回复： The issue of incorrect width estimation in UNION queries

ok

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

Regards

Wei Sun

------------------ 原始邮件 ------------------
发件人: "David Rowley" <dgrowleyml(at)gmail(dot)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(at)qq(dot)com&gt;;
抄送:&nbsp;"pgsql-bugs"<pgsql-bugs(at)lists(dot)postgresql(dot)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(at)qq(dot)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