RE: [HACKERS] views and group by (formerly: create view as selec t distinct)

This implies that the "group by" clause is not supported in views.  I have
created views that use the group by clause and they appear to work.  I have
not verified the content of the records.  I would like to know more about
what Jan means when he says that "group by" is not supported in views?  Does
it mean that the content of the results could be unexpected or are they
conditions where they may work and other conditions where they don't work?
More info would be greatly appreciated.

Thanks, Michael
-----Original Message-----From:	jwieck@debis.com [SMTP:jwieck@debis.com]Sent:	Monday, April 26, 1999 9:35 AMTo:	rbrad@hpb50023.boi.hp.comCc:	pgsql-hackers@postgreSQL.orgSubject:	Re: [HACKERS] create view as select distinct (fwd)
>> This is on the TODO list.>> I actually have a solution that seems to work fine, but I wanted
to run it past> the backend guru's after we have finished the 6.5 beta.>> Sorry I din't get it finished before the beta started.>> -Ryan
    I wonder how it does!
    Have the following:
      CREATE TABLE t1 (a int4, b text);      CREATE TABLE t2 (c int4, d text);      CREATE VIEW v2 AS SELECT DISTINCT ON c * FROM t2;
    Populate them with:
      t1:      1    'one'      1    'ena'      2    'two'      2    'thio'      3    'three'      3    'tria'      4    'four'      4    'tessera'
      t2:      1    'I'      1    'eins'      2    'II'      2    'zwei'      3    'III'      3    'drei'
    Now you do
      SELECT t1.a, t1.b, v2.d FROM t1, v2        WHERE t1.a = v2.c;
    Does  that  work  and  produce the correct results? Note that    there are more than one correct results. The DISTINCT  SELECT    from t2 already has. But in any case, the above SELECT should    present 6 rows (all the rows of t1 from 1 to  33  in  english    and  greek) and column d must show either the roman or german    number.
    To make it more complicated, add table  t3  and  populate  it    with more languages.  Then setup
      CREATE VIEW v3 AS SELECT DISTINCT ON e * FROM t3;
    and expand the above SELECT to a join over t1, v2, v3.
    Finally,  think  about  a view that is a DISTINCT SELECT over    multiple tables.  Now you build another view as  SELECT  from    the  first  plus  some  other  table  and  make  the new view    DISTINCT again.
    The same kind of problem causes that views  currently  cannot    have ORDER BY or GROUP BY clauses. All these clauses can only    appear once per query, so there is no room where the  rewrite    system  can place multiple different ones.  Implementing this    requires first dramatic changes to the querytree layout and I    think it needs subselecting RTE's too.

Sorry - Jan
--

#======================================================================## It's easier to get forgiveness for being wrong than for being
right. ## Let's break this rule - forgive me.
##======================================== jwieck@debis.com (Jan
Wieck) #