title: How to optimize Core Data query for full text search
body: <p>Can I optimize a Core Data query when searching for matching words in a text?  (This question also pertains to the wisdom of custom SQL versus Core Data on an iPhone.)</p>
  
  <p>I'm working on a new (iPhone) app that is a handheld reference tool for a scientific database.  The main interface is a standard searchable table view and I want as-you-type response as the user types new words.  Words matches must be prefixes of words in the text.  The text is composed of 100,000s of words.</p>
  
  <p>In my prototype I coded SQL directly.  I created a separate "words" table containing every word in the text fields of the main entity.  I indexed words and performed searches along the lines of</p>
  
  <pre><code>SELECT id, * FROM textTable 
    JOIN (SELECT DISTINCT textTableId FROM words 
           WHERE word BETWEEN 'foo' AND 'fooz' ) 
      ON id=textTableId
   LIMIT 50
  </code></pre>
  
  <p>This runs very fast.  Using an IN would probably work just as well, i.e.</p>
  
  <pre><code>SELECT * FROM textTable
   WHERE id IN (SELECT textTableId FROM words 
                 WHERE word BETWEEN 'foo' AND 'fooz' ) 
   LIMIT 50
  </code></pre>
  
  <p>The LIMIT is crucial and allows me to display results quickly.  I notify the user that there are too many to display if the limit is reached.  This is kludgy.</p>
  
  <p>I've spent the last several days pondering the advantages of moving to Core Data, but I worry about the lack of control in the schema, indexing, and querying for an important query.</p>
  
  <p>Theoretically an NSPredicate of <code>textField MATCHES '.*\bfoo.*'</code> would just work, but I'm sure it will be slow.  This sort of text search seems so common that I wonder what is the usual attack?  Would you create a words entity as I did above and use a predicate of "word BEGINSWITH 'foo'"?  Will that work as fast as my prototype?  Will Core Data automatically create the right indexes?  I can't find any explicit means of advising the persistent store about indexes.</p>
  
  <p>I see some nice advantages of Core Data in my iPhone app.  The faulting and other memory considerations allow for efficient database retrievals for tableview queries without setting arbitrary limits.  The object graph management allows me to easily traverse entities without writing lots of SQL.  Migration features will be nice in the future. On the other hand, in a limited resource environment (iPhone) I worry that an automatically generated database will be bloated with metadata, unnecessary inverse relationships, inefficient attribute datatypes, etc.</p>
  
  <p>Should I dive in or proceed with caution?</p>