Create array index on bit_prefs (#2644)

db/migrate/20160917180923_add_index_to_users_bit_prefs.rb

@@ -0,0 +1,24 @@
+class AddIndexToUsersBitPrefs < ActiveRecord::Migration
+  def up
+    execute "set statement_timeout = 0"
+    execute <<-'SQL'
+      CREATE OR REPLACE FUNCTION bit_position_array(x bigint)
+      RETURNS integer[] AS $BODY$
+      select array_agg(i) 
+        from generate_series(0, floor(log(2, x))::integer) i
+       where (x & (1::bigint << i)) > 0;
+      $BODY$ LANGUAGE sql IMMUTABLE
+    SQL
+    
+    execute <<-'SQL'
+      CREATE INDEX index_users_on_bit_prefs_array
+       ON users USING gin (bit_position_array(bit_prefs) _int4_ops)
+    SQL
+  end
+
+  def down
+    execute "set statement_timeout = 0"
+    execute "DROP INDEX index_users_on_bit_prefs_array"
+    execute "DROP FUNCTION bit_position_array(x bigint)"
+  end
+end