EXPLAIN only for sqlite3

1 files changed, 25 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index ee5d10859c..ae5eaa5f1b 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -50,6 +50,31 @@ module ActiveRecord
         @connection.encoding.to_s
       end
 
+      # Returns true.
+      def supports_explain?
+        true
+      end
+
+      # DATABASE STATEMENTS ======================================
+
+      def explain(arel, binds = [])
+        sql = "EXPLAIN QUERY PLAN #{to_sql(arel, binds)}"
+        ExplainPrettyPrinter.new.pp(exec_query(sql, 'EXPLAIN', binds))
+      end
+
+      class ExplainPrettyPrinter
+        # Pretty prints the result of a EXPLAIN QUERY PLAN in a way that resembles
+        # the output of the SQLite shell:
+        #
+        #   0|0|0|SEARCH TABLE users USING INTEGER PRIMARY KEY (rowid=?) (~1 rows)
+        #   0|1|1|SCAN TABLE posts (~100000 rows)
+        #
+        def pp(result) # :nodoc:
+          result.rows.map do |row|
+            row.join('|')
+          end.join("\n") + "\n"
+        end
+      end
     end
   end
 end