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10.9.2 Switchable Optimizations

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,index_condition_pushdown=on,
                    mrr=on,mrr_cost_based=on,block_nested_loop=on,
                    batched_key_access=off,materialization=on,semijoin=on,
                    loosescan=on,firstmatch=on,duplicateweedout=on,
                    subquery_materialization_cost_based=on,
                    use_index_extensions=on,condition_fanout_filter=on,
                    derived_merge=on,use_invisible_indexes=off,skip_scan=on,
                    hash_join=on,subquery_to_derived=off,
                    prefer_ordering_index=on,hypergraph_optimizer=off,
                    derived_condition_pushdown=on,hash_set_operations=on
1 row in set (0.00 sec)

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
`opt_name=default`	Set the named optimization to its default value
`opt_name=off`	Disable the named optimization
`opt_name=on`	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, leaving the value of optimizer_switch unchanged.

The following list describes the permissible opt_name flag names, grouped by optimization strategy:

Batched Key Access Flags
- batched_key_access (default off)
  Controls use of BKA join algorithm.
For batched_key_access to have any effect when set to on, the mrr flag must also be on. Currently, the cost estimation for MRR is too pessimistic. Hence, it is also necessary for mrr_cost_based to be off for BKA to be used.
For more information, see Section 10.2.1.12, “Block Nested-Loop and Batched Key Access Joins”.
Block Nested-Loop Flags
- block_nested_loop (default on)
  Controls use of hash joins, as do the BNL and NO_BNL optimizer hints.
For more information, see Section 10.2.1.12, “Block Nested-Loop and Batched Key Access Joins”.
Condition Filtering Flags
- condition_fanout_filter (default on)
  Controls use of condition filtering.
For more information, see Section 10.2.1.13, “Condition Filtering”.
Derived Condition Pushdown Flags
- derived_condition_pushdown (default on)
  Controls derived condition pushdown.
For more information, see Section 10.2.2.5, “Derived Condition Pushdown Optimization”
Derived Table Merging Flags
- derived_merge (default on)
  Controls merging of derived tables and views into outer query block.
The derived_merge flag controls whether the optimizer attempts to merge derived tables, view references, and common table expressions into the outer query block, assuming that no other rule prevents merging; for example, an ALGORITHM directive for a view takes precedence over the derived_merge setting. By default, the flag is on to enable merging.
For more information, see Section 10.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”.
Engine Condition Pushdown Flags
- engine_condition_pushdown (default on)
  Controls engine condition pushdown.
For more information, see Section 10.2.1.5, “Engine Condition Pushdown Optimization”.
Hash Join Flags
- hash_join (default on)
  Has no effect in MySQL 9.1. Use the block_nested_loop flag, instead.
For more information, see Section 10.2.1.4, “Hash Join Optimization”.
Index Condition Pushdown Flags
- index_condition_pushdown (default on)
  Controls index condition pushdown.
For more information, see Section 10.2.1.6, “Index Condition Pushdown Optimization”.
Index Extensions Flags
- use_index_extensions (default on)
  Controls use of index extensions.
For more information, see Section 10.3.10, “Use of Index Extensions”.
Index Merge Flags
- index_merge (default on)
  Controls all Index Merge optimizations.
- index_merge_intersection (default on)
  Controls the Index Merge Intersection Access optimization.
- index_merge_sort_union (default on)
  Controls the Index Merge Sort-Union Access optimization.
- index_merge_union (default on)
  Controls the Index Merge Union Access optimization.
For more information, see Section 10.2.1.3, “Index Merge Optimization”.
