什么是Index Merge
The Index Merge access method retrieves rows with multiple range scans and merges their results into one. This access method merges index scans from a single table only, not scans across multiple tables. The merge can produce unions, intersections, or unions-of-intersections of its underlying scans.
举例说明
• If your query has a complex WHERE clause with deep AND/OR nesting and MySQL does not choose the optimal plan, try distributing terms using the following identity transformations:
• Index Merge is not applicable to full-text indexes.
In EXPLAIN output, the Index Merge method appears as index_merge in the type column. In this case, the key column contains a list of indexes used, and key_len contains a list of the longest key parts for those indexes. The Index Merge access method has several algorithms, which are displayed in the Extra field of EXPLAIN output:Using intersect(…) 或者Using union(…)或者Using sort_union(…)
The Index Merge access method has three algorithms
This access algorithm is applicable when a WHERE clause is converted to several range conditions on different keys combined with AND, and each condition is one of the following:
• An N-part expression of this form, where the index has exactly N parts (that is, all index parts are covered):
• Any range condition over the primary key of an InnoDB table.
例如：
The Index Merge intersection algorithm performs simultaneous（同时的、同步的） scans on all used indexes and produces the intersection of row sequences that it receives from the merged index scans.
If all columns used in the query are covered by the used indexes, full table rows are not retrieved (EXPLAIN output contains Using index in Extra field in this case).
If the used indexes do not cover all columns used in the query, full rows are retrieved only when the range conditions for all used keys are satisfied.
If one of the merged conditions is a condition over the primary key of an InnoDB table, it is not used for row retrieval, but is used to filter out rows retrieved using other conditions.
The criteria（标准、原则） for this algorithm are similar to those for the Index Merge intersection algorithm. The algorithm is applicable when the table’s WHERE clause is converted to several range conditions on different keys combined with OR, and each condition is one of the following：
• An N-part expression of this form, where the index has exactly N parts (that is, all index parts are covered):
• Any range condition over a primary key of an InnoDB table.
• A condition for which the Index Merge intersection algorithm is applicable
例如：
This access algorithm is applicable when the WHERE clause is converted to several range conditions combined by OR, but the Index Merge union algorithm is not applicable.
例如：
The difference between the sort-union algorithm and the union algorithm is that the sort-union algorithm must first fetch row IDs for all rows and sort them before returning any rows.
—转载MySQL官网文档
Original: https://www.cnblogs.com/xuliuzai/p/15542411.html
Author: 东山絮柳仔
Title: MySQL学习笔记（七）–Index Merge
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