Ecto.Adapter.Schema.delete

You're seeing just the callback delete, go back to Ecto.Adapter.Schema module for more information.
Link to this callback

delete(adapter_meta, schema_meta, filters, options)

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Specs

delete(adapter_meta(), schema_meta(), filters(), options()) ::
  {:ok, fields()} | {:invalid, constraints()} | {:error, :stale}

Deletes a single struct with the given filters.

While filters can be any record column, it is expected that at least the primary key (or any other key that uniquely identifies an existing record) be given as a filter. Therefore, in case there is no record matching the given filters, {:error, :stale} is returned.