[basex-talk] Optimizing query that reads nodes from two databases

Hi all,

I'm working on a query that reads from two databases -- OriginalDB and UpdateDB. The structure of each database is the same:

<data> <item id="1">...</item> <item id="2">...</item> ... </data>

OriginalDB has 2,305,570 items and UpdateDB has 307,019.

The query gets all distinct item ids from the two databases, then gets the item with a matching id (if there is one) from each database, and returns the id and both nodes in a JSON object. Here's the query:

let $originalDB := db:get('OriginalDB')/data/item let $updateDB := db:get('UpdateDB')/data/item for $id in distinct-values(($originalDB/@id, $updateDB/@id)) let $original := $originalDB[@id = $id] let $update := $updateDB[@id = $id] return json:serialize(map { 'id': xs:long($id), 'original': $original, 'update': $update }, map { 'indent': 'no' })

In its current state this query is very slow -- it's returned only 35 JSON objects in ~30 minutes. How can I go about optimizing it to best take advantage of each database's indices?

Thanks in advance, Matt