What is slow query? How to optimize?

1. What is slow query ？

The slow query , seeing the name of a thing one thinks of its function , It's a slow query , So what is a slow query ？ Different companies have different definitions of slow query .

In general , We took the inquiry time longer than 1s The query of is called slow query .

2. Optimize SQL To reduce slow queries

2.1. Index optimization

Many slow queries are due to SQL It is caused by not following the appropriate index . however , Even with the index , If not used correