Move `explain` into `AbstractMysqlAdapter`

Common methods in both mysql adapters are should be added to
`AbstractMysqlAdapter`, but some methods had been added to
`Mysql2Adapter`. (8744632f, 0306f82e, #14359)

Some methods already moved from `Mysql2Adapter` to
`AbstractMysqlAdapter`. (#17601, #17998)

Common methods in both mysql adapters are remaining only the `explain`
method in `Mysql2Adapter`.

1 files changed, 27 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/mysql/explain_test.rb b/activerecord/test/cases/adapters/mysql/explain_test.rb
new file mode 100644
index 0000000000..f75386abb9
--- /dev/null
+++ b/activerecord/test/cases/adapters/mysql/explain_test.rb
@@ -0,0 +1,27 @@
+require "cases/helper"
+require 'models/developer'
+require 'models/computer'
+
+module ActiveRecord
+  module ConnectionAdapters
+    class MysqlAdapter
+      class ExplainTest < ActiveRecord::TestCase
+        fixtures :developers
+
+        def test_explain_for_one_query
+          explain = Developer.where(:id => 1).explain
+          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
+          assert_match %r(developers |.* const), explain
+        end
+
+        def test_explain_with_eager_loading
+          explain = Developer.where(:id => 1).includes(:audit_logs).explain
+          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
+          assert_match %r(developers |.* const), explain
+          assert_match %(EXPLAIN for: SELECT `audit_logs`.* FROM `audit_logs` WHERE `audit_logs`.`developer_id` = 1), explain
+          assert_match %r(audit_logs |.* ALL), explain
+        end
+      end
+    end
+  end
+end