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Include a preload joins vs. no joins discussion in the preload docs #4517

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Oct 7, 2024
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Include a preload joins vs. no joins discussion in the preload docs #4517

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79e6a29
Include a preload joins vs. no joins discussion in the preload docs
PragTob Oct 2, 2024
a38584c
Remove discussion of database load after feedback
PragTob Oct 7, 2024
e7d7648
Update lib/ecto/query.ex
josevalim Oct 7, 2024
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24 changes: 24 additions & 0 deletions lib/ecto/query.ex
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Expand Up @@ -2667,6 +2667,30 @@ defmodule Ecto.Query do
where: l.inserted_at > c.updated_at,
preload: [:author, comments: {c, likes: l}]

## Choosing Between Preloading with Joins vs. Separate Queries
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I wasn't sure whether to make it a separate section with a heading or not, but I ended up doing it because it got too big to just be part of the normal text.

I also put it "first" as it seemed most relevant to the preceeding setion flow-wise. Happy to re-evaluate and change!

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Deciding between preloading associations via joins, a single large
query, (`preload: [comments: c]`) or separate smaller queries
(`preload: [:comments]`) depends on the specific use case.
Here are some factors to guide your decision:

* **Joins reduce database round trips:** By fetching data in a single
query, joins can minimize database round trips, potentially reducing
overall latency.
* **Potential for data duplication:** Joins may lead to duplicated
data in the result set, which requires more processing by Ecto
and consumes more bandwidth when transmitting the results.
* **Parallelism with separate queries:** When using separate queries
outside of a transaction, Ecto can parallelize the preload queries,
which can speed up the overall operation.

In general, a good default is to only use joins in preloads if you're
already joining the associations in the main query. For example,
in the last query in the section above, comments and likes are already
joined, so they are included in the preload.
However, the author is not joined in the main query, so it is preloaded
via a separate query.
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## Preload queries

Preload also allows queries to be given, allowing you to filter or
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