17.8.11 Configuring the Merge Threshold for Index Pages (original) (raw)

17.8.11 Configuring the Merge Threshold for Index Pages

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 anUPDATE operation,InnoDB attempts to merge the index page with a neighboring index page. The defaultMERGE_THRESHOLD value is 50, which is the previously hardcoded value. The minimumMERGE_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 defaultMERGE_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 thatInnoDB 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. AMERGE_THRESHOLD value defined for an individual index takes priority over a MERGE_THRESHOLD value defined for the table. If undefined, theMERGE_THRESHOLD value defaults to 50.

Setting MERGE_THRESHOLD for a Table

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

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_tableoption_ COMMENT clause with ALTER TABLE:

CREATE TABLE t1 (
   id INT,
  KEY id_index (id)
);

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_indexoption_ COMMENT clause with CREATE TABLE,ALTER TABLE, orCREATE INDEX, as shown in the following examples:

CREATE TABLE t1 (  
   id INT,  
  KEY id_index (id) COMMENT 'MERGE_THRESHOLD=40'  
);  
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';  
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 theMERGE_THRESHOLD value forGEN_CLUST_INDEX by settingMERGE_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 theINNODB_INDEXES table. For example:

mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_INDEXES WHERE NAME='id_index' \G
*************************** 1. row ***************************
       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 the_tableoption_ COMMENT clause:

mysql> SHOW CREATE TABLE t2 \G
*************************** 1. row ***************************
       Table: t2
Create Table: CREATE TABLE `t2` (
  `id` int(11) DEFAULT NULL,
  KEY `id_index` (`id`) COMMENT 'MERGE_THRESHOLD=40'
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

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_indexoption_ COMMENT clause:

mysql> SHOW INDEX FROM t2 \G
*************************** 1. row ***************************
        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

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

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:

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 usingINNODB_METRICS counters, seeSection 17.15.6, “InnoDB INFORMATION_SCHEMA Metrics Table”.