From: Arne Roland <A.Roland@index.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Guo <riguo@pivotal.io>
Cc: pgsql-hackers@lists.postgresql.org <pgsql-hackers@lists.postgresql.org>
Subject: Re: Partial join
Date: Thu, 1 Aug 2019 16:29:21 +0000
Message-ID: <563d1a984c2c41b5b126318aafceab11@index.de> (raw)
In-Reply-To: <5928.1564668894@sss.pgh.pa.us>
References: <9dc03e949e9b4327a0d074da9ab44318@index.de>
<dd01626a46a342f7b1d811cf38fcebe6@index.de>
<CAN_9JTxQtJr2RHLKAfDCJoZFF+qsUMAF5JF+BVyAVcmcC2kSQA@mail.gmail.com>
<5928.1564668894@sss.pgh.pa.us>
"Tom Lane" <tgl@sss.pgh.pa.us> wrote:
> Uh ... why? The pushed-down restrictions should result in pruning
> away any prunable partitions at the scan level, leaving nothing for
> the partitionwise join code to do.
It seems reasonable to me that the join condition can no longer be verified, since 'sc.sl = sg.sl' is now replaced by 'sg.sl = 5' so the join condition can no longer be validated.
It's true that the pruning would prune everything but one partition, in case we'd just have a single column partition key. But we don't. I don't see how pruning partitions should help in this case, since we are left with multiple partitions for both relations.
Regards
Arne
________________________________
From: Tom Lane <tgl@sss.pgh.pa.us>
Sent: Thursday, August 1, 2019 4:14:54 PM
To: Richard Guo
Cc: Arne Roland; pgsql-hackers@lists.postgresql.org
Subject: Re: Partial join
Richard Guo <riguo@pivotal.io> writes:
> For the third query, a rough investigation shows that, the qual 'sl =
> 5' and 'sc.sl = sg.sl' will form an equivalence class and generate two
> implied equalities: 'sc.sl = 5' and 'sg.sl = 5', which can be pushed
> down to the base rels. One consequence of the deduction is when
> constructing restrict lists for the joinrel, we lose the original
> restrict 'sc.sl = sg.sl', and this would fail the check
> have_partkey_equi_join(), which checks if there exists an equi-join
> condition for each pair of partition keys. As a result, this joinrel
> would not be considered as an input to further partitionwise joins.
> We need to fix this.
Uh ... why? The pushed-down restrictions should result in pruning
away any prunable partitions at the scan level, leaving nothing for
the partitionwise join code to do.
regards, tom lane
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Cc: A.Roland@index.de, tgl@sss.pgh.pa.us, riguo@pivotal.io, pgsql-hackers@lists.postgresql.org
Subject: Re: Partial join
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