Added to TODO:
Add support for WITH RECURSIVE ... CYCLE
* http://archives.postgresql.org/pgsql-hackers/2008-10/msg00291.php
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Tom Lane wrote:
> I looked a bit at the SQL:2008 spec for a CYCLE clause for WITH
> RECURSIVE. It is interesting to see that it is just syntactic sugar,
> because *they spell out how to expand it into regular SQL*. More,
> they defined it in such a way that it's hard to optimize at all,
> because the "path" column is exposed to the user; you don't really
> have any choice about how to do it. There are some ugly and unnecessary
> choices in there too, like insisting that the cycle mark column be
> char(1).
>
> So I am not feeling very excited about implementing the syntax per se
> (and I note that DB2 doesn't seem to have done so either). Instead
> we should document some examples of how to do cycle detection at the
> SQL level. However, it would be nice if the spec's approach to cycle
> detection actually worked well in Postgres. There are a couple of
> things we seem to be missing, according to some experiments I just
> did with trying to translate the spec's code into Postgres:
>
> * The spec assumes that ARRAY[ROW(some columns)] works, ie, that you can
> have an array of an anonymous record type. We don't allow that right
> now, but it seems like a useful thing to have --- at least as a
> transient value within a query. I'm not sure there's a case for
> allowing such things to go to disk.
>
> * The spec writes this to detect whether a row of an anonymous record
> type is present in an array of that same anonymous record type:
> ROW(some columns) IN (SELECT P.* FROM TABLE(array variable) P)
> We haven't got the TABLE() syntax; you can sort of emulate it with a SRF
> but only for arrays of named rowtypes. For an anonymous rowtype,
> it's very unclear to me how the rowtype would be communicated at
> parse time so that the P.* notation could be expanded properly.
>
> * Instead of the above, we could try to make
> ROW(some columns) = ANY (array variable)
> work. This is shorter than the above syntax and would presumably have
> a lot less overhead too. But it doesn't work right now, not even for
> named rowtypes much less anonymous ones.
>
> I'm thinking that addressing these pieces would be a generally good
> thing to do, above and beyond potential uses in recursive queries.
>
> regards, tom lane
>
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-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
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