MySQL 8.4 Release Notes
Storage engines collect statistics about tables for use by the optimizer. Table statistics are based on value groups, where a value group is a set of rows with the same key prefix value. For optimizer purposes, an important statistic is the average value group size.
MySQL uses the average value group size in the following ways:
To estimate how many rows must be read for each
refaccessTo estimate how many rows a partial join produces, that is, the number of rows produced by an operation of the form
(...) JOIN tbl_name ON tbl_name.key = expr
As the average value group size for an index increases, the index is less useful for those two purposes because the average number of rows per lookup increases: For the index to be good for optimization purposes, it is best that each index value target a small number of rows in the table. When a given index value yields a large number of rows, the index is less useful and MySQL is less likely to use it.
The average value group size is related to table cardinality,
which is the number of value groups. The
SHOW INDEX statement displays a
cardinality value based on N/S, where
N is the number of rows in the table
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 <=>
NN.
However, for a join based on the = operator,
NULL is different from
non-NULL values:
expr1 =
expr2expr1 or
expr2 (or both) are
NULL. This affects
ref accesses for comparisons
of the form tbl_name.key =
exprexpr 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, allNULLvalues are treated as identical (that is, they all form a single value group).If the
NULLvalue group size is much higher than the average non-NULLvalue 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-NULLvalues. Consequently, thenulls_equalmethod may cause the optimizer not to use the index forrefaccesses when it should.When the variable is set to
nulls_unequal,NULLvalues are not considered the same. Instead, eachNULLvalue forms a separate value group of size 1.If you have many
NULLvalues, this method skews the average value group size downward. If the average non-NULLvalue group size is large, countingNULLvalues each as a group of size 1 causes the optimizer to overestimate the value of the index for joins that look for non-NULLvalues. Consequently, thenulls_unequalmethod may cause the optimizer to use this index forreflookups when other methods may be better.When the variable is set to
nulls_ignored,NULLvalues 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:
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_methodand issue anANALYZE TABLEstatement
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, some of those statements modify the table, MySQL may collect statistics. (This may occur for bulk inserts or deletes, or some
ALTER TABLEstatements, for example.) If this happens, the statistics are collected using whatever valueinnodb_stats_methodormyisam_stats_methodhas at the time. Thus, if you collect statistics using one method, but the system variable is set to the other method when a table's statistics are collected automatically later, the other method is used.There is no way to tell which method was used to generate statistics for a given table.
These variables apply only to
InnoDBandMyISAMtables. Other storage engines have only one method for collecting table statistics. Usually it is closer to thenulls_equalmethod.