Don't run explain on slow queries for database adapters that don't support it

activerecord/lib/active_record/explain.rb

@@ -6,11 +6,12 @@ module ActiveRecord
       base.mattr_accessor :auto_explain_threshold_in_seconds, instance_accessor: false
     end
 
-    # If auto explain is enabled, this method triggers EXPLAIN logging for the
+    # If the database adapter supports explain and auto explain is enabled,
-    # queries triggered by the block if it takes more than the threshold as a
+    # this method triggers EXPLAIN logging for the queries triggered by the
-    # whole. That is, the threshold is not checked against each individual
+    # block if it takes more than the threshold as a whole. That is, the
-    # query, but against the duration of the entire block. This approach is
+    # threshold is not checked against each individual query, but against the
-    # convenient for relations.
+    # duration of the entire block. This approach is convenient for relations.
+
     #
     # The available_queries_for_explain thread variable collects the queries
     # to be explained. If the value is nil, it means queries are not being
@@ -21,7 +22,7 @@ module ActiveRecord
 
       threshold = auto_explain_threshold_in_seconds
       current   = Thread.current
-      if threshold && current[:available_queries_for_explain].nil?
+      if connection.supports_explain? && threshold && current[:available_queries_for_explain].nil?
         begin
           queries = current[:available_queries_for_explain] = []
           start = Time.now

activerecord/lib/active_record/railtie.rb

@@ -136,6 +136,13 @@ module ActiveRecord
       end
     end
 
+    initializer "active_record.validate_explain_support" do |app|
+      if app.config.active_record[:auto_explain_threshold_in_seconds] &&
+        !ActiveRecord::Base.connection.supports_explain?
+        warn "auto_explain_threshold_in_seconds is set but will be ignored because your adapter does not support this feature. Please unset the configuration to avoid this warning."
+      end
+    end
+
     # Expose database runtime to controller for logging.
     initializer "active_record.log_runtime" do |app|
       require "active_record/railties/controller_runtime"

activerecord/test/cases/explain_test.rb

@@ -108,6 +108,16 @@ if ActiveRecord::Base.connection.supports_explain?
       assert_equal expected, base.exec_explain(queries)
     end
 
+    def test_unsupported_connection_adapter
+      connection.stubs(:supports_explain?).returns(false)
+
+      base.logger.expects(:warn).never
+
+      with_threshold(0) do
+        Car.where(:name => 'honda').to_a
+      end
+    end
+
     def test_silence_auto_explain
       base.expects(:collecting_sqls_for_explain).never
       base.logger.expects(:warn).never