# Query performance with large(r) inputs

**URL:** <https://forum.datomic.com/t/query-performance-with-large-r-inputs/1508>\
**Category:** Datomic Pro\
**Created:** [July 1, 2020, 5:00pm UTC](https://forum.datomic.com/t/query-performance-with-large-r-inputs/1508 "2020-07-01T17:00:11Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![bhurlow](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.datomic.com/bhurlow/32/181_2.png) [@bhurlow](https://forum.datomic.com/u/bhurlow)\
**Post date:** [July 1, 2020, 5:00pm UTC](https://forum.datomic.com/t/query-performance-with-large-r-inputs/1508/1 "2020-07-01T17:00:11Z")

</div>

Our team has been debugging a slow query and I wanted to ask a question about why one query variant is slower than the other

```auto
(comment
  (def members
    (d/q
     '[:find [?u ...]
       :in $ ?b
       :where [?u :user/follows ?b]]
     (d/db @conn)
     board-id)))

(def query1
  '[:find ?u (count ?views)
    :in $ % [?u ...] ?b
    :where
    (or
     [?b :board/admins ?instructors]
     [?b :board/created-by ?instructors])
    [?post :post/author ?instructors]
    [?post :post/board ?b]
    [?post :post/views ?views]
    [?views :post-view/viewer ?u]
    [(!= ?u ?instructors)]])

(def query2
  '[:find ?viewer (count ?views)
    :in $ % [?u ...] ?b
    :where
    (or
     [?b :board/admins ?instructors]
     [?b :board/created-by ?instructors])
    [?post :post/author ?instructors]
    [?post :post/board ?b]
    [?post :post/views ?views]
    [?views :post-view/viewer ?viewer]
    [?viewer :user/follows ?b]
    [(!= ?viewer ?instructors)]])

(comment
  ;; Elapsed time: 6489.551387 msecs
  (time
   (d/q
    query1
    (d/db @conn)
    (yd.rules/rules)
    (take 250 members)
    board-id))
  ;; "Elapsed time: 87.187617 msecs"
  (time
   (d/q
    query2
    (d/db @conn)
    (yd.rules/rules)
    (set (take 250 members))
    board-id)))

```

The queries shown above are aiming to find the count of views on posts made by instructors for each user. In query1 we pass in a subset of the “members” of the board, who are just users with the `:user/follows` relation as shown. In query2, the input is ignored in favor of an additional binding `[?viewer :user/follows ?b]` which should constrain the results to produce the same output (not withstanding the`take`).

We did identify some `:where` clauses which can be moved around to increase the performance, but I’m a bit confused about why the version which accepts the input vector would be a factor of ~75x slower. The members collection is around 7000 eids. My guess here is that since the input is de-structured it has to be probed or scanned against each `:post-view/viewer` value, whereas the version in query2 is binding a set and thus can do a set whatever-the-big-o intersection operation is.

Is there any reason passing in a large-ish collection is not recommended? Is it possible to pass in a set as a query input?

I should note that putting `[?views :post-view/viewer ?u]` as the first where clause did bring the query down to around `564ms` for the whole input collection, but it’s still slower than the no input version by about 5x

Any suggestions?

---

<div class="post-metadata">

**Author:** ![m0smithtb](https://avatars.discourse-cdn.com/v4/letter/m/e19adc/32.png) [@m0smithtb](https://forum.datomic.com/u/m0smithtb)\
**Post date:** [July 2, 2020, 3:28pm UTC](https://forum.datomic.com/t/query-performance-with-large-r-inputs/1508/2 "2020-07-02T15:28:47Z")

</div>

Try putting

> [@bhurlow](#):
>
> ` [?views :post-view/viewer ?u]`  
> `[?post :post/views ?views]`  
> `[?post :post/board ?b]`  
> ` [?post :post/author ?instructors]`  
> ` (or`  
> ` [?b :board/admins ?instructors]`  
> ` [?b :board/created-by ?instructors])`

As ?u is passed in this will unify. Reorder to put the (or …) further down.

---

<div class="post-metadata">

**Author:** ![nikolayandr](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.datomic.com/nikolayandr/32/375_2.png) [@nikolayandr](https://forum.datomic.com/u/nikolayandr)\
**Post date:** [July 11, 2025, 12:17pm UTC](https://forum.datomic.com/t/query-performance-with-large-r-inputs/1508/3 "2025-07-11T12:17:19Z")

</div>

Hi @bhurlow, were you able to solve your issue? I have a similar one [Performance degradation and heap overflow with large collection binding input](https://forum.datomic.com/t/performance-degradation-and-heap-overflow-with-large-collection-binding-input/2572), but I have not found a suitable solution yet. Thanks