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* UNION does not append
@ 2025-10-07 22:27 PG Doc comments form <[email protected]>
2025-10-08 08:33 ` Re: UNION does not append Laurenz Albe <[email protected]>
2025-10-08 11:29 ` Re: UNION does not append David G. Johnston <[email protected]>
0 siblings, 2 replies; 3+ messages in thread
From: PG Doc comments form @ 2025-10-07 22:27 UTC (permalink / raw)
To: [email protected]; +Cc: [email protected]
The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/18/queries-union.html
Description:
I'm very confused about this statement in the documentation of UNION: "UNION
effectively appends the result of query2 to the result of query1".
Because, if I append one item to a list, it is still the same list. But in
the case of UNION, it's ""copied"" into a new anonymous derived-table. Alias
don't work anymore, which I would expect if I just append some rows. Or is
it like "effectively" means that this statement is not to be taken
literally? In that case I would really appreciate a more detailed
description, that after the UNION with tables a new anonymous derived-table
is returned and the original first table can't be accessed anymore. Thank
you for taking your time to read this even if you disagree.
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: UNION does not append
2025-10-07 22:27 UNION does not append PG Doc comments form <[email protected]>
@ 2025-10-08 08:33 ` Laurenz Albe <[email protected]>
1 sibling, 0 replies; 3+ messages in thread
From: Laurenz Albe @ 2025-10-08 08:33 UTC (permalink / raw)
To: [email protected]; [email protected]
On Tue, 2025-10-07 at 22:27 +0000, PG Doc comments form wrote:
> Page: https://www.postgresql.org/docs/18/queries-union.html
>
> I'm very confused about this statement in the documentation of UNION: "UNION
> effectively appends the result of query2 to the result of query1".
> Because, if I append one item to a list, it is still the same list. But in
> the case of UNION, it's ""copied"" into a new anonymous derived-table. Alias
> don't work anymore, which I would expect if I just append some rows. Or is
> it like "effectively" means that this statement is not to be taken
> literally? In that case I would really appreciate a more detailed
> description, that after the UNION with tables a new anonymous derived-table
> is returned and the original first table can't be accessed anymore. Thank
> you for taking your time to read this even if you disagree.
I can understand your confusion. Yes, "effectively" means that you shouldn't
take it literally, but UNION does behave *as if* the second result is appended
to the first one.
Look at this:
SELECT * FROM (VALUES (1), (5), (3)) AS t1(x)
UNION ALL
SELECT * FROM (VALUES (2), (4), (3)) AS t2(y);
x
═══
1
5
3
2
4
3
(6 rows)
The column alias is "x", which is the alias from the first result.
And look at the execution plan:
EXPLAIN
SELECT * FROM (VALUES (1), (5), (3)) AS t1(x)
UNION ALL
SELECT * FROM (VALUES (2), (4), (3)) AS t2(y);
QUERY PLAN
═════════════════════════════════════════════════════════════════════
Append (cost=0.00..0.11 rows=6 width=4)
-> Values Scan on "*VALUES*" (cost=0.00..0.04 rows=3 width=4)
-> Values Scan on "*VALUES*_1" (cost=0.00..0.04 rows=3 width=4)
(3 rows)
So I'd say that the documentation is alright.
Yours,
Laurenz Albe
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: UNION does not append
2025-10-07 22:27 UNION does not append PG Doc comments form <[email protected]>
@ 2025-10-08 11:29 ` David G. Johnston <[email protected]>
1 sibling, 0 replies; 3+ messages in thread
From: David G. Johnston @ 2025-10-08 11:29 UTC (permalink / raw)
To: [email protected] <[email protected]>; [email protected] <[email protected]>
On Tuesday, October 7, 2025, PG Doc comments form <[email protected]>
wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/18/queries-union.html
> Description:
>
> I'm very confused about this statement in the documentation of UNION:
> "UNION
> effectively appends the result of query2 to the result of query1".
> Because, if I append one item to a list, it is still the same list. But in
> the case of UNION, it's ""copied"" into a new anonymous derived-table.
> Alias
> don't work anymore, which I would expect if I just append some rows. Or is
> it like "effectively" means that this statement is not to be taken
> literally? In that case I would really appreciate a more detailed
> description, that after the UNION with tables a new anonymous derived-table
> is returned and the original first table can't be accessed anymore. Thank
> you for taking your time to read this even if you disagree.
>
Yes, if you place a set-operation using query into a subquery context the
subquery has to be given its own name: and the individual from clause
relations of the contained query are not exposed. This is just how
subqueries work.
If you don’t place it in a subquery the rows are just sent to the client.
In both cases the word append simply means “using the column structure of
the first component query”. Though Imsuppose that is strictly “column
names” since types can be changed to a more encompassing one if needed.
David J.
^ permalink raw reply [nested|flat] 3+ messages in thread
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2025-10-07 22:27 UNION does not append PG Doc comments form <[email protected]>
2025-10-08 08:33 ` Laurenz Albe <[email protected]>
2025-10-08 11:29 ` David G. Johnston <[email protected]>
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