> On 14. Jan 2022, at 13:21, Peter Eisentraut
wrote:
>
> There is nothing in there that says that certain branches of the
UNION in a recursive query mean certain things. In fact, it doesn't even
require the query to contain a UNION at all. It just says to iterate on
evaluating the query
> On 14. Jan 2022, at 13:21, Peter Eisentraut
> wrote:
>
> There is nothing in there that says that certain branches of the UNION in a
> recursive query mean certain things. In fact, it doesn't even require the
> query to contain a UNION at all. It just says to iterate on evaluating the
> I have been studying this a bit more. I don't understand your argument here.
> Why would this query have different semantics than, say
>
> WITH RECURSIVE t(n) AS (
> VALUES (1)
> UNION ALL
> VALUES (2)
> UNION ALL
> SELECT n+1 FROM t WHERE n < 100
> ) SELECT * FROM t
> I studied up on the theory and terminology being discussed here. I conclude
> that what the latest version of this patch is doing (allowing multiple
> recursive references, but only in a linear-recursion way) is sound and useful.
That's great to hear!
> I haven't looked closely at the
The documentation was not up to date anymore with the most recent changes.
This version of the patch fixes that.
Best,
–– Denis
0008-Allow-multiple-recursive-self-references.patch
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> I am not sure if this patch should introduce such a rewriting.
I have thought about this again. I think it is reasonable that this patch
introduces
such a rewriting.
> This well-formedness check apparently needs to be enhanced to allow for more
> than two branches in the union.
The new
> This well-formedness check apparently needs to be enhanced to allow for more
> than two branches in the union.
The UNION operators' left associativity causes this problem. Currently, the
recursive term
must be enclosed in parentheses to make this example work:
> WITH RECURSIVE nodes(x) AS
> Maybe the variable selfrefcountL can be renamed slightly (e.g.
> curr_selfrefcount) so that the code is easier to read.
Yes, you are absolutely right. Thanks for the suggestion.
The new version renames this variable.
Best wishes,
-- Denis
> The tests fail when you build with assertions enabled (configure
> --enable-cassert).
Thank you for pointing that out. The new version of this patch fixes that.
The tests are working properly now. All style related issues are fixed as well.
Best wishes,
-- Denis
> In the next version of the patch, would you be so kind as to include
> updated documentation of the feature and at least one regression test
> of same?
As requested, this new version of the patch contains regression tests and
documentation.
Best wishes,
-- Denis
> The patch does not apply on Head anymore, could you rebase and post a patch.
Sure thing. Here's the new patch.
Best wishes,
-- Denis
v3-0001-Allow-multiple-recursive-self-references.patch
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Sorry, I didn't append the patch properly.
Best wishes,
--Denis
v2-0001-Allow-multiple-recursive-self-references.patch
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Based on Toms feedback, and due to the fact that SQL:2021 forbidsnon-linear recursion, version 2 of the patch allows only linearrecursion. Therefore, later SQL committee decisions on non-linearrecursion should not be problematic.> [LIN] PostgreSQL does not allow multiple references to the
Thanks for the feedback, Tom.
> Tom Lane writes:
> [...]
> As far as I can see, the spec flat-out forbids this. In SQL:2021,
> it's discussed in 7.17 syntax rule 3) j) ix), which
> defines [linear recursion]
(Aside: We don't have a copy of the SQL:2021 specification here (all
we've got here
has implemented this, except MariaDB. Tested
> here:
There are a few recent DBMSs that I know of that support this: HyPer, Umbra,
DuckDB, and NoisePage.
I'm sure there are some more examples. Still, you are right, many other DBMSs
do not support this – yet.
--
Denis Hirn
16
32
64
128
(8 rows)
This feature deviates only slightly from the current WITH RECURSIVE, and
requires very little changes (~10 loc). Any thoughts on this?
--
Denis Hirn
0001-Allow-multiple-recursive-self-references.patch
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