README.md in upsert-2.2.0 vs README.md in upsert-2.2.1

@@ -21,11 +21,11 @@
 You pass a __selector__ that uniquely identifies a row, whether it exists or not. You also pass a __setter__, attributes that should be set on that row.
 
 Syntax inspired by [mongo-ruby-driver's update method](http://api.mongodb.org/ruby/1.6.4/Mongo/Collection.html#update-instance_method).
 
 ### Basic
-    
+
 ```ruby
 connection = Mysql2::Client.new([...])
 table_name = :pets
 upsert = Upsert.new connection, table_name
 # N times...
@@ -57,14 +57,16 @@
 
 Batch mode is tested to be about 80% faster on PostgreSQL, MySQL, and SQLite3 than other ways to emulate upsert (see the tests, which fail if they are not faster).
 
 ### Native Postgres upsert
 
-`INSERT ... ON CONFLICT DO UPDATE` is used when Postgres 9.5+ is detected and *unique indexes are in place.*
+`INSERT ... ON CONFLICT DO UPDATE` is used when Postgres 9.5+ is detected and *unique constraint are in place.*
 
-If you don't have unique indexes, it will fall back to the classic Upsert gem user-defined function, which does not require indexes.
+**Note: ** You must have a **unique constraint** on the column(s) you're using as a selector.  A unique index won't work.  See https://github.com/seamusabshere/upsert/issues/98#issuecomment-295341405 for more information and some ways to check.
 
+If you don't have unique constraints, it will fall back to the classic Upsert gem user-defined function, which does not require a constraint.
+
 ### ActiveRecord helper method
 
 ```ruby
 require 'upsert/active_record_upsert'
 # N times...
@@ -171,28 +173,28 @@
 CREATE PROCEDURE upsert_pets_SEL_name_A_tag_number_SET_name_A_tag_number(`name_sel` varchar(255), `tag_number_sel` int(11), `name_set` varchar(255), `tag_number_set` int(11))
 BEGIN
   DECLARE done BOOLEAN;
   REPEAT
     BEGIN
-      -- If there is a unique key constraint error then 
+      -- If there is a unique key constraint error then
       -- someone made a concurrent insert. Reset the sentinel
       -- and try again.
       DECLARE ER_DUP_UNIQUE CONDITION FOR 23000;
       DECLARE ER_INTEG CONDITION FOR 1062;
       DECLARE CONTINUE HANDLER FOR ER_DUP_UNIQUE BEGIN
         SET done = FALSE;
       END;
-      
+
       DECLARE CONTINUE HANDLER FOR ER_INTEG BEGIN
         SET done = TRUE;
       END;
 
       SET done = TRUE;
       SELECT COUNT(*) INTO @count FROM `pets` WHERE `name` = `name_sel` AND `tag_number` = `tag_number_sel`;
       -- Race condition here. If a concurrent INSERT is made after
       -- the SELECT but before the INSERT below we'll get a duplicate
       -- key error. But the handler above will take care of that.
-      IF @count > 0 THEN 
+      IF @count > 0 THEN
         -- UPDATE table_name SET b = b_SET WHERE a = a_SEL;
         UPDATE `pets` SET `name` = `name_set`, `tag_number` = `tag_number_set` WHERE `name` = `name_sel` AND `tag_number` = `tag_number_sel`;
       ELSE
         -- INSERT INTO table_name (a, b) VALUES (k, data);
         INSERT INTO `pets` (`name`, `tag_number`) VALUES (`name_set`, `tag_number_set`);