You're seeing just the function explain, go back to Ecto.Adapters.SQL module for more information.
Link to this function

explain(repo, operation, queryable, opts \\ [])

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  pid() | Ecto.Repo.t() | Ecto.Adapter.adapter_meta(),
  :all | :update_all | :delete_all,
  opts :: Keyword.t()
) :: String.t() | Exception.t()

Executes an EXPLAIN statement or similar for the given query according to its kind and the adapter in the given repository.


# Postgres
iex> Ecto.Adapters.SQL.explain(Repo, :all, Post)
"Seq Scan on posts p0  (cost=0.00..12.12 rows=1 width=443)"

iex> Ecto.Adapters.SQL.explain(Repo, :all, from(p in Post, where: p.title == "title")) |> IO.puts()
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
|  1 | SIMPLE      | p0    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |    100.0 | Using where |

# Shared opts
iex> Ecto.Adapters.SQL.explain(Repo, :all, Post, analyze: true, timeout: 20_000)
"Seq Scan on posts p0  (cost=0.00..11.70 rows=170 width=443) (actual time=0.013..0.013 rows=0 loops=1)\nPlanning Time: 0.031 ms\nExecution Time: 0.021 ms"

It's safe to execute it for updates and deletes, no data change will be committed:

iex> Ecto.Adapters.SQL.explain(Repo, :update_all, from(p in Post, update: [set: [title: "new title"]]))
"Update on posts p0  (cost=0.00..11.70 rows=170 width=449)\n  ->  Seq Scan on posts p0  (cost=0.00..11.70 rows=170 width=449)"

This function is also available under the repository with name explain:

iex> Repo.explain(:all, from(p in Post, where: p.title == "title"))
"Seq Scan on posts p0  (cost=0.00..12.12 rows=1 width=443)\n  Filter: ((title)::text = 'title'::text)"


Built-in adapters support passing opts to the EXPLAIN statement according to the following:

AdapterSupported opts
Postgrexanalyze, verbose, costs, settings, buffers, timing, summary

Postgrex: Check PostgreSQL doc for version compatibility.

MyXQL: EXTENDED and PARTITIONS opts were deprecated and are enabled by default.

Also note that:

  • Currently :map, :yaml, and :text format options are supported for PostgreSQL. :map is the deserialized JSON encoding. The last two options return the result as a string.
  • Any other value passed to opts will be forwarded to the underlying adapter query function, including Repo shared options such as :timeout;
  • Non built-in adapters may have specific behavior and you should consult their own documentation.