You can configure the MERGE_THRESHOLD value for index pages. 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. The minimum MERGE_THRESHOLD value is 1 and the maximum value is 50.
您可以为索引页配置MERGE_THRESHOLD值。如果一个索引页的“page-full”百分比低于MERGE_THRESHOLD值，当一行被删除，或者当一行被更新操作缩短时，InnoDB尝试将索引页合并到一个相邻的索引页。默认的MERGE_THRESHOLD值为50，这是之前硬编码的值。最小的MERGE_THRESHOLD值为1，最大值为50。
When the “page-full” percentage for an index page falls below 50%, which is the default MERGE_THRESHOLD setting, InnoDBattempts to merge the index page with a neighboring page. If both pages are close to 50% full, a page split can occur soon after the pages are merged. If this merge-split behavior occurs frequently, it can have an adverse affect on performance. To avoid frequent merge-splits, you can lower the MERGE_THRESHOLD value so that InnoDB attempts page merges at a lower“page-full” percentage. Merging pages at a lower page-full percentage leaves more room in index pages and helps reduce merge-split behavior.
当索引页的“page-full百分比低于50%(这是默认的MERGE_THRESHOLD设置)时，innodbtry试图将索引页与相邻页面合并。如果两个页面都接近50%满，那么在页面合并后不久就会发生页面分割。如果这种合并分割行为频繁发生，则会对性能产生不利影响。为了避免频繁的合并-分割，您可以降低MERGE_THRESHOLD值，以便InnoDB尝试将页面合并到一个更低的“page-full”百分比。以较低的page-full百分比合并页面会在索引页中留下更多空间，并有助于减少合并拆分行为。
The MERGE_THRESHOLD for index pages can be defined for a table or for individual indexes. A MERGE_THRESHOLD value defined for an individual index takes priority over a MERGE_THRESHOLD value defined for the table. If undefined, the MERGE_THRESHOLD value defaults to 50.
可以为表或单个索引定义索引页的MERGE_THRESHOLD。为单个索引定义的MERGE_THRESHOLD值优先于为表定义的MERGE_THRESHOLD值。如果没有定义，MERGE_THRESHOLD值默认为50。
Setting MERGE_THRESHOLD for a Table
为表设置MERGE_THRESHOLD
You can set the MERGE_THRESHOLD value for a table using the table_option COMMENT clause of the CREATE TABLEstatement. For example:
您可以使用CREATE table语句的table_option COMMENT子句为表设置MERGE_THRESHOLD值。例如:
CREATE TABLE t1 (
id INT,
KEY id_index (id)
) COMMENT='MERGE_THRESHOLD=45';
You can also set the MERGE_THRESHOLD value for an existing table using the table_option COMMENT clause with ALTER TABLE:
您还可以使用ALTER table的表COMMENT子句设置现有表的MERGE_THRESHOLD值:
CREATE TABLE t1 (
id INT,
KEY id_index (id)
);
ALTER TABLE t1 COMMENT='MERGE_THRESHOLD=40';
Setting MERGE_THRESHOLD for Individual Indexes
为单个索引设置MERGE_THRESHOLD
To set the MERGE_THRESHOLD value for an individual index, you can use the index_option COMMENT clause with CREATE TABLE, ALTER TABLE, or CREATE INDEX, as shown in the following examples:
要为单个索引设置MERGE_THRESHOLD值，可以使用CREATE TABLE, ALTER TABLE, 或者 CREATE INDEX的所有选项COMMENT子句，如下面的示例所示:
- Setting MERGE_THRESHOLD for an individual index using CREATE TABLE:
- 使用CREATE TABLE为单个索引设置MERGE_THRESHOLD:
CREATE TABLE t1 (
id INT,
KEY id_index (id) COMMENT 'MERGE_THRESHOLD=40'
); - Setting MERGE_THRESHOLD for an individual index using ALTER TABLE:
- 使用ALTER TABLE为单个索引设置MERGE_THRESHOLD:
CREATE TABLE t1 (
id INT,
KEY id_index (id)
);
ALTER TABLE t1 DROP KEY id_index;
ALTER TABLE t1 ADD KEY id_index (id) COMMENT 'MERGE_THRESHOLD=40';
- Setting MERGE_THRESHOLD for an individual index using CREATE INDEX:
- 使用CREATE index为单个索引设置MERGE_THRESHOLD:
CREATE TABLE t1 (id INT);
CREATE INDEX id_index ON t1 (id) COMMENT 'MERGE_THRESHOLD=40';
Note
You cannot modify the MERGE_THRESHOLD value at the index level for GEN_CLUST_INDEX, which is the clustered index created by InnoDB when an InnoDB table is created without a primary key or unique key index. You can only modify the MERGE_THRESHOLD value for GEN_CLUST_INDEX by setting MERGE_THRESHOLD for the table.
