I was having some performance issues with an Oracle query, so I downloaded a trial of the Quest SQL Optimizer for Oracle, which made some changes that dramatically improved the query's performance. I'm not exactly sure why the recommended query had such an improvement; can anyone provide an explanation?

Before:

SELECT t1.version_id, 
       t1.id, 
       t2.field1, 
       t3.person_id, 
       t2.id 
  FROM table1 t1, 
       table2 t2, 
       table3 t3 
 WHERE t1.id = t2.id 
   AND t1.version_id = t2.version_id 
   AND t2.id = 123
   AND t1.version_id = t3.version_id 
   AND t1.VERSION_NAME <> 'AA' 
 order by t1.id

• Plan Cost: 831
• Elapsed Time: 00:00:21.40
• Number of Records: 40,717

After:

SELECT /*+ USE_NL_WITH_INDEX(t1) */ t1.version_id, 
       t1.id, 
       t2.field1, 
       t3.person_id, 
       t2.id 
  FROM table2 t2, 
       table3 t3,
       table1 t1 
 WHERE t1.id = t2.id + 0
   AND t1.version_id = t2.version_id + 0
   AND t2.id = 123
   AND t1.version_id = t3.version_id + 0
   AND t1.VERSION_NAME || '' <> 'AA' 
   AND t3.version_id = t2.version_id + 0
 order by t1.id

• Plan Cost: 686
• Elapsed Time: 00:00:00.95
• Number of Records: 40,717

Questions:

1. Why does re-arranging the order of the tables in the FROM clause help?

2. Why does adding + 0 to the WHERE clause comparisons help?

3. Why does || '' <> 'AA' in the WHERE clause VERSION_NAME comparison help? Is this a more efficient way of handling possible nulls on this column?