Controls index condition pushdown. 

For more information, see Section 8.2.1.6, “Index Condition Pushdown Optimization” . 

•Index Extensions Flags 

• use_index_extensions  (default on ) 

Controls use of index extensions. 

For more information, see Section 8.3.10, “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 8.2.1.3, “Index Merge Optimization” . 

•Index Visibility Flags 

• use_invisible_indexes  (default off ) 

Controls use of invisible indexes. 

For more information, see Section 8.3.12, “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 optimzation 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. Prior to MySQL 8.0.21, it ws not possible to disable this optimization, but in MySQL 8.0.21 and later, while it remains the default behavior, it can be disabled by setting the prefer_ordering_index  flag to off . 

For more information and examples, see Section 8.2.1.19, “LIMIT Query Optimization” . 

•Multi-Range Read Flags 

