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Many changes have been made to the InnoDB engine and the Optimizer since 10.5. These changes have affected the output of the `EXPLAIN` and a lot of new items have been added which are missing documentation and details for instance: https://mariadb.com/kb/en/explain/
As an example, refer to the following plan.
+------+-------------+-------+------------+---------------------------+---------------------------+---------+-------+--------------+---------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+------------+---------------------------+---------------------------+---------+-------+--------------+---------------------------------+
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| 1 | SIMPLE | deal | ref|filter | dealref_idx,transref_idx1 | transref_idx1|dealref_idx | 14|50 | const | 1906508 (0%) | Using where; Using rowid filter |
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+------+-------------+-------+------------+---------------------------+---------------------------+---------+-------+--------------+---------------------------------+
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Here under the column `type`, we can see `ref|filter`. What does this mean? Similarly, the `key` column has two indexes separated by a PIPE, is this referring to a MERGE index or something else? Finally, the `rows` column has a `(0%)`. What does this represent?
This is just an example, and we need all the changes in the EXPLAIN/EXPLAIN JSON documented with details for the proper reference and understanding.
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