From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Geier <david@swarm64.com>
cc: pgsql-hackers@postgresql.org
Subject: Re: search_plan_tree(): handling of non-leaf CustomScanState nodes
 causes segfault
In-reply-to: <478756.1611069597@sss.pgh.pa.us>
References: <0253344d-9bdd-11c4-7f0d-d88c02cd7991@swarm64.com>
 <347100.1610995608@sss.pgh.pa.us>
 <3ad851cf-ce22-ace0-726d-56209e214981@swarm64.com>
 <416507.1611009730@sss.pgh.pa.us>
 <00d461cf-c919-6a3e-2dda-c1b5d2207f3d@swarm64.com>
 <478756.1611069597@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 19 Jan 2021 10:19:57 -0500"
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Date: Tue, 19 Jan 2021 11:53:02 -0500
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I wrote:
> Now that I look at this, I strongly wonder whether whoever added
> MergeAppend support here understood what they were doing.  That
> looks broken, because child nodes will typically be positioned on
> tuples, whether or not the current top-level output came from them.
> So I fear we could get a false-positive confirmation that some
> tuple matches WHERE CURRENT OF.

Urgh, indeed it's buggy.  With the attached test script I get

...
BEGIN
DECLARE CURSOR
 f1 | f2  
----+-----
  1 | one
(1 row)

UPDATE 1
UPDATE 1
UPDATE 1
COMMIT
 f1 |     f2      
----+-------------
  1 | one updated
(1 row)

 f1 |     f2      
----+-------------
  2 | two updated
(1 row)

 f1 |      f2       
----+---------------
  3 | three updated
(1 row)

where clearly only the row with f1=1 should have updated
(and if you leave off ORDER BY, so as to get a Merge not
MergeAppend plan, indeed only that row updates).

I shall go fix this, and try to improve the evidently-inadequate
comments in search_plan_tree.

			regards, tom lane


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Content-Description: where-current-of-merge-append.sql

drop table if exists t1,t2,t3;

create table t1 (f1 int unique, f2 text);
insert into t1 values (1, 'one');
create table t2 (f1 int unique, f2 text);
insert into t2 values (2, 'two');
create table t3 (f1 int unique, f2 text);
insert into t3 values (3, 'three');

explain
select * from t1 union all select * from t2 union all select * from t3
order by 1;

begin;

declare c cursor for
select * from t1 union all select * from t2 union all select * from t3
order by 1;

fetch 1 from c;

update t1 set f2 = f2 || ' updated' where current of c;
update t2 set f2 = f2 || ' updated' where current of c;
update t3 set f2 = f2 || ' updated' where current of c;

commit;

table t1;
table t2;
table t3;

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