How to return results together with update operations in BaseX?

Question

I recognized that (insert/delete)-XQueries executed with the BaseX client always returning an empty string. I find this very confusing or unintuitive.

Is there a way to find out if the query was "successful" without querying the database again (and using potentially buggy "transitive" logic like "if I deleted a node, there must be 'oldNodeCount-1' nodes in the XML")?

Answer 1

XQuery Update statements do not return anything -- that's how they are defined. But you're not the only one who does not like those restrictions, and BaseX added two ways around this limitation:

Returning Results

By default, it is not possible to mix different types of expressions in a query result. The outermost expression of a query must either be a collection of updating or non-updating expressions. But there are two ways out:

• The BaseX-specific update:output() function bridges this gap: it caches the results of its arguments at runtime and returns them after all updates have been processed. The following example performs an update and returns a success message:

  update:output("Update successful."), insert node <c/> into doc('factbook')/mondial
  

• With the MIXUPDATES option, all updating constraints will be turned off. Returned nodes will be copied before they are modified by updating expressions. An error is raised if items are returned within a transform expression.

If you want to modify nodes in main memory, you can use the transform expression.

The transform expression will not help you, as you seem to modify the data on disk. Enabling MIXUPDATES allows you to both update the document and return something at the same time, for example running something like

let $node := <c/>
return ($node, insert node $node into doc('factbook')/mondial)

MIXUPDATES allows you to return something which can be further processed. Results are copied before being returned, if you run multiple updates operations and do not get the expected results, make sure you got the concept of the pending update list.

The db:output() function intentionally breaks its interface contract: it is defined to be an updating function (not having any output), but at the same time it prints some information to the query info. You cannot further process these results, but the output can help you debugging some issues.

Pending Update List

Both ways, you will not be able to have an immediate result from the update, you have to add something on your own -- and be aware updates are not visible until the pending update list is applied, ie. after the query finished.

Compatibility

Obviously, these options are BaseX-specific. If you strongly require compatible and standard XQuery, you cannot use these expressions.