Thread:

On 12 September 2016 at 14:02, Pavel Stehule <pavel.stehule@gmail.com> wrote:
> Hi
>
> There is some opened questions - the standard (and some other databases)
> requires entering XPath expression as string literal.
>
> I am thinking so it is too strong not necessary limit - (it enforces dynamic
> query in more cases), so I allowed the expressions there.

I agree. There's no reason not to permit expressions there, and there
are many other places where we have similar extensions.

> Another questions is when these expressions should be evaluated. There are
> two possibilities - once per query, once per input row. I selected "once per
> input row mode" - it is simpler to implement it, and it is consistent with
> other "similar" generators - see the behave and related discussion to
> "array_to_string" and evaluation of separator argument. The switch to "once
> per query" should not be hard - but it can be strange for users, because
> some his volatile expression should be stable.

I would've expected once per query. There's no way the expressions can
reference the row data, so there's no reason to evaluate them each
time.

The only use case I see for evaluating them each time is - maybe -
DEFAULT. Where maybe there's a use for nextval() or other volatile
functions. But honestly, I think that's better done explicitly in a
post-pass, i.e.

select uuid_generate_v4(), x.*
from (
  xmltable(.....) x
);

in cases where that's what the user actually wants.

There's no other case I can think of where expressions as arguments to
set-returning functions are evaluated once per output row.

--
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

On 12 September 2016 at 14:29, Pavel Stehule <pavel.stehule@gmail.com> wrote:

>> I would've expected once per query. There's no way the expressions can
>> reference the row data, so there's no reason to evaluate them each
>> time.
>
> I disagree - it is hypothetical situation but it is possible
>
> if somebody store documents like
>
> id, xml
> =====
> id = 1, xml = <doc id = 1> ....<>
> id = 2, xml = <doc id = 2> ....
>
> Then evaluating one per query doesn't allow to use any reference to other
> columns, and doesn't to build expressions like PATH (...[@id= ' || id || ']

Referencing columns on the same evaluation level? I dunno about that.
You're relying on strict order of evaluation which is pretty unusual
for SQL.

I guess this is why full XQuery would be desirable, but that's a whole
different business.

I would personally expect this sort of thing to be handled by a second
pass; isn't that part of why it's so easy to return xml fields from
xmltable?

Evaluating expressions each time seems likely to be bad for
performance, but I guess it's not going to make a big difference
compared to all the XML crud, so I don't have a super strong opinion
here.

Either way, it's crucial that the behaviour be documented.

> DEFAULT should be evaluated per output row - anybody can use volatile
> function there - example: when I have not data - use some random there

That would be consistent with how we handle DEFAULT on a table, so I
agree. It's a departure from what we do normally, but we didn't have
table functions before either.

--
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services