An explain query does not raise the `ReadOnlyError` if preventing writes

activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb

@@ -19,7 +19,7 @@ def query(sql, name = nil) # :nodoc:
           execute(sql, name).to_a
         end
 
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :set, :show, :release, :savepoint, :rollback) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :explain, :select, :set, :show, :release, :savepoint, :rollback) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -67,7 +67,7 @@ def query(sql, name = nil) #:nodoc:
           end
         end
 
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :set, :show, :savepoint, :rollback) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :explain, :select, :set, :show, :savepoint, :rollback) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -209,7 +209,7 @@ def disable_referential_integrity # :nodoc:
       # DATABASE STATEMENTS ======================================
       #++
 
-      READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :pragma, :savepoint, :rollback) # :nodoc:
+      READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :explain, :select, :pragma, :savepoint, :rollback) # :nodoc:
       private_constant :READ_QUERY
 
       def write_query?(sql) # :nodoc:

activerecord/test/cases/base_test.rb

@@ -1530,4 +1530,12 @@ def test_protected_environments_are_stored_as_an_array_of_string
       assert_equal bird, Bird.where(name: "Bluejay").first
     end
   end
+
+  test "an explain query does not raise if preventing writes" do
+    Bird.create!(name: "Bluejay")
+
+    ActiveRecord::Base.connection.while_preventing_writes do
+      assert_queries(2) { Bird.where(name: "Bluejay").explain }
+    end
+  end
 end