streaming and sequential flwor

[Matthias Brantner]

If a FLWOR has a sequential return clause, all of the binding sequences of the clauses are materialized before the return clause is executed. This behavior is correct according to the latest Scripting specification and avoids confusion if the return clause would modify any input to one of the clauses.

However, the downside is that there is currently no way to stream the input. For example, in the following query, $contents is completely materialized before the sequential local:test function is invoked.

variable $xmlcontents := fetch:content("big-file.xml");

let $contents := parse-xml:parse(
      $xmlcontents,
      <opt:options>
        <opt:parse-external-parsed-entity opt:skip-root-nodes="1" />
      </opt:options>)
for $w at $pos in $contents
return
{
    local:test();
    $w
}

There has been an extensive discussion on zorba-dev (subject "Streaming and Sequential FLWOR -> no good") on this topic but no satisfying solution has been found, yet.

[Chris Hillery (ceejatec)]

I am assigning to Ghislain simply to ensure that it stays on my planning spreadsheet.

[Matthias Brantner (matthias-brantner)]

The discussion resulted in the following:
- a sequential flwor shouldn't do materialization by default
- a pragma should be available in order to turn materialization on