High ratio of rows returned to index
In MySQL, the number of rows returned by the query exceeds 20% of the total number of rows, and the execution plan will not take the index. Let's look at an example to query information with an id greater than 1.
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Mysql > explain select * from test06 where id > 1
+-- +
| | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+-- +
| | 1 | SIMPLE | test06 | ALL | idx_test03_id | NULL | NULL | NULL | 2009559 | Using where |
When the proportion of returned rows is high, the optimizer thinks that full table scanning saves time. Indexes are not used.
