MySQL 5.1 Reference Manual  /  ...  /  How to Avoid Full Table Scans How to Avoid Full Table Scans

The output from EXPLAIN shows ALL in the type column when MySQL uses a table scan to resolve a query. This usually happens under the following conditions:

  • The table is so small that it is faster to perform a table scan than to bother with a key lookup. This is common for tables with fewer than 10 rows and a short row length.

  • There are no usable restrictions in the ON or WHERE clause for indexed columns.

  • You are comparing indexed columns with constant values and MySQL has calculated (based on the index tree) that the constants cover too large a part of the table and that a table scan would be faster. See Section, “How MySQL Optimizes WHERE Clauses”.

  • You are using a key with low cardinality (many rows match the key value) through another column. In this case, MySQL assumes that by using the key it probably will do many key lookups and that a table scan would be faster.

For small tables, a table scan often is appropriate and the performance impact is negligible. For large tables, try the following techniques to avoid having the optimizer incorrectly choose a table scan:

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User Comments
  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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