From: | Yeb Havinga <yebhavinga(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | pgsql-hackers(at)postgreSQL(dot)org |
Subject: | Re: EquivalenceClasses and subqueries and PlaceHolderVars, oh my |
Date: | 2012-03-15 09:16:03 |
Message-ID: | 4F61B353.2010700@gmail.com |
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Lists: | pgsql-hackers |
On 2012-03-15 02:29, Tom Lane wrote:
>
> explain select * from
> (select thousand as t1, tenthous as t2 from tenk1 a
> union all
> select 42 as t1, 42 as t2 from tenk1 b) c
> order by t1, t2;
>
> There is an EquivalenceClass for each of "t1" and "t2", and if we don't
> do something like wrapping the constants with distinct PHVs, then
> add_child_rel_equivalences will end up pushing identical constants into
> both ECs, thus totally bollixing the fundamental rule that any expression
> should match at most one EC.
I'm having a hard time imagining that add_child_rel_equivalences is not
just plain wrong. Even though it will only add child equivalence members
to a parent eq class when certain conditions are met, isn't it the case
that since a union (all) is addition of tuples and not joining, any kind
of propagating restrictions on a append rel child member to other areas
of the plan can cause unwanted results, like the ones currently seen?
regards,
Yeb Havinga
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