Datomic query performance improvements - clojure

Question

I have a schema that looks similar to this in a Datomic database:
; --- tenant
{:db/id #db/id[:db.part/db]
:db/ident :tenant/guid
:db/unique :db.unique/identity
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :tenant/name
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :tenant/taks
:db/valueType :db.type/ref
:db/cardinality :db.cardinality/many
:db.install/_attribute :db.part/db}
; --- task
{:db/id #db/id[:db.part/db]
:db/ident :task/guid
:db/unique :db.unique/identity
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :task/createdAt
:db/valueType :db.type/instant
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :task/name
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :task/subtasks
:db/valueType :db.type/ref
:db/cardinality :db.cardinality/many
:db.install/_attribute :db.part/db}
; --- subtask
{:db/id #db/id[:db.part/db]
:db/ident :subtask/guid
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db/unique :db.unique/identity
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :subtask/type
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :subtask/startedAt
:db/valueType :db.type/instant
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :subtask/completedAt
:db/valueType :db.type/instant
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :subtask/participants
:db/valueType :db.type/ref
:db/cardinality :db.cardinality/many
:db.install/_attribute :db.part/db}
; --- participant
{:db/id #db/id[:db.part/db]
:db/ident :participant/guid
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db/unique :db.unique/identity
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :participant/name
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
The tasks are pretty static over time but subtasks are added and removed on average about once per 5 minutes per task. I would say that each task on average has about 40 subtasks at any given time containing (almost always but there are a few exceptions) one participant. My sole purpose of using Datomic is to be able to see how tasks have evolved over time, i.e. I'd like to see the what a task looked like at a given time. To achieve I'm currently doing something similar to this:
(defn find-tasks-by-tenant-at-time
[conn tenant-guid ^long time-epoch]
(let [db-conn (-> conn d/db (d/as-of (Date. time-epoch)))
task-ids (->> (d/q '[:find ?taskIds
:in $ ?tenantGuid
:where
[?tenantId :tenant/guid ?tenantGuid]
[?tenantId :tenant/tasks ?taskIds]]
db-conn tenant-guid)
vec flatten)
task-entities (map #(d/entity db-conn %) task-ids)
dtos (map (fn [task]
(letfn [(participant-dto [participant]
{:id (:participant/guid participant)
:name (:participant/name participant)})
(subtask-dto [subtask]
{:id (:subtask/guid subtask)
:type (:subtask/type subtask)
:participants (map participant-dto (:subtask/participants subtask))})]
{:id (:task/guid task)
:name (:task/name task)
:subtasks (map subtask-dto (:task/subtasks task))})) task-entities)]
dtos))
Unfortunately this is extremely slow. It can take almost 60 seconds to return from this function if there are many tasks for a tenant (say 20) each containing roughly 40 subtasks. Am I doing something obviously wrong here? Is it possible to speed this up?
Update:
The entire dataset is roughly 2 Gb and the peer has 3.5Gb of memory (but it doesn't seem to make any difference if I decrease it to say 1.5 Gb) and the transactor has 1 Gb of memory. I'm using Datomic Free.

Answer 1

Before you start profiling etc. you could replace
[:find ?taskIds ...]
by
[:find (pull ?task-entity [*]) ...]
to reduce the number of round-trips to the peer and thus get rid of the map statement for task-entities. In a second step replace [*] with the appropriate set of keys you really want to pull for each entity.