a24ad, which added a FlattenedSubLink
node type and needed to fix them up here:
+ * We also have to fix the relid sets of any FlattenedSubLink nodes in
+ * the parent query. (This could perhaps be done by ResolveNew, but it
Then when I got rid of FlattenedSubLink in e549722a8, I neglected
to remove that check. So I think maybe we don't need it, but I've
not tested.
regards, tom lane
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