title: Oracle LEADING hint -- why is this required?
body: <p>Suddenly (but unfortunately I don't know when "suddenly" was; I know it ran fine at some point in the past) one of my queries started taking 7+ seconds instead of milliseconds to execute.  I have 1 local table and 3 tables being accessed via a DB link.  The 3 remote tables are joined together, and one of them is joined with my local table.</p>
  
  <p>The local table's where clause only takes a few millis to execute on its own, and only returns a few (10's or 100's at the most) records.  The 3 remote tables have many hundreds of thousands, possibly millions, of records between them, and if I join them appropriately I get tens or hundreds of thousands of records.</p>
  
  <p>I am only joining with the remote tables so that I can pull out a few pieces of data related to each record in my local table.</p>
  
  <p>What appears to be happening, however, is that Oracle joins the remote tables together first and then my local table to that mess at the end.  This is always going to be a bad idea, especially given the data set that exists right now, so I added a <code>/*+ LEADING(local_tab remote_tab_1) */</code> hint to my query and it now returns in milliseconds.</p>
  
  <p>I compared the explain plans and they are almost identical, save for a single <code>BUFFER SORT</code> on one of the remote tables.</p>
  
  <p>I'm wondering what might cause Oracle to approach this the wrong way?  Is it an index issue?  What should I be looking for?</p>