Revert "Merge pull request rails#6197 from blakesmith/connection_adap…

…ters_without_explain_support"

This reverts commit 29d564a.

This Rails upstream commit, added after 3.2.12 and included in 3.2.13, mistakenly opens a database connection to check for auto explain support.
This causes issues if you use threads as each will make a database connection for any queries, even cached ones.

It turns out that the upstream Rails 4-0-stable branch has this issue fixed as they removed the auto explain feature entirely: rails#9400
That pull references this issue, rails#9385, where a database connection was required during assets precompiling following the commit we are reverting.

activerecord/lib/active_record/explain.rb

@@ -11,12 +11,11 @@ def self.extended(base)
       end
     end
 
-    # If the database adapter supports explain and auto explain is enabled,
-    # this method triggers EXPLAIN logging for the queries triggered by the
-    # block if it takes more than the threshold as a whole. That is, the
-    # threshold is not checked against each individual query, but against the
-    # duration of the entire block. This approach is convenient for relations.
-
+    # If auto explain is enabled, this method triggers EXPLAIN logging for the
+    # queries triggered by the block if it takes more than the threshold as a
+    # whole. That is, the threshold is not checked against each individual
+    # query, but against the 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
@@ -27,7 +26,7 @@ def logging_query_plan # :nodoc:
 
       threshold = auto_explain_threshold_in_seconds
       current   = Thread.current
-      if connection.supports_explain? && threshold && current[:available_queries_for_explain].nil?
+      if threshold && current[:available_queries_for_explain].nil?
         begin
           queries = current[:available_queries_for_explain] = []
           start = Time.now

activerecord/lib/active_record/railtie.rb

@@ -83,13 +83,6 @@ class Railtie < Rails::Railtie
       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

@@ -98,16 +98,6 @@ def test_exec_explain_with_binds
       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