(mysql.info) how-to-avoid-table-scan

 
 7.2.15 How to Avoid 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  where-optimizations.
 
    * 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:
 
    * Use `ANALYZE TABLE TBL_NAME' to update the key distributions for
      the scanned table. See  analyze-table.
 
    * Use `FORCE INDEX' for the scanned table to tell MySQL that table
      scans are very expensive compared to using the given index:
 
           SELECT * FROM t1, t2 FORCE INDEX (INDEX_FOR_COLUMN)
             WHERE t1.COL_NAME=t2.COL_NAME;
 
      See  select.
 
    * Start `mysqld' with the -max-seeks-for-key=1000 option or use `SET
      max_seeks_for_key=1000' to tell the optimizer to assume that no
      key scan causes more than 1,000 key seeks. See 
      server-system-variables.
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