ADQL evolution: OFFSET?

[Markus Demleitner]

Dear DAL list,

One of the users of DaCHS' ADQL engine requested the addition of
OFFSET, and indeed I think there's at least three good reasons to do
so:

(1) It aids harvesting, which in particular with obscore is IMHO a
    very nice thing to have
(2) It enables pagination UIs bolted on top of an ADQL service
(3) The OFFSET 0 hack that helps in several database systems to set a
    confused query planner right.

Given that, and given I've missed OFFSET even back in 2010 or so
(when I wrote the example
http://dc.g-vo.org/tap/examples#Crossmatchforaguidestar), I'm a bit
surprised OFFSET hasn't been part of my TAP implementation notes.

I don't remember why I dropped it -- perhaps because, equally
surprisingly, OFFSET is not part of SQL92.  Nevertheless, I'd like to
propose the addition of (at first optional, mandatory starting 3.0)
support for OFFSET in ADQL 2.1.

This would entail giving a new rule

  <offset_spec> ::= "OFFSET" <unsigned_integer>

and changing the query_specification production to

  <query_specification> ::=
    "SELECT" [ <set_quantifier> ] [ <set_limit> ] 
      <select_list> <table_expression> [ <offset_spec> ]

-- which would correspond to the postgres syntax of offset.

Since it's not part of SQL 92, a bit of prose would be required in
the body of the spec, which I'd be happy to contribute if people
don't protest.

But: How does everyone else feel about this?  In particular: Is this
an issue for non-postgres databases (and I'm mainly thinking of MySQL
and Oracle here; SQLlite has the feature and even syntax as proposed
here)?

Cheers,

          Markus