Thread: Functions in sort order - undocumented restriction
The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/9.5/static/queries-order.html Description: This works: select distinct o.bid, organisation, posttown, replace(case when phone ='' then null else trim(phone) end, ' ', ''), phone, o.active, website, email, (select count(*) from notes as n where n.bid = o.bid) as nn from organisations as o right join notes as n on o.bid = n.bid where true order by replace(case when phone ='' then null else trim(phone) end, ' ', '') nulls last ; This does not work: select distinct (o.bid), organisation, posttown, replace(case when postcode ='' then null else trim(postcode) end, ' ', '') as pc, phone, o.active, website, email, (select count(*) from notes as n where n.bid = o.bid) as nn from organisations as o right join notes as n on o.bid = n.bid where true order by replace(case when phone ='' then null else trim(phone) end, ' ', '') nulls last ; ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list LINE 1: ...n notes as n on o.bid = n.bid where true order by replace(ca... The documentation does not explain the restriction, nor that, or why, you cannot use 'as' to rename the field and then cite the renamed version.
On Saturday, February 10, 2018, PG Doc comments form <noreply@postgresql.org> wrote:
The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/9.5/static/queries-order. html
Description:
This works:
select distinct o.bid, organisation, posttown, replace(case when phone =''
then null else trim(phone) end, ' ', ''), phone, o.active, website, email,
(select count(*) from notes as n where n.bid = o.bid) as nn from
organisations as o right join notes as n on o.bid = n.bid where true order
by replace(case when phone ='' then null else trim(phone) end, ' ', '')
nulls last ;
This does not work:
select distinct (o.bid), organisation, posttown, replace(case when postcode
='' then null else trim(postcode) end, ' ', '') as pc, phone, o.active,
website, email, (select count(*) from notes as n where n.bid = o.bid) as nn
from organisations as o right join notes as n on o.bid = n.bid where true
order by replace(case when phone ='' then null else trim(phone) end, ' ',
'') nulls last ;
ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select
list
I suppose that error message should say "FOR SELECT DISTINCT ON, ORDER BY ..."
LINE 1: ...n notes as n on o.bid = n.bid where true order by replace(ca...
The documentation does not explain the restriction,
It is explained in the SELECT command documentation.
"""
The DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s). The ORDER BY clause will normally contain additional expression(s) that determine the desired precedence of rows within each DISTINCT ON group.
"""
nor that, or why, you
cannot use 'as' to rename the field and then cite the renamed version.
You can and it does with limitations.
"Note that an output column name has to stand alone, that is, it cannot be used in an expression"
You are correct that the "why" behind some of the decisions and limitations is not always covered in great detail.
I suppose that the sorting docs could cover DSTINCT ON, it just needs someone willing to write it up. Since the SELECT docs cover the material it is a usability issue as opposed to a correctness one. Also, that chapter is somewhat of an introductory piece and distinct on is somewhat of an intermediate query concept. Adding it there might be considered too much information at that point in the docs. Given the directness of the error and the coverage in the command reference the status quo has merit.
David J.
=?utf-8?q?PG_Doc_comments_form?= <noreply@postgresql.org> writes:
> This does not work:
> select distinct (o.bid), organisation, posttown, replace(case when postcode
> ='' then null else trim(postcode) end, ' ', '') as pc, phone, o.active,
> website, email, (select count(*) from notes as n where n.bid = o.bid) as nn
> from organisations as o right join notes as n on o.bid = n.bid where true
> order by replace(case when phone ='' then null else trim(phone) end, ' ',
> '') nulls last ;
> ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
What do you find unclear about this error message?
The reason seems clear enough to me: if the ordering expression isn't one
of the values that are being de-duplicated on, then there isn't a unique
value to associate with each surviving row for sorting purposes.
regards, tom lane
The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/9.5/static/queries-order. html
Description:
This works:
select distinct o.bid, organisation, posttown, replace(case when phone =''
then null else trim(phone) end, ' ', ''), phone, o.active, website, email,
(select count(*) from notes as n where n.bid = o.bid) as nn from
organisations as o right join notes as n on o.bid = n.bid where true order
by replace(case when phone ='' then null else trim(phone) end, ' ', '')
nulls last ;
This does not work:
select distinct (o.bid), organisation, posttown, replace(case when postcode
='' then null else trim(postcode) end, ' ', '') as pc, phone, o.active,
website, email, (select count(*) from notes as n where n.bid = o.bid) as nn
from organisations as o right join notes as n on o.bid = n.bid where true
order by replace(case when phone ='' then null else trim(phone) end, ' ',
'') nulls last ;
ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select
list
LINE 1: ...n notes as n on o.bid = n.bid where true order by replace(ca...
The documentation does not explain the restriction, nor that, or why, you
cannot use 'as' to rename the field and then cite the renamed version.
Let me get some clarity now since my first reply saw an "ON" clause where there isn't one...
What was the point of adding parens around o.bid in the second query? (I assumed it was to do DISTINCT ON but you didn't add the word ON)
Why did you change replace(case when phone...) in the first query with replace (case when postcode...) in the second query - but only in the select-list? (phone no longer exists in the select-list and thus you get the error.)
The following works just fine with an alias in the ORDER BY:
WITH vals (v,t,o) AS ( VALUES (1,'a',10),(2,'c',20),(3,'d',30),(3,'b',40),(4,'e',50) )
SELECT DISTINCT v, replace(t || '2', ' ', '') AS v2
FROM vals ORDER BY v2
Adding "o" to the ORDER BY provokes the error you have been seeing.
Tom has explained why this is the case. In short, DISTINCT seems to effectively remove the ability to specify "arbitrary expressions from input column values" in the ORDER BY - though you can still specify non-arbitrary expressions, as long as they are the same expressions that are used in the select-list.
David J.