SQLite CVSTrac

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  it in yourself rather than letting the optimizer do it for
  you.</p>
  
+ <H2>A SELECT which only accesses the index of a table, not the table row data</H2>
+ 
+ <p>Checked only with version 2.8.18! When you only want to check if something exists in a table which has an index(e.g. an index on "hour"), you might do this like so:</p>
+ 
+ <pre>
+      SELECT hour FROM appointments WHERE hour=5 LIMIT 1;
+      - or worse -
+      SELECT * FROM appointments WHERE hour=5 LIMIT 1;
+ </pre>
+ 
+ But both first check the index and then get the value of "hour" from the table row, not from the index itself. So what is quicker is simply this:
+ 
+ <pre>
+      SELECT 1 FROM appointments WHERE hour=5 LIMIT 1;
+ </pre>
+ 
+ Now SQlite doesn't have to read the value and just puts the number 1 in the result row so you know there are appointments at 5.
+ This is mostly usable when you for example give an overview of a day and have to indicate on what hours there are appointments. Personally I used this on a not so powerful embedded system, so it be not so noticable on a PC platform of course.
  
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sqlite - Performance Tuning