Shulou(Shulou.com)06/01 Report--

Possible_keys, key, and key_len are all null. It can be seen that there are no indexes available on the table.

To calculate the degree of differentiation, the closer to 1, the better, which should be placed on the left side of the federated index.

Explain after the federated index is built:

The more pages you turn, the slower you are, and further optimize:

Premise of optimization: comment_id is the primary key of the product review table and has an overlay index

Principle:

Using the overlay index, take out the primary key comment_id, sort it, take out the desired data, and then sort it with the comment table through the primary key to extract other fields. The data overhead of this approach is: the result of the io + index paging of the index, that is, the io of the 15 rows of data corresponding to the table, is much less than the io before optimization. The advantage is that each page turn consumes basically the same resources and time.

Adapt to the scene:

When the query and sort fields, that is, the fields involved in the where clause and the order by clause, have corresponding override indexes. And it also adapts to this situation when the intermediate result set is very large.

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