Just tried my own advice and this unfortunately won’t work!

Docs say that

• :function - purges all messages with the “function/arity” Remember that if you want to purge log calls from a dependency, the dependency must be recompiled.

So I first modified my logger config to find the logging function:

config :logger, :console,
  format: "$time\n[$level] $metadata\n$message\n\n",
  metadata: [:mfa, :file, :line]

That resulted in

[debug] mfa=Ecto.Adapters.SQL.log/4 file=lib/ecto/adapters/sql.ex line=938
QUERY OK …

But the ecto_sql library at line 938 invokes Logger.log/3 which warns that

The macros debug/2, info/2, notice/2, warning/2, error/2, critical/2, alert/2, and emergency/2 are preferred over this macro as they can automatically eliminate the call to Logger altogether at compile time if desired (see the documentation for the Logger module).

I added an explicit :debug case option to the log/4 source code in my deps folder and recompiled as instructed with mix deps.compile ecto_sql but it’s still logging?

:debug ->
  IO.puts("SQL debug")
  Logger.debug(
    fn -> log_iodata(measurements, metadata) end,
    ansi_color: sql_color(query_string)
  )

config.exs

config :logger, :console,
  format: "$time\n[$level] $metadata\n$message\n\n",
  metadata: [:mfa, :file, :line],
  compile_time_purge_matching: [
    [module: Ecto.Adapters.SQL, function: "log/4", level_lower_than: :info]
  ]

iex(1)> Repo.all(User)
SQL debug
14:02:05.547
[debug] mfa=Ecto.Adapters.SQL.log/4 file=lib/ecto/adapters/sql.ex line=939 
QUERY OK source="users" ...

Hopefully someone with more experience can provide some insight

Ok, this works, at least for me, and in my particular case.

Given log: false in the config, I made a deliberate error in a query (idd instead of id):

  def get_page_data(id) do
    Quire.Repo.one(from(p in Page, where: p.idd == ^id, select: p))
  end

Ecto failed and printed the following error:

[error] #PID<0.1021.0> running QuireWeb.Endpoint (connection #PID<0.967.0>, stream id 8) terminated
Server: localhost:4000 (http)
Request: GET /admin/pages/33
** (exit) an exception was raised:
    ** (Ecto.QueryError) lib/quire/cms.ex:186: field `idd` in `where` does not exist in schema Quire.CMS.Page in query:

from p0 in Quire.CMS.Page,
  where: p0.idd == ^33,
  select: p0

No other debug logs. At this point this probably covers 99.999% of my needs.

@KristerV Give it a try. Quite possible if it wasn’t working for you it was a bug that was solved at one point.

You should set your log level in your dev.exs file. See Logger — Logger v1.12.3 and Logger — Logger v1.12.3 for more info.