Expanded Program Option Modifiers
MySQL has the concept of options modifiers which is a simple way to modify either the way that MySQL interprets an option or the way the option behaves. Option modifiers are used by simply prepending the name of the modifier and a dash “-” before the actual configuration option name. For example specifying –maximum-query_cache_size=4M on the mysqld commad line or specifying maximum-query_cache_size=4M in the my.cnf will prevent any client from setting the query_cache_size value larger than 4MB.
- Currently MySQL supports five existing option modifiers:
- disable [disable-<option_name>] disables or ignores option_name.
- enable [enable-<option_name>] enables option_name.
- loose [loose-<option_name>] - mysqld will not exit with an error if it does not recognize option_name, but instead it will issue only a warning.
- maximum [maximum-<option_name>=<value>] indicates that a client can not set the value of option_name greater than the limit specified. If the client does attempt to set the value of option_name greater than the limit, the option_name will simply be set to the defined limit.
- skip [skip-<option_name>] skips or ignores option_name.
- In order to offer more control over option visibility, access and range limits, the following new option modifiers have been added by Percona Server:
- minimum [minimum-<option_name>=<value>] indicates that clients can not set the value of option_name to less than the limit specified. If the client does attempt to set the value of option_name lesser than the limit, the option_name will simply be set to the defined limit.
- hidden [hidden-<option_name>=<TRUE/FALSE>] indicates that clients can not see or modify the value of option_name.
- readonly [readonly-<option_name>=<TRUE/FALSE>] indicates that clients can see the value of option_name but can not modify the value.
Combining the options
Some of the option modifiers may be used together in the same option specification, example:
--skip-loose-<option_name>
--loose-readonly-<option_name>=<T/F>
--readonly-<option_name>=<T/F>
--hidden-<option_name>=<T/F>
Examples
Adding the following option to the my.cnf will set the minimum limit on query_cache_size
minimum-query_cache_size = 4M
Trying to set up bigger value will work correctly, but if we try to set it up with smaller than the limit, defined minimum limit will be used and warning (1292) will be issued:
Initial query_cache_size size:
mysql> show variables like 'query_cache_size';
+------------------+---------+
| Variable_name | Value |
+------------------+---------+
| query_cache_size | 8388608 |
+------------------+---------+
1 row in set (0.00 sec)
Setting up bigger value:
mysql> set global query_cache_size=16777216;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'query_cache_size';
+------------------+----------+
| Variable_name | Value |
+------------------+----------+
| query_cache_size | 16777216 |
+------------------+----------+
1 row in set (0.00 sec)
Setting up smaller value:
mysql> set global query_cache_size=1048576;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+-------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------+
| Warning | 1292 | Truncated incorrect query_cache_size value: '1048576' |
+---------+------+-------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show variables like 'query_cache_size';
+------------------+---------+
| Variable_name | Value |
+------------------+---------+
| query_cache_size | 4194304 |
+------------------+---------+
1 row in set (0.00 sec)
Adding following option to my.cnf will make query_cache_size hidden.
hidden-query_cache_size=1
mysql> show variables like 'query_cache%';
+------------------------------+---------+
| Variable_name | Value |
+------------------------------+---------+
| query_cache_limit | 1048576 |
| query_cache_min_res_unit | 4096 |
| query_cache_strip_comments | OFF |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |
+------------------------------+---------+
5 rows in set (0.00 sec)
Adding following option to my.cnf will make query_cache_size read-only
readonly-query_cache_size=1
Trying to change the variable value will result in error:
mysql> show variables like 'query_cache%';
+------------------------------+---------+
| Variable_name | Value |
+------------------------------+---------+
| query_cache_limit | 1048576 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 8388608 |
| query_cache_strip_comments | OFF |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |
+------------------------------+---------+
6 rows in set (0.00 sec)
mysql> set global query_cache_size=16777216;
ERROR 1238 (HY000): Variable 'query_cache_size' is a read only variable
