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15.8.11 Configuring the Merge Threshold for Index Pages

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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.

When the “page-full” percentage for an index page falls below 50%, which is the default MERGE_THRESHOLD setting, InnoDB attempts 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.

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.

Setting MERGE_THRESHOLD for a Table

You can set the MERGE_THRESHOLD value for a table using the table_option COMMENT clause of the CREATE TABLE statement. For example:

  1. CREATE TABLE t1 (
  2.    id INT,
  3.   KEY id_index (id)
  4. ) 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:

  1. CREATE TABLE t1 (
  2.    id INT,
  3.   KEY id_index (id)
  4. );
  5.  
  6. ALTER TABLE t1 COMMENT='MERGE_THRESHOLD=40';

 

Setting MERGE_THRESHOLD for Individual Indexes

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:

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.

 

Querying the MERGE_THRESHOLD Value for an Index

The current MERGE_THRESHOLD value for an index can be obtained by querying the INNODB_INDEXES table. For example:

  1. mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_INDEXES WHERE NAME='id_index' \G
  2. *************************** 1. row ***************************
  3.        INDEX_ID: 91
  4.            NAME: id_index
  5.        TABLE_ID: 68
  6.            TYPE: 0
  7.        N_FIELDS: 1
  8.         PAGE_NO: 4
  9.           SPACE: 57
  10. MERGE_THRESHOLD: 40

You can use SHOW CREATE TABLE to view the MERGE_THRESHOLD value for a table, if explicitly defined using the table_option COMMENT clause:

  1. mysql> SHOW CREATE TABLE t2 \G
  2. *************************** 1. row ***************************
  3.        Table: t2
  4. Create Table: CREATE TABLE `t2` (
  5.   `id` int(11) DEFAULT NULL,
  6.   KEY `id_index` (`id`) COMMENT 'MERGE_THRESHOLD=40'
  7. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
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.

Likewise, you can use SHOW INDEX to view the MERGE_THRESHOLD value for an index, if explicitly defined using the index_option COMMENT clause:

  1. mysql> SHOW INDEX FROM t2 \G
  2. *************************** 1. row ***************************
  3.         Table: t2
  4.    Non_unique: 1
  5.      Key_name: id_index
  6.  Seq_in_index: 1
  7.   Column_name: id
  8.     Collation: A
  9.   Cardinality: 0
  10.      Sub_part: NULL
  11.        Packed: NULL
  12.          Null: YES
  13.    Index_type: BTREE
  14.       Comment:
  15. Index_comment: MERGE_THRESHOLD=40

 

Measuring the Effect of MERGE_THRESHOLD Settings

The INNODB_METRICS table provides two counters that can be used to measure the effect of a MERGE_THRESHOLD setting on index page merges.

  1. mysql> SELECT NAME, COMMENT FROM INFORMATION_SCHEMA.INNODB_METRICS
  2.        WHERE NAME like '%index_page_merge%';
  3. +-----------------------------+----------------------------------------+
  4. | NAME                        | COMMENT                                |
  5. +-----------------------------+----------------------------------------+
  6. | index_page_merge_attempts   | Number of index page merge attempts    |
  7. | index_page_merge_successful | Number of successful index page merges |
  8. +-----------------------------+----------------------------------------+

When lowering the MERGE_THRESHOLD value, the objectives are:

  • A smaller number of page merge attempts and successful page merges

  • A similar number of page merge attempts and successful page merges

A MERGE_THRESHOLD setting that is too small could result in large data files due to an excessive amount of empty page space.

For information about using INNODB_METRICS counters, see Section 15.14.6, “InnoDB INFORMATION_SCHEMA Metrics Table”.


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