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how-to-avoid-table-scan

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User Comments
User comments in this section are, as the name implies, provided by MySQL users. The MySQL documentation team is not responsible for, nor do they endorse, any of the information provided here.
  Posted by Becky D on February 28, 2011
We ran into a condition where we were trying to join on 2 varchar fields that used a different collation. One field was utf-8 and the other was latin1. This was causing a full table scan on a join between the 2 tables. Once we converted the fields to all be utf-8, query went from 9 seconds down to 0.
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