Index Visibility Flags
- use_invisible_indexes (default off)
  Controls use of invisible indexes.
For more information, see Section 10.3.12, “Invisible Indexes”.
Limit Optimization Flags
- prefer_ordering_index (default on)
  Controls whether, in the case of a query having an ORDER BY or GROUP BY with a LIMIT clause, the optimizer tries to use an ordered index instead of an unordered index, a filesort, or some other optimization. This optimization is performed by default whenever the optimizer determines that using it would allow for faster execution of the query.
  Because the algorithm that makes this determination cannot handle every conceivable case (due in part to the assumption that the distribution of data is always more or less uniform), there are cases in which this optimization may not be desirable. This optimization can be disabled by setting the prefer_ordering_index flag to off.
For more information and examples, see Section 10.2.1.19, “LIMIT Query Optimization”.
Multi-Range Read Flags
- mrr (default on)
  Controls the Multi-Range Read strategy.
- mrr_cost_based (default on)
  Controls use of cost-based MRR if mrr=on.
For more information, see Section 10.2.1.11, “Multi-Range Read Optimization”.
Semijoin Flags
- duplicateweedout (default on)
  Controls the semijoin Duplicate Weedout strategy.
- firstmatch (default on)
  Controls the semijoin FirstMatch strategy.
- loosescan (default on)
  Controls the semijoin LooseScan strategy (not to be confused with Loose Index Scan for GROUP BY).
- semijoin (default on)
  Controls all semijoin strategies.
  This also applies to the antijoin optimization.
The semijoin, firstmatch, loosescan, and duplicateweedout flags enable control over semijoin strategies. The semijoin flag controls whether semijoins are used. If it is set to on, the firstmatch and loosescan flags enable finer control over the permitted semijoin strategies.
If the duplicateweedout semijoin strategy is disabled, it is not used unless all other applicable strategies are also disabled.
If semijoin and materialization are both on, semijoins also use materialization where applicable. These flags are on by default.
For more information, see Optimizing IN and EXISTS Subquery Predicates with Semijoin Transformations.
Set Operations Flags
- hash_set_operations (default on)
  Enables the hash table optimization for set operations involving EXCEPT and INTERSECT); enabled by default. Otherwise, temporary table based de-duplication is used, as in previous versions of MySQL.
  The amount of memory used for hashing by this optimization can be controlled using the set_operations_buffer_size system variable; increasing this generally results in faster execution times for statements using these operations.
Skip Scan Flags
- skip_scan (default on)
  Controls use of Skip Scan access method.
For more information, see Skip Scan Range Access Method.
Subquery Materialization Flags
- materialization (default on)
  Controls materialization (including semijoin materialization).
- subquery_materialization_cost_based (default on)
  Use cost-based materialization choice.
The materialization flag controls whether subquery materialization is used. If semijoin and materialization are both on, semijoins also use materialization where applicable. These flags are on by default.
The subquery_materialization_cost_based flag enables control over the choice between subquery materialization and IN-to-EXISTS subquery transformation. If the flag is on (the default), the optimizer performs a cost-based choice between subquery materialization and IN-to-EXISTS subquery transformation if either method could be used. If the flag is off, the optimizer chooses subquery materialization over IN-to-EXISTS subquery transformation.
For more information, see Section 10.2.2, “Optimizing Subqueries, Derived Tables, View References, and Common Table Expressions”.

