• COMMENT ’ string ’ 

Index definitions can include an optional comment of up to 1024 characters. 

The MERGE_THRESHOLD  for index pages can be configured for individual indexes using the index_option COMMENT  clause of the CREATE INDEX  statement. For example: 

CREATE TABLE t1 (id INT); CREATE INDEX id_index ON t1 (id) COMMENT ’MERGE_THRESHOLD=40’; 

If the page-full percentage for an index page falls below the MERGE_THRESHOLD  value when a row is deleted or when a row is shortened by an update operation, InnoDB  attempts to merge the index page with a neighboring index page. The default MERGE_THRESHOLD  value is 50, which is the previously hardcoded value. 

MERGE_THRESHOLD  can also be defined at the index level and table level using CREATE TABLE and ALTER TABLE  statements. For more information, see Section 15.8.11, “Configuring the Merge Threshold for Index Pages” . 

• VISIBLE , INVISIBLE 

Specify index visibility. Indexes are visible by default. An invisible index is not used by the optimizer. Specification of index visibility applies to indexes other than primary keys (either explicit or implicit). For more information, see Section 8.3.12, “Invisible Indexes” . 

• ENGINE_ATTRIBUTE  and SECONDARY_ENGINE_ATTRIBUTE  options (available as of MySQL 8.0.21) are used to specify index attributes for primary and secondary storage engines. The options are reserved for future use. 

Permitted values are a string literal containing a valid JSON  document or an empty string (’’). Invalid JSON  is rejected. 

CREATE INDEX i1 ON t1 (c1) ENGINE_ATTRIBUTE=’{" key ":" value "}’; 

ENGINE_ATTRIBUTE  and SECONDARY_ENGINE_ATTRIBUTE  values can be repeated without error. In this case, the last specified value is used. 

ENGINE_ATTRIBUTE  and SECONDARY_ENGINE_ATTRIBUTE  values are not checked by the server, nor are they cleared when the table’s storage engine is changed. 

## Table Copying and Locking Options 

ALGORITHM  and LOCK  clauses may be given to influence the table copying method and level of concurrency for reading and writing the table while its indexes are being modified. They have the same meaning as for the ALTER TABLE  statement. For more information, see Section 13.1.9, “ALTER TABLE Statement” 

NDB Cluster supports online operations using the same ALGORITHM=INPLACE  syntax used with the standard MySQL Server. See Section 23.5.11, “Online Operations with ALTER TABLE in NDB Cluster” , for more information. 

## 13.1.16 CREATE LOGFILE GROUP Statement 

CREATE LOGFILE GROUP logfile_group ADD UNDOFILE ’ undo_file ’ [INITIAL_SIZE [=] initial_size ] [UNDO_BUFFER_SIZE [=] undo_buffer_size ] [REDO_BUFFER_SIZE [=] redo_buffer_size ] 

