Re: Typmod associated with multi-row VALUES constructs - Mailing list pgsql-hackers
| From | David G. Johnston |
|---|---|
| Subject | Re: Typmod associated with multi-row VALUES constructs |
| Date | |
| Msg-id | CAKFQuwYOeesXJ1bH31b2MKx1UStEzrpYe=tSAO2-eg1Ai4=Eww@mail.gmail.com Whole thread Raw |
| In response to | Re: Typmod associated with multi-row VALUES constructs (Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>) |
| Responses |
Re: Typmod associated with multi-row VALUES constructs
Re: Typmod associated with multi-row VALUES constructs |
| List | pgsql-hackers |
feel free to s/typemod/typmod/ ... my fingers don't want to drop the "e"
Hello,
At Mon, 5 Dec 2016 18:59:42 -0700, "David G. Johnston" <david.g.johnston@gmail.com> wrote in <CAKFQuwYXzBQNpH5L=AHJzOjOZCZSzRvF9qiA0wwt_QZmAuYmEA@mail.gm ail.com>
(It's not typo but my poor wording... Sorry.)
Mmm. I think the typemod of a row should not be applied onto
other rows, and the same can be said for types. But type of the
first row is applied to all of the rest rows, though... Does it
make sense?
Yes. All rows in a given relation must end up with the exact same type and it isn't friendly to fail when a implicit conversion from unknown is possible.
But what I wanted to say was not that but the something like the
following.
select pg_typeof('12345'::varchar(1));
pg_typeof
-------------------
character varying
A value seemingly doesn't have typmod. So it seems reasonable
that VALUES cannot handle typmod. It seems reasonable regardless
of how it is acutually implemented.
This is an artifact of functions - the typemod associated with the value '12345' is lost when that value is passed into the function pg_typeof. Thus it is impossible to write a SQL query the reports the typemod of a literal or column reference. Nonetheless it is there in reality. Just see the original CREATE TABLE AS example for proof. The created table's column is shown (using direct catalog queries) to contain typemod value of 20 - which it could only have gotten from the first values rows which contained a casted literal.
> > Even though I'm not sure about SQL standard here but my
> > feeling is something like the following.
> >
> > | FROM (
> > | VALUES (row 1), .. (row n))
> > | AS foo (colname *type*, ..)
> >
> > for this case,
> >
> > | create temporary table product_codes as select *
> > | from (
> > | values
> > | ('abcdefg'),
> > | ('012345678901234567ABCDEFGHIJKLMN')
> > | ) csv_data (product_code character varying(20));
> >
> > Myself have gotten errors for this false syntax several times:(
> >
>
> Only the function in from form of this accepts a column definition in lieu
> of a simple alias. Regardless of the merits of this idea it would not be
> backpatch-able and wouldn't resolve the current valid syntax problem.
Yeah.. It wouldn't be back-patchable. I personally think that it
is not necessary to be "solve"d, since a value doesn't rigged
with typmod.
But I undetstand what we want to solve here is how to change
VALUES's behavior to perfectly coerce the row values to the types
explicity applied in the first row. Right?
Actually, no, since it is not possible to coerce "perfectly". Since any attempt at doing so could fail it is only possible to scan every row and compare its typemod to the first row's typemod. Ideally (but maybe not in reality) as soon as a discrepancy is found stop and discard the typemod. If every row passes you can retain the typemod. That is arguably the perfect solution. The concern is that "scan every row" could be very expensive - though in writing this I'm thinking that you'd quickly find a non-match even in a large dataset - and so a less perfect but still valid solution is to simply discard the typemod if there is more than one row. My thought was that if you are going to discard typemod in the n > 1 case for consistency you should discard the typemod in the n = 1 case as well.
That is, in a nutshell, options 1, 2, and 3 in order.
The "fault" in #1 that #4 attempted to fix was that VALUES are often hand entered and so the inexperienced would like to only type in a cast one the first row and have it will apply to all subsequent rows. That would be a feature request, though. Your capability to add type information to any FROM alias is likewise a feature request - solving the overall problem by giving the author a place to specify the desired type explicitly without having to pollute the query with excessive casts or subqueries.
David J.
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