The optimizer_switch system
variable enables control over optimizer behavior. Its value is
a set of flags, each of which has a value of
on or off to indicate
whether the corresponding optimizer behavior is enabled or
disabled. This variable has global and session values and can
be changed at runtime. The global default can be set at server
startup.
To see the current set of optimizer flags, select the variable value:
mysql> SELECT @@optimizer_switch\G
*************************** 1. row ***************************
@@optimizer_switch: index_merge=on,index_merge_union=on,
index_merge_sort_union=on,
index_merge_intersection=on,
engine_condition_pushdown=on
To change the value of
optimizer_switch, assign a
value consisting of a comma-separated list of one or more
commands:
SET [GLOBAL|SESSION] optimizer_switch='command[,command]...';
Each command value should have one
of the forms shown in the following table.
| Command Syntax | Meaning |
|---|---|
default | Reset every optimization to its default value |
| Set the named optimization to its default value |
| Disable the named optimization |
| Enable the named optimization |
The order of the commands in the value does not matter,
although the default command is executed
first if present. Setting an
opt_name flag to
default sets it to whichever of
on or off is its default
value. Specifying any given
opt_name more than once in the
value is not permitted and causes an error. Any errors in the
value cause the assignment to fail with an error and the value
of optimizer_switch remains
unchanged.
The following table lists the permissible
opt_name flag names, grouped by
optimization strategy.
| Optimization | Flag Name | Meaning |
|---|---|---|
| Index Merge | index_merge | Controls all Index Merge optimizations |
index_merge_intersection | Controls the Index Merge Intersection Access optimization | |
index_merge_sort_union | Controls the Index Merge Sort-Union Access optimization | |
index_merge_union | Controls the Index Merge Union Access optimization | |
| Condition Pushdown | engine_condition_pushdown | Controls engine condition pushdown |
The flag for condition pushdown was added in MySQL 5.5.3.
For information about Index Merge, see Section 7.13.2, “Index Merge Optimization”. For information about engine condition pushdown, see Section 7.13.3, “Engine Condition Pushdown Optimization”.
When you assign a value to
optimizer_switch, flags that
are not mentioned keep their current values. This makes it
possible to enable or disable specific optimizer behaviors in
a single statement without affecting other behaviors. The
statement does not depend on what other optimizer flags exist
and what their values are. Suppose that all Index Merge
optimizations are enabled:
mysql> SELECT @@optimizer_switch\G
*************************** 1. row ***************************
@@optimizer_switch: index_merge=on,index_merge_union=on,
index_merge_sort_union=on,
index_merge_intersection=on,
engine_condition_pushdown=on
If the server is using the Index Merge Union or Index Merge Sort-Union access methods for certain queries and you want to check whether the optimizer will perform better without them, set the variable value like this:
mysql>SET optimizer_switch='index_merge_union=off,index_merge_sort_union=off';mysql>SELECT @@optimizer_switch\G*************************** 1. row *************************** @@optimizer_switch: index_merge=on,index_merge_union=off, index_merge_sort_union=off, index_merge_intersection=on, engine_condition_pushdown=on