Datomic queries and laziness

Question

I was surprised to find that query results in datomic are not lazy when entities.

Is there an obvious rationale for this choice that I do not see? It seems reasonable that someone wants to want to (call some-fn (take 100 query-result-containing-millions)), but it will make you evaluate the whole set of entity identifiers, no?

Is there a way to get lazy seq (entity identifiers) directly back from the request, or do they always need to be loaded into memory first, from laziness, accessible only through the entity?

Answer 1

You can use datomic.api/datoms fn to access objects in a lazy way.

Please note that you need to specify the type of index when calling datoms , and the types of indexes available to you depend on the type of attribute you are interested in. For example, the :avet index is only available if your attribute has :db/index set in the schema, and the :vaet is only available if your attribute is of the type :db.type/ref .

We use something similar at work (note: for this there must be a ref-attr attribute ref-attr ):

 (defn datoms-by-ref-value "Returns a lazy seq of all the datoms in the database matching the given reference attribute value." [db ref-attr value] (d/datoms db :vaet value ref-attr)) 

The datoms documentation datoms bit sparse, but with some trial error, you can probably decide what you need. There's a post from August Lilleaas about using an index :avet (which requires an index for an attribute in a datomic schema), which I found somewhat useful.