您不能修改GEN_CLUST_INDEX的索引级别上的MERGE_THRESHOLD值，这是InnoDB创建的聚集索引，当创建一个InnoDB表时，没有主键或惟一键索引。您只能通过设置表的MERGE_THRESHOLD来修改GEN_CLUST_INDEX的MERGE_THRESHOLD值。
Querying the MERGE_THRESHOLD Value for an Index
查询索引的MERGE_THRESHOLD值
The current MERGE_THRESHOLD value for an index can be obtained by querying the INNODB_SYS_INDEXES table. For example:
可以通过查询INNODB_SYS_INDEXES表来获得索引的当前MERGE_THRESHOLD值。例如:
mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_SYS_INDEXES WHERE NAME='id_index' \G
INDEX_ID: 91
NAME: id_index
TABLE_ID: 68
TYPE: 0
N_FIELDS: 1
PAGE_NO: 4
SPACE: 57
MERGE_THRESHOLD: 40
You can use SHOW CREATE TABLE to view the MERGE_THRESHOLD value for a table, if explicitly defined using thetable_option COMMENT clause:
如果使用表选项COMMENT子句显式定义，您可以使用SHOW CREATE TABLE查看表的MERGE_THRESHOLD值:
mysql> SHOW CREATE TABLE t2 \G
Table: t2
Create Table: CREATE TABLE t2 (id int(11) DEFAULT NULL,
KEY id_index (id) COMMENT 'MERGE_THRESHOLD=40'
) ENGINE=InnoDB DEFAULT CHARSET=latin1
Note
A MERGE_THRESHOLD value defined at the index level takes priority over a MERGE_THRESHOLD value defined for the table. If undefined, MERGE_THRESHOLD defaults to 50% (MERGE_THRESHOLD=50, which is the previously hardcoded value.
在索引级别定义的MERGE_THRESHOLD值优先于为表定义的MERGE_THRESHOLD值。如果没有定义，MERGE_THRESHOLD默认为50% (MERGE_THRESHOLD=50，这是之前硬编码的值。
Likewise, you can use SHOW INDEX to view the MERGE_THRESHOLD value for an index, if explicitly defined using theindex_option COMMENT clause:
同样，如果使用索引选项COMMENT子句显式定义，您可以使用SHOW INDEX查看索引的MERGE_THRESHOLD值:
mysql> SHOW INDEX FROM t2 \G
Table: t2
Non_unique: 1
Key_name: id_index
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment: MERGE_THRESHOLD=40
Measuring the Effect of MERGE_THRESHOLD Settings
测量MERGE_THRESHOLD设置的效果
The INNODB_METRICS table provides two counters that can be used to measure the effect of a MERGE_THRESHOLD setting on index page merges.
INNODB_METRICS表提供了两个计数器，可用于度量MERGE_THRESHOLD设置对索引页面合并的影响。
mysql> SELECT NAME, COMMENT FROM INFORMATION_SCHEMA.INNODB_METRICS
WHERE NAME like '%index_page_merge%';
+-----------------------------+----------------------------------------+
| NAME | COMMENT |
+-----------------------------+----------------------------------------+
| index_page_merge_attempts | Number of index page merge attempts |
| index_page_merge_successful | Number of successful index page merges |
+-----------------------------+----------------------------------------+
When lowering the MERGE_THRESHOLD value, the objectives are:
降低MERGE_THRESHOLD值时，目标为
- A smaller number of page merge attempts and successful page merges
- 较少的页面合并尝试和成功的页面合并
- A similar number of page merge attempts and successful page merges
- 类似数量的页面合并尝试和成功的页面合并
A MERGE_THRESHOLD setting that is too small could result in large data files due to an excessive amount of empty page space.
一个太小的MERGE_THRESHOLD设置可能会由于过多的空页空间而导致大数据文件。
For information about using INNODB_METRICS counters, see Section 14.15.6, “InnoDB INFORMATION_SCHEMA Metrics Table”.
关于INNODB_METRICS计算器的更多信息，请看Section 14.15.6, “InnoDB INFORMATION_SCHEMA Metrics Table”.
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