and S  is the average value group size. That ratio yields an approximate number of value groups in the table. 

For a join based on the <=>  comparison operator, NULL  is not treated differently from any other value: NULL <=> NULL , just as N  <=> N  for any other N . 

However, for a join based on the =  operator, NULL  is different from non- NULL  values: expr1  = expr2  is not true when expr1  or expr2  (or both) are NULL . This affects ref  accesses for comparisons of the form tbl_name.key  = expr : MySQL does not access the table if the current value of expr  is NULL , because the comparison cannot be true. 

For =  comparisons, it does not matter how many NULL  values are in the table. For optimization purposes, the relevant value is the average size of the non- NULL  value groups. However, MySQL does not currently enable that average size to be collected or used. 

For InnoDB  and MyISAM  tables, you have some control over collection of table statistics by means of the innodb_stats_method  and myisam_stats_method  system variables, respectively. These variables have three possible values, which differ as follows: 

•When the variable is set to nulls_equal , all NULL  values are treated as identical (that is, they all form a single value group). 

If the NULL  value group size is much higher than the average non- NULL  value group size, this method skews the average value group size upward. This makes index appear to the optimizer to be less useful than it really is for joins that look for non- NULL  values. Consequently, the nulls_equal  method may cause the optimizer not to use the index for ref  accesses when it should. 

•When the variable is set to nulls_unequal , NULL  values are not considered the same. Instead, each NULL  value forms a separate value group of size 1. 

If you have many NULL  values, this method skews the average value group size downward. If the average non- NULL  value group size is large, counting NULL  values each as a group of size 1 causes the optimizer to overestimate the value of the index for joins that look for non- NULL  values. Consequently, the nulls_unequal  method may cause the optimizer to use this index for ref  lookups when other methods may be better. 

•When the variable is set to nulls_ignored , NULL  values are ignored. 

If you tend to use many joins that use <=>  rather than = , NULL  values are not special in comparisons and one NULL  is equal to another. In this case, nulls_equal  is the appropriate statistics method. 

The innodb_stats_method  system variable has a global value; the myisam_stats_method  system variable has both global and session values. Setting the global value affects statistics collection for tables from the corresponding storage engine. Setting the session value affects statistics collection only for the current client connection. This means that you can force a table’s statistics to be regenerated with a given method without affecting other clients by setting the session value of myisam_stats_method . 

To regenerate MyISAM  table statistics, you can use any of the following methods: 

•Execute myisamchk --stats_method= method_name  --analyze 

•Change the table to cause its statistics to go out of date (for example, insert a row and then delete it), and then set myisam_stats_method  and issue an ANALYZE TABLE  statement 

Some caveats regarding the use of innodb_stats_method  and myisam_stats_method : 

•You can force table statistics to be collected explicitly, as just described. However, MySQL may also collect statistics automatically. For example, if during the course of executing statements for a table, 

