implements AR::Relation#explain

This is a first implementation, EXPLAIN is highly
dependent on the database and I have made some
compromises.

On one hand, the method allows you to run the most
common EXPLAIN and that's it. If you want EXPLAIN
ANALYZE in PostgreSQL you need to do it by hand.

On the other hand, I've tried to construct a string
as close as possible to the ones built by the
respective shells. The rationale is that IMO the
user should feel at home with the output and
recognize it at first sight. Per database.

I don't know whether this implementation is going
to work well. Let's see whether people like it.

3 files changed, 71 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/mysql2/explain_test.rb b/activerecord/test/cases/adapters/mysql2/explain_test.rb
new file mode 100644
index 0000000000..8ea777b72b
--- /dev/null
+++ b/activerecord/test/cases/adapters/mysql2/explain_test.rb
@@ -0,0 +1,23 @@
+require "cases/helper"
+require 'models/developer'
+
+module ActiveRecord
+  module ConnectionAdapters
+    class Mysql2Adapter
+      class ExplainTest < ActiveRecord::TestCase
+        fixtures :developers
+
+        def test_explain_for_one_query
+          explain = Developer.where(:id => 1).explain
+          assert_match %(developers | const), explain
+        end
+
+        def test_explain_with_eager_loading
+          explain = Developer.where(:id => 1).includes(:audit_logs).explain
+          assert_match %(developers | const), explain
+          assert_match %(audit_logs | ALL), explain
+        end
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/adapters/postgresql/explain_test.rb b/activerecord/test/cases/adapters/postgresql/explain_test.rb
new file mode 100644
index 0000000000..0d599ed37f
--- /dev/null
+++ b/activerecord/test/cases/adapters/postgresql/explain_test.rb
@@ -0,0 +1,25 @@
+require "cases/helper"
+require 'models/developer'
+
+module ActiveRecord
+  module ConnectionAdapters
+    class PostgreSQLAdapter
+      class ExplainTest < ActiveRecord::TestCase
+        fixtures :developers
+
+        def test_explain_for_one_query
+          explain = Developer.where(:id => 1).explain
+          assert_match %(QUERY PLAN), explain
+          assert_match %(Index Scan using developers_pkey on developers), explain
+        end
+
+        def test_explain_with_eager_loading
+          explain = Developer.where(:id => 1).includes(:audit_logs).explain
+          assert_match %(QUERY PLAN), explain
+          assert_match %(Index Scan using developers_pkey on developers), explain
+          assert_match %(Seq Scan on audit_logs), explain
+        end
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/adapters/sqlite3/explain_test.rb b/activerecord/test/cases/adapters/sqlite3/explain_test.rb
new file mode 100644
index 0000000000..97be9f14e9
--- /dev/null
+++ b/activerecord/test/cases/adapters/sqlite3/explain_test.rb
@@ -0,0 +1,23 @@
+require "cases/helper"
+require 'models/developer'
+
+module ActiveRecord
+  module ConnectionAdapters
+    class SQLite3Adapter
+      class ExplainTest < ActiveRecord::TestCase
+        fixtures :developers
+
+        def test_explain_for_one_query
+          explain = Developer.where(:id => 1).explain
+          assert_match %(SEARCH TABLE developers USING INTEGER PRIMARY KEY), explain
+        end
+
+        def test_explain_with_eager_loading
+          explain = Developer.where(:id => 1).includes(:audit_logs).explain
+          assert_match %(SEARCH TABLE developers USING INTEGER PRIMARY KEY), explain
+          assert_match %(SCAN TABLE audit_logs), explain
+        end
+      end
+    end
+  end
+end