Subquery Transformation Flags

subquery_to_derived (default off)

The optimizer is able in many cases to transform a scalar subquery in a SELECT, WHERE, JOIN, or HAVING clause into a left outer joins on a derived table. (Depending on the nullability of the derived table, this can sometimes be simplified further to an inner join.) This can be done for a subquery which meets the following conditions:

The subquery does not make use of any nondeterministic functions, such as RAND().
The subquery is not an ANY or ALL subquery which can be rewritten to use MIN() or MAX().
The parent query does not set a user variable, since rewriting it may affect the order of execution, which could lead to unexpected results if the variable is accessed more than once in the same query.
The subquery should not be correlated, that is, it should not reference a column from a table in the outer query, or contain an aggregate that is evaluated in the outer query.

This optimization can also be applied to a table subquery which is the argument to IN, NOT IN, EXISTS, or NOT EXISTS, that does not contain a GROUP BY.

The default value for this flag is off, since, in most cases, enabling this optimization does not produce any noticeable improvement in performance (and in many cases can even make queries run more slowly), but you can enable the optimization by setting the subquery_to_derived flag to on. It is primarily intended for use in testing.

Example, using a scalar subquery:

d
mysql> CREATE TABLE t1(a INT);

mysql> CREATE TABLE t2(a INT);

mysql> INSERT INTO t1 VALUES ROW(1), ROW(2), ROW(3), ROW(4);

mysql> INSERT INTO t2 VALUES ROW(1), ROW(2);

mysql> SELECT * FROM t1
    ->     WHERE t1.a > (SELECT COUNT(a) FROM t2);
+------+
| a    |
+------+
|    3 |
|    4 |
+------+

mysql> SELECT @@optimizer_switch LIKE '%subquery_to_derived=off%';
+-----------------------------------------------------+
| @@optimizer_switch LIKE '%subquery_to_derived=off%' |
+-----------------------------------------------------+
|                                                   1 |
+-----------------------------------------------------+

mysql> EXPLAIN SELECT * FROM t1 WHERE t1.a > (SELECT COUNT(a) FROM t2)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 4
     filtered: 33.33
        Extra: Using where
*************************** 2. row ***************************
           id: 2
  select_type: SUBQUERY
        table: t2
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: NULL

mysql> SET @@optimizer_switch='subquery_to_derived=on';


mysql> SELECT @@optimizer_switch LIKE '%subquery_to_derived=off%';
+-----------------------------------------------------+
| @@optimizer_switch LIKE '%subquery_to_derived=off%' |
+-----------------------------------------------------+
|                                                   0 |
+-----------------------------------------------------+

mysql> SELECT @@optimizer_switch LIKE '%subquery_to_derived=on%';
+----------------------------------------------------+
| @@optimizer_switch LIKE '%subquery_to_derived=on%' |
+----------------------------------------------------+
|                                                  1 |
+----------------------------------------------------+

mysql> EXPLAIN SELECT * FROM t1 WHERE t1.a > (SELECT COUNT(a) FROM t2)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: <derived2>
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1
     filtered: 100.00
        Extra: NULL
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 4
     filtered: 33.33
        Extra: Using where; Using join buffer (hash join)
*************************** 3. row ***************************
           id: 2
  select_type: DERIVED
        table: t2
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: NULL

As can be seen from executing SHOW WARNINGS immediately following the second EXPLAIN statement, with the optimization enabled, the query SELECT * FROM t1 WHERE t1.a > (SELECT COUNT(a) FROM t2) is rewritten in a form similar to what is shown here:

SELECT t1.a FROM t1
    JOIN  ( SELECT COUNT(t2.a) AS c FROM t2 ) AS d
            WHERE t1.a > d.c;

Example, using a query with IN (subquery):

mysql> DROP TABLE IF EXISTS t1, t2;

mysql> CREATE TABLE t1 (a INT, b INT);
mysql> CREATE TABLE t2 (a INT, b INT);

mysql> INSERT INTO t1 VALUES ROW(1,10), ROW(2,20), ROW(3,30);
mysql> INSERT INTO t2
    ->    VALUES ROW(1,10), ROW(2,20), ROW(3,30), ROW(1,110), ROW(2,120), ROW(3,130);

mysql> SELECT * FROM t1
    ->     WHERE   t1.b < 0
    ->             OR
    ->             t1.a IN (SELECT t2.a + 1 FROM t2);
+------+------+
| a    | b    |
+------+------+
|    2 |   20 |
|    3 |   30 |
+------+------+

mysql> SET @@optimizer_switch="subquery_to_derived=off";

mysql> EXPLAIN SELECT * FROM t1
    ->             WHERE   t1.b < 0
    ->                     OR
    ->                     t1.a IN (SELECT t2.a + 1 FROM t2)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: Using where
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: t2
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6
     filtered: 100.00
        Extra: Using where

mysql> SET @@optimizer_switch="subquery_to_derived=on";

mysql> EXPLAIN SELECT * FROM t1
    ->             WHERE   t1.b < 0
    ->                     OR
    ->                     t1.a IN (SELECT t2.a + 1 FROM t2)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: NULL
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: <derived2>
   partitions: NULL
         type: ref
possible_keys: <auto_key0>
          key: <auto_key0>
      key_len: 9
          ref: std2.t1.a
         rows: 2
     filtered: 100.00
        Extra: Using where; Using index
*************************** 3. row ***************************
           id: 2
  select_type: DERIVED
        table: t2
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6
     filtered: 100.00
        Extra: Using temporary

