Re: SELECT * in a CREATE VIEW statement doesn't update column set automatically
От | Merlin Moncure |
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Тема | Re: SELECT * in a CREATE VIEW statement doesn't update column set automatically |
Дата | |
Msg-id | AANLkTimQwI46evYngZhlzcrHaB7sKR5BcJaH1yAWE4AP@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: SELECT * in a CREATE VIEW statement doesn't update column set automatically (Jim Nasby <decibel@decibel.org>) |
Список | pgsql-hackers |
On Mon, May 17, 2010 at 2:15 PM, Jim Nasby <decibel@decibel.org> wrote: > On May 6, 2010, at 4:29 PM, Merlin Moncure wrote: >> On Thu, May 6, 2010 at 3:23 PM, Andrew Dunstan <andrew@dunslane.net> wrote: >>> And many places regard "select *" in anything other than throw-away queries >>> as bad practice anyway. I have seen people get bitten by it over and over >>> again, and I have worked at companies where it is explicitly forbidden in >>> coding standards. >> >> In terms of application queries I generally agree. However, I think >> this rule does not apply to server side definitions, especially in >> regards to views and/or composite types. There are cases where you >> _want_ the view to be define as 'all fields of x'...In fact, it's >> pretty typical IMNSHO. It may be possible to expose this behavior. >> >> I'd like to see: >> select * from foo >> -- and -- >> select (foo).* >> exhibit different behaviors -- ().* is more a type operator, returning >> all the fields of foo, than a field list expression. This gives us a >> cool loophole to exploit for views that really want to be defined with >> *: >> create view particular_foos as select (foo).* from foo where something = true; >> create view something_complex as select (foo).*, (func(foo.field)).*; >> -- execute func() just one time please! >> >> The something_complex case above is a real problem in how it behaves >> currently -- sometimes without a hassle free workaround. Am I off my >> rocker? :-) I've made this point many times (prob got annoying a long >> time ago) but I'm curious if you guys agree... > > What you're suggesting makes sense to me. > > What is the composite type workaround you mentioned? This is definitely an issue I face at work and would love a more elegantsolution than drop and re-create the view. Well, the workaround I was specifically talking about was dealing with the problem of composite type return from functions executing the function multiple times: select (func()).*; This gets expanded to select func().f1, func().f2, etc. This is the behavior I think has to go. if func returns foo and foo has 6 columns, func gets executed 6 times for each row. The workaround is this: select (q).f.* from (select func() as f) q; the problem here is that forcing the function call into a subquery can be awkward in non trival queries -- it causes other problems. What you are probably looking for is to be able to add columns to a view without recreating it: create table foo(...); create view v as select foo from foo; now you can just do: select (foo).* from v; small disclaimer: I don't actually do this much, it might cause other issues. postgres is pretty smart about detecting how composite type changes cascade to other structures. This is an exception! postgres=# create table foo(a int, b int, c int); CREATE TABLE postgres=# create view v as select foo from foo; CREATE VIEW postgres=# create view vv as select (v).foo.c; CREATE VIEW postgres=# insert into foo select 1,2,3; INSERT 0 1 postgres=# insert into foo select 2,4,6; INSERT 0 1 alter table foo drop column c; -- uh oh ALTER TABLE postgres=# select * from v; -- this seems ok foo -------(1,2)(2,4) postgres=# select * from vv; -- urk! postgres=# \d+ vv View "public.vv"Column | Type | Modifiers | Storage | Description --------+---------+-----------+---------+-------------c | integer | | plain | View definition:SELECT (v.foo)."........pg.dropped.3........" AS c FROM v; I don't actually mind this so much TBH...feature not bug. I hesitated fixing this because I was terrified someone might actually fix it. merlin
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