Re: Killing off removed rels properly

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: David Rowley <dgrowleyml(at)gmail(dot)com>
Cc: Alexander Lakhin <exclusion(at)gmail(dot)com>, Richard Guo <guofenglinux(at)gmail(dot)com>, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Killing off removed rels properly
Date: 2023-02-20 19:13:35
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I wrote:
>> It looks like we're somehow triggering the elide-a-left-join code
>> when we shouldn't?

So the reason why we see this with a partitioned target table and not
with a regular target table reduces to this bit in preprocess_targetlist:

* For non-inherited UPDATE/DELETE/MERGE, register any junk column(s)
* needed to allow the executor to identify the rows to be updated or
* deleted. In the inheritance case, we do nothing now, leaving this to
* be dealt with when expand_inherited_rtentry() makes the leaf target
* relations. (But there might not be any leaf target relations, in which
* case we must do this in distribute_row_identity_vars().)
if ((command_type == CMD_UPDATE || command_type == CMD_DELETE ||
command_type == CMD_MERGE) &&
/* row-identity logic expects to add stuff to processed_tlist */
root->processed_tlist = tlist;
add_row_identity_columns(root, result_relation,
target_rte, target_relation);
tlist = root->processed_tlist;

This happens before join removal, so that we see use of the row identity
columns of a regular table as a reason not to do join removal. But
expand_inherited_rtentry will happen after that, too late to stop join

I think the best fix is just to teach join removal that it mustn't
remove the result relation, as attached (needs a regression test).

I'm a little inclined to back-patch this, even though I think it's
probably unreachable in v15. It's a cheap enough safety measure,
and at the very least it will save a few cycles deciding that we
can't remove the target table of a MERGE.

regards, tom lane

Attachment Content-Type Size
dont-elide-MERGE-target-rel.patch text/x-diff 732 bytes

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