Spike to solve https://github.com/rails/rails/issues/21216

patch.diff

diff --git a/activerecord/lib/active_record/explain.rb b/activerecord/lib/active_record/explain.rb
index 727a9be..1d9dde0 100644
--- a/activerecord/lib/active_record/explain.rb
+++ b/activerecord/lib/active_record/explain.rb
@@ -16,14 +16,14 @@ module ActiveRecord
     # Makes the adapter execute EXPLAIN for the tuples of queries and bindings.
     # Returns a formatted string ready to be logged.
     def exec_explain(queries) # :nodoc:
-      str = queries.map do |sql, bind|
+      str = queries.map do |sql, bind, connection_id|
         [].tap do |msg|
           msg << "EXPLAIN for: #{sql}"
           unless bind.empty?
             bind_msg = bind.map {|col, val| [col.name, val]}.inspect
             msg.last << " #{bind_msg}"
           end
-          msg << connection.explain(sql, bind)
+          msg << get_connection_for_explain(connection_id).explain(sql, bind)
         end.join("\n")
       end.join("\n")
 
@@ -34,5 +34,18 @@ module ActiveRecord
 
       str
     end
+
+    private
+    def get_connection_for_explain(connection_id)
+      explain_connection = nil
+      ::ActiveRecord::Base.connection_handler.connection_pool_list.each do |handler|
+        explain_connection = handler.connections.detect do |conn|
+          conn.object_id == connection_id
+        end
+
+        break if explain_connection
+      end
+      explain_connection || connection
+    end
   end
 end
diff --git a/activerecord/lib/active_record/explain_subscriber.rb b/activerecord/lib/active_record/explain_subscriber.rb
index 9adabd7..8a31fc4 100644
--- a/activerecord/lib/active_record/explain_subscriber.rb
+++ b/activerecord/lib/active_record/explain_subscriber.rb
@@ -9,7 +9,7 @@ module ActiveRecord
 
     def finish(name, id, payload)
       if ExplainRegistry.collect? && !ignore_payload?(payload)
-        ExplainRegistry.queries << payload.values_at(:sql, :binds)
+        ExplainRegistry.queries << payload.values_at(:sql, :binds, :connection_id)
       end
     end