Ecommerce and Vespa: filtering customer's wishlist - vespa

Question

Let's suppose an ecommerce site has wishlisting functionality. Some users wishlist a lot of products (on a scale of tens of thousands). Total amount of products is in millions. We want to implement functionality where a customer can filter those products like in catalogue.
When we looked into implementing this using Elasticsearch, the best way we've found was using terms lookup. Like, we create a document for each user's wishlist, and then filter out products we need using this document. After that all filtering/etc is done regularly. The problem here is that Elasticsearch cannot sort those products properly — i.e. how document specifies it, since we want to sort by wishlisting time.
This is when I decided to look into Vespa. But after reading documentation I still have no idea what's the best way to implement this. This looks like a problem needing "join" to my rdbms-infected mind. :)
Cardinalities of data is like so:
millions of products
hundreds of thousands of users
tens of thousands of wishlisted items
So... any ideas how to implement or pointers on what to read?

Answer 1

In Vespa you need two document types for this (if you want to store the wishlist in Vespa that is, it's not a requirement)
Retrieved the whislist for given user, either from Vespa using get api of Vespa or from another storage solution.
Retrieve and rank using DotProductItem over the product id field along with a ranking profile.
/search/?yq=select * from products where dotProduct(product_id, {"a":3, "b":2});&ranking=wishlist&hits=10
In this case a is a more recent product in the wishlist then b. ranking profile to go with it:
rank-profile wishlist {
first-phase {
expression:rawScore(product_id)
}
}
You can also use WAND to speed up the search and retrieve only the most recent/top ranking hits in the wishlist. The above example retrieves all and ranks all.
See
https://docs.vespa.ai/en/reference/query-language-reference.html
https://docs.vespa.ai/en/using-wand-with-vespa.html

Answer 2

You can do this by using a WeightedSet item where the item tokens are an id of the products and the weight is the timestamp you want to sort by, see https://docs.vespa.ai/documentation/reference/query-language-reference.html#weightedset, or see https://docs.vespa.ai/documentation/multivalue-query-operators.html#weightedset-example to create it in a Searcher (recommended).
To rank by the timestamp, use a rank profile which just ranks by the match weight, e.g attributeMatch(name).totalWeight
(Regarding sorting, you could also just retrieve all the matches in a Searcher, resort in code, remove those below the fold and then fill() with the summary data. That will scale fine to a few tens of k as long as you do it before filling.)