Horton

回复： The issue of incorrect width estimation in UNION queries

Wei Sun <936739278@qq.com>

View thread
From: Wei Sun <936739278@qq.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: pgsql-bugs <pgsql-bugs@lists.postgresql.org>
Date: 2025-09-24T11:27:11Z
Lists: pgsql-bugs

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Improve planner's width estimates for set operations

ok



Thank you for the reply，currently, we can also temporarily work around this issue by adding a hint,&nbsp;
look forward to the fix in the master branch.


Regards

Wei Sun


------------------&nbsp;原始邮件&nbsp;------------------
发件人:                                                                                                                        "David Rowley"                                                                                    <dgrowleyml@gmail.com&gt;;
发送时间:&nbsp;2025年9月24日(星期三) 晚上6:47
收件人:&nbsp;"Wei Sun"<936739278@qq.com&gt;;
抄送:&nbsp;"pgsql-bugs"<pgsql-bugs@lists.postgresql.org&gt;;
主题:&nbsp;Re: The issue of incorrect width estimation in UNION queries



On Wed, 24 Sept 2025 at 18:13, sunw.fnst <936739278@qq.com&gt; wrote:
&gt;&nbsp;&nbsp;&nbsp; -&gt;&nbsp; Append&nbsp; (cost=0.00..4082.00 rows=200000 width=38) (actual time=0.016..39.050 rows=200000 loops=1)
&gt;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -&gt;&nbsp; Seq Scan on t1&nbsp; (cost=0.00..1541.00 rows=100000 width=11) (actual time=0.015..10.056 rows=100000 loo
&gt; ps=1)
&gt;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -&gt;&nbsp; Seq Scan on t2&nbsp; (cost=0.00..1541.00 rows=100000 width=11) (actual time=0.018..10.654 rows=100000 loo
&gt; ps=1)

This happens because in generate_union_paths() -&gt; create_pathtarget()
-&gt; set_pathtarget_cost_width() in get_expr_width(), the varno for the
union's RelOptInfo is 0, which results in using the get_typavgwidth()
result.

set_append_rel_size() does a better job of this for the UNION ALL code
path. Maybe we can copy the relevant parts of that.

Also, I think if we're doing this, then it'd be a master-only fix.
Changing this could result in plan changes in the back-branches, which
we normally try to avoid.&nbsp; The attached patch is against master. I
need to spend a bit longer on this as generate_nonunion_paths() might
need the same treatment. I've just run out of time for tonight.

David