Fl4m3Ph03n1x

Ecto multiple streams in 1 transaction

Background

PS: the following situation describes an hypothetical scenario, where I own a company that sells things to customers.

I have an Ecto query that is so big, that my machine cannot handle it. With billions of results returned, there is probably not enough RAM in the world that can handle it.

The solution here (or so my research indicates) is to use streams. Streams were made for potentially infinite sets of results, which would fit my use case.

Problem

So lets imagine that I want to delete All users that bought a given item. Maybe that item was not really legal in their country, and now me, the poor guy in IT, has to fix things so the world doesn’t come down crashing.

Naive way:

item_id = "123asdasd123"

purchase_ids =
      Purchases
      |> where([p], p.item_id == ^item_id)
      |> select([p], p.id)
      |> Repo.all()

Users
    |> where([u], u.purchase_id in ^purchase_ids)
    |> Repo.delete_all()

This is the naive way. I call it naive, because of 2 issues:

We have so many purchases, that the machine’s memory will overflow (looking at purchase_ids query)
purchase_ids will likely have more than 100K ids, so the second query (where we delete things) will fail as it hits Postgres parameters limit of 32K: https://stackoverflow.com/a/42251312/1337392

What can I say, our product is highly addictive and very well priced!
Our customers simply cant get enough of it. Don’t know why. Nope. No reason comes to mind. None at all.

With these problems in mind, I cannot help my customers and grow my empire, I mean, little home owned business.

I did find this possible solution:

Stream way:

item_id = "123asdasd123"

purchase_ids =
      Purchases
      |> where([p], p.item_id == ^item_id)
      |> select([p], p.id)

stream = Repo.stream(purchase_ids)

Repo.transacion(fn -> 
  ids = Enum.to_list(stream)

  Users
    |> where([u], u.purchase_id in ^ids)
    |> Repo.delete_all()
end)

Questions

However, I am not convinced this will work:

I am using Enum.to_list and saving everything into a variable, placing everything into memory again. So I am not gaining any advantage by using Repo.stream.
I still have too many ids for my Repo.delete_all to work without blowing up

I guess the one advantage here is that this now a transaction, so either everything goes or nothing goes.

So, the following questions arise:

How do I properly make use of streams in this scenario?
Can I delete items by streaming parameters (ids) or do I have to manually batch them?
Can I stream ids to Repo.delete_all ?

15 comments

#ecto #stream #transactions

0 2865 15

2022-08-08 14:49:55 UTC

Marked As Solved

benwilson512

Author of Craft GraphQL APIs in Elixir with Absinthe

The SQL for this would be:

DELETE FROM users u
USING purchases p
WHERE u.purchase_id = p.id and p.item_id = $1

In Ecto you can do this as simply as:

query = from u in Users,
  join: p in assoc(u, :purchase),
  where: p.item_id == ^item_id

Repo.delete_all(query)

Post #4

Also Liked

benwilson512

Author of Craft GraphQL APIs in Elixir with Absinthe

What you’re missing is that it is only returned if you ask it to be returned. By default, it returns nothing at all except a count of the entities deleted.

Post #10

LostKobrakai

What @benwilson512 suggested is even better than a subquery, but for more information on subqueries see:

Post #5

benwilson512

Author of Craft GraphQL APIs in Elixir with Absinthe

Yeah I’m making some assumptions about your User schema cause it isn’t posted here. I’m assuming that if you have a purchase_id then you’d have a belongs_to :purchase, Purchase. If that isn’t true then you’ll need to change the join.

Post #12

Last Post!

dimitarvp

(EDIT: Sorry, just now saw you actually know about this. Still, leaving it in).

While I completely support @benwilson512’s recommendation and I feel the same – that PG should be able to handle a humongous DELETE command – I’ll still give you an alternative if you truly feel you want an iterable solution in Elixir. Taking the latter part of your “Stream way” snippet and modifying it:

Repo.transaction(fn -> 
  stream
  |> Stream.chunk_every(1000)
  |> Enum.each(fn batch ->
    Users
    |> where([u], u.purchase_id in ^ids)
    |> Repo.delete_all()
  end)
end, timeout: :infinity) # or something like two hours?

You are right to be worried that Enum.to_list might crash the VM. If you suspect anything more than 5_000 records, usually it’s not at all recommended to try and get them into memory because e.g. big binaries might stick around and render garbage collecting ineffective while still having a BEAM VM OS process taking a huge amount of RAM.

Thus, use Stream as intended and chew through work at your own pace. You could also do an SQL count query beforehand and have a progress % while you’re deleting the records in batches.

Post #16