Under what circumstances will the index fail?
The situations where the index fails are: 1. When calculating the column or using a function; 2. When using the reverse operation or link operation; 3. When using or in the where statement, if a column is not index, then other columns will be invalid.
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Under what circumstances do you need to use an index?
(1) Columns are often used in where conditions
(2) There are a large number of null values in the column
(3) The table has hardly been modified
(4) The amount of data is very large, only 2-4% of the data is selected
Under what circumstances will the index become invalid?
(1) If the column is calculated or a function is used, the index of the column will become invalid
(2) Mismatching the data type will cause index failure
(3) IS NULL or IS NOT NULL is used in the where statement, which will cause index failure
(4) Reverse operation is used, which The index will not work
(5) If the link operation is used, the index will not work
(6) When using OR in WHERE, if one column does not have an index, then the other columns The index will not work
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