(mysql.info.gz) How to avoid table scan

(mysql.info.gz) LIMIT optimization

 
 7.2.13 How to Avoid Table Scans
 -------------------------------
 
 The output from `EXPLAIN' will show `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's faster to do a table scan than a
      key lookup.  This is a common case 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.   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 will probably do a lot of key lookups and that
      a table scan would be faster.
 
 For small tables, a table scan often is appropriate. 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.   `ANALYZE TABLE' 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' SELECT.
 
           SELECT * FROM t1, t2 FORCE INDEX (INDEX_FOR_COLUMN)
           WHERE t1.COL_NAME=t2.COL_NAME;
 
    * 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 will cause more than 1,000 key seeks.   Server
      system variables.

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