nightfury17200
Operating on large amount of data
hello all, I want to operate on large amount of data where I pass a query to get some records and then for each record I have to perform a costly operation (creating large number of redis keys(~1000) for each record data and then deleting it) so here I want to use a stream to make the strain less. My code till now is-
Repo.transaction(
fn ->
query
|> Repo.stream(max_rows: @query_batch)
|> Stream.chunk_every(@query_batch)
|> Stream.each(fn x ->
Enum.to_list(x) |> Enum.each(fn y -> delete_from_redis(y) end)
end)
|> Enum.to_list()
end,
timeout: :infinity
)
here in this code when I tried to do -
Stream.each(x, fn y -> delete_from_redis(y) end)
in place of-
Enum.to_list(x) |> Enum.each(fn y -> delete_from_redis(y) end)
my code did not worked, please explain the reason for that also please tell me what can I do to make my current code more efficient?
Marked As Solved
al2o3cr
Stream.each returns a new stream that calls the supplied function as each element is generated. It doesn’t iterate by itself.
Example:
iex(1)> a = [1,2,3]
[1, 2, 3]
iex(2)> stream = Stream.each(a, &IO.inspect/1)
#Stream<[enum: [1, 2, 3], funs: [#Function<38.58486609/1 in Stream.each/2>]]>
iex(3)> Stream.run(stream)
1
2
3
:ok
iex(4)> Enum.to_list(stream)
1
2
3
[1, 2, 3]
Note that in the second line, stream is bound as a Stream struct but nothing has been IO.inspect-ed yet.
Stream.run causes the stream to compute values but discards them and returns :ok, while Enum.to_list accumulates and returns the values.
Also Liked
benwilson512
Your overall stream does, but you’re doing another Stream.each inside a Stream.each and that inner one isn’t doing anything:
|> Stream.each(fn x ->
Stream.each(x, fn y -> delete_from_redis(y) end)
end)
The inner Stream.each should probably be an Enum.each since the x list is already loaded into memory, streaming here doesn’t accomplish anything.
al2o3cr
Re: your specific question:
Stream.each doesn’t run any computation, you have to call a function like Enum.to_list or Stream.run to make Stream do things.
A more general thought: this code returns all the rows from query which neutralizes a lot of the benefit from streaming; Stream.run will do the computation but ignore the result, if that’s your intent.
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nightfury17200
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