10.3.8 InnoDB和MyISAM索引统计信息收集
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
ref
access -
To 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
for any other N
<=> N
N
.
However, for a join based on the =
operator, NULL
is different from non-NULL
values:
is not true when expr1
= expr2
expr1
or expr2
(or both) are NULL
. This affects ref
accesses for comparisons of the form
: MySQL does not access the table if the current value of tbl_name.key
= expr
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
, allNULL
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, thenulls_equal
method may cause the optimizer not to use the index forref
accesses when it should. -
When the variable is set to
nulls_unequal
,NULL
values are not considered the same. Instead, eachNULL
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, countingNULL
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, thenulls_unequal
method may cause the optimizer to use this index forref
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:
-
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 anANALYZE 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, some of those statements modify the table, MySQL may collect statistics. (This may occur for bulk inserts or deletes, or some
ALTER TABLE
statements, for example.) If this happens, the statistics are collected using whatever valueinnodb_stats_method
ormyisam_stats_method
has 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
InnoDB
andMyISAM
tables. Other storage engines have only one method for collecting table statistics. Usually it is closer to thenulls_equal
method.