title: Query JOIN or not? (optimization)
body: <p>i was wondering if to use 2 tables is better then using 1 single table .</p>
  
  <p>Scenario:</p>
  
  <p>i have a simple user table and a simple user_details table. i can JOIN tables and select both records.</p>
  
  <p>But i was wondering if to merge 2 table into 1 single table.</p>
  
  <p>what if i have 2milions users records in both tables?</p>
  
  <p>in terms of speed and exec time is better to have a single table when selecting records?</p>