title: Can foreign keys hurt query performance
body: <p>As I understood from <a href="https://stackoverflow.com/questions/507179/does-foreign-key-improve-query-performance">this post</a>, there are some scenarios where foreign keys can improve query performance. </p>
  
  <p>I've heard the opposite claim though, that because of referential integrity checks, foreign keys can actually hurt query performance. Under which conditions (if at all) is this true?</p>
  
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  <p>1) The term query seems to be misleading. I am interested in all kinds of performance penalties.</p>
  
  <p>2) Does anyone have any real-world numbers about the negative impact on INSERT, DELETE or UPDATE statements (I know it depends on the specific system, but nevertheless any kind of real-world measurements would be appreciated)?</p>