Mysql explain index_merge

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 use and key_len contains a list of the longest key parts for those indexes. The of the scans are then merged. When using EXPLAIN , if index_merge is the plan chosen by the optimizer, it will show up in the type column.

The USE INDEX is useful in case the EXPLAIN shows that the Query Optimizer uses the wrong index from the list of possible indexes. In this tutorial, you have learned how to use the MySQL USE INDEX hint to instruct the Query Optimizer to use the only list of specified indexes to find rows in a table. EXPLAIN returns a row of information for each table used in the SELECT statement. It lists the tables in the output in the order that MySQL would read them while processing the statement.

MySQL resolves all joins using a nested-loop join method. When EXPLAIN is used with an explainable statement , MySQL displays information from the optimizer about the statement execution plan. That is, MySQL explains how it would process the statement, including information about how tables are joined and in which order. The most effective index for the query is a composite index that includes all three fields, for example: (object_i user_i location_id).

Since there is no such index , MySQL does its best to get most of the information from existing indexes. Thanks for the explanation. It makes no sense to use a separate index on b just to get its value, it’s an extra lookup.

Might as well go for the row data, which is a more generic case since many queries will select more columns anyway. The KEY column of the explain will contain the keys used. IN subquery returns only one result from the.

On the other han the order of columns in a composite index does matter. But it does not happen to matter in this case. Description: index_merge _innodb.

The first two tests are disabled because of non deterministic explain output. In MySQL , when you submit an SQL query, the query optimizer will try to make an optimal query execution plan. To determine the best possible plan, the query optimizer makes use of many parameters. One of the most important parameters for choosing which index to use is stored key distribution which is also known as cardinality. Using EXPLAIN to Write Better MySQL Queries When you issue a query, the MySQL Query Optimizer tries to devise an optimal plan for query execution.

You can see information about the plan by. The new optimization can use intersection, union or sort-union algorithms. EXPLAIN of the query shows that index _ merge method is used and the Extra co.

Stack Exchange Network Stack Exchange network consists of 1QA communities including Stack Overflow , the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. MySQL using a index _ merge and intersect instead of a ref and where. I have a query that is producing different optimizer on development and production (same database, same data). Index Merge shows used_key_parts For multi-part keys, tabular EXPLAIN shows key_length column and leaves the user to do column-size arithmetic to figure out how many key parts are used. It is very easy to disable index merge if you do not want it to run, however I do not know of the hint in MySQL which would allow forcing using index merge when MySQL does not think it should be used.

I hope hint would be added at some point. Finally let me mention the case when Index merge works much better than multiple column indexes. The key column of EXPLAIN ‘s output will contain the keys used. Since MyRocks query plan is less stable than InnoDB, it skips using explain and just verifies data correctness.

The EXPLAIN statement provides information about how MySQL executes statements.