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

explain_query(connection, query, params, opts)

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  query :: String.t(),
  params :: Keyword.t(),
  opts :: Keyword.t()
) :: {:ok, term()} | {:error, Exception.t()}

Executes an EXPLAIN query or similar depending on the adapter to obtains statistics of the given query.

Receives the connection, query, params for the query, and all opts including those related to the EXPLAIN statement and shared opts.

Must execute the explain query and return the result.