Checking and simplifying the result of SHOW WARNINGS after executing EXPLAIN on this query shows that, when the subquery_to_derived flag enabled, SELECT * FROM t1 WHERE t1.b < 0 OR t1.a IN (SELECT t2.a + 1 FROM t2) is rewritten in a form similar to what is shown here:

SELECT a, b FROM t1
    LEFT JOIN (SELECT DISTINCT a + 1 AS e FROM t2) d
    ON t1.a = d.e
    WHERE   t1.b < 0
            OR
            d.e IS NOT NULL;

Example, using a query with EXISTS (subquery) and the same tables and data as in the previous example:

mysql> SELECT * FROM t1
    ->     WHERE   t1.b < 0
    ->             OR
    ->             EXISTS(SELECT * FROM t2 WHERE t2.a = t1.a + 1);
+------+------+
| a    | b    |
+------+------+
|    1 |   10 |
|    2 |   20 |
+------+------+

mysql> SET @@optimizer_switch="subquery_to_derived=off";

mysql> EXPLAIN SELECT * FROM t1
    ->             WHERE   t1.b < 0
    ->                     OR
    ->                     EXISTS(SELECT * FROM t2 WHERE t2.a = t1.a + 1)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: Using where
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: t2
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6
     filtered: 16.67
        Extra: Using where

mysql> SET @@optimizer_switch="subquery_to_derived=on";

mysql> EXPLAIN SELECT * FROM t1
    ->             WHERE   t1.b < 0
    ->                     OR
    ->                     EXISTS(SELECT * FROM t2 WHERE t2.a = t1.a + 1)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: NULL
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: <derived2>
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6
     filtered: 100.00
        Extra: Using where; Using join buffer (hash join)
*************************** 3. row ***************************
           id: 2
  select_type: DERIVED
        table: t2
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6
     filtered: 100.00
        Extra: Using temporary

If we execute SHOW WARNINGS after running EXPLAIN on the query SELECT * FROM t1 WHERE t1.b < 0 OR EXISTS(SELECT * FROM t2 WHERE t2.a = t1.a + 1) when subquery_to_derived has been enabled, and simplify the second row of the result, we see that it has been rewritten in a form which resembles this:

SELECT a, b FROM t1
LEFT JOIN (SELECT DISTINCT 1 AS e1, t2.a AS e2 FROM t2) d
ON t1.a + 1 = d.e2
WHERE   t1.b < 0
        OR
        d.e1 IS NOT NULL;

For more information, see Section 10.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”, as well as Section 10.2.1.19, “LIMIT Query Optimization”, and Optimizing IN and EXISTS Subquery Predicates with Semijoin Transformations.

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,index_condition_pushdown=on,
                    mrr=on,mrr_cost_based=on,block_nested_loop=on,
                    batched_key_access=off,materialization=on,semijoin=on,
                    loosescan=on, firstmatch=on,
                    subquery_materialization_cost_based=on,
                    use_index_extensions=on,condition_fanout_filter=on,
                    derived_merge=on,use_invisible_indexes=off,skip_scan=on,
                    hash_join=on,subquery_to_derived=off,
                    prefer_ordering_index=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 can 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,index_condition_pushdown=on,
                    mrr=on,mrr_cost_based=on,block_nested_loop=on,
                    batched_key_access=off,materialization=on,semijoin=on,
                    loosescan=on, firstmatch=on,
                    subquery_materialization_cost_based=on,
                    use_index_extensions=on,condition_fanout_filter=on,
                    derived_merge=on,use_invisible_indexes=off,skip_scan=on,
                    hash_join=on,subquery_to_derived=off,
                    prefer_ordering_index=on

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