MySQL optimization: from ten seconds to three hundred milliseconds

Today's article is still a record of the problem solving process.When I tested the test code yesterday, I found that a function takes about ten seconds to execute.I went to see my own code logic. After reading the code completely, I felt that there are two reasons for the long time: one is the writing problem of mysql, and the other may be the problem of cyclically querying the database.The code Mengmeng will not be uploaded, which may involve the company's privacy issues

The following is the glued sql part. You can see that this sql is more than 7 seconds. The second picture is the result after using the explain executor. You can see that only two of the five tables are indexed.The table is not indexed, and you can see that the level of the type column is also ALL.
The resulting values, from good to bad, are: system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL.Generally speaking, it is necessary to ensure that the query reaches at least the range level, preferably ref, otherwise performance problems may occur.So this sql needs to be optimized.

Optimization method:
1. First, optimize by combining the two columns of type+key, that is, by adding an index to optimize.Typically I would add indexes to the fields used in the where and order by clauses.So I added four indexes (deviceId, name, del_flag, project_id) to the d table, and the c table also added an index to prdfession.

2, and then optimize according to the Extra column. You can see that "Using temporary" and "Using filesort" appear in the two columns above.
"Using temporary": It means that if the fields in order by or group by in sql come from other tables instead of the first table in the join order, a temporary table will be created.Solution: You can try to create a joint index.
"Using filesort": refers to the fact that the amount of field data in the order by query exceeds the default value of mysql, which causes the fields in the order by to not be indexed, and the file query is used, which is also the cause of the performance bottleneck.Solution: You can try to implement the sorting function in the code, and do not do sorting in sql.
After creating a common index and a joint index (project_id + name), look at the execution speed of sql.

You can see that the execution speed has been improved immediately. When the explain command is executed, it will also be found that the type column has reached the ref level, and the extra column has no "Using temporary" and "Using filesort", which affects performance.symbols of.
Now the optimization of sql is completed, if the speed is still slow, consider optimizing part of the code logic.

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