title: MS SQL Serv: Why is my query running so slow?
body: <p>Self-taught, small business DIYer here. Running MS Sql Server '14. </p>
  
  <p>I have ran this query now 4 times a month for almost 2 years. However the last two months the query run time has become unbearably long. What use to run in 2-3 minutes now takes 24+ hours. </p>
  
  <p>I import weekly files into a monthly table and then dedupe that table against the previous 2 months tables to get a unique file for export. The volume of the tables has stayed consistent as is the data type, the index and the allowable nulls.</p>
  
  <p>It is not computing power. I can substitute last January's file in for this January's and the query runs in minutes. Again, this started with December 2016's file. Any previous file runs very quickly.</p>
  
  <p>Any ideas? Thanks in advance!</p>
  
  <p>Query:</p>
  
  <pre><code>SELECT [2017_JAN].*
   FROM [dbo].[2017_JAN]
    LEFT JOIN [dbo].[2016_NOV]
     ON [2016_NOV].[ID] = [2017_JAN].[ID]
    LEFT JOIN [dbo].[2016_DEC]
     ON [2016_DEC].[ID] = [2017_JAN].[ID]
  WHERE   [2016_NOV].[ID]  IS NULL
      and [2016_DEC].[ID]  IS NULL
  </code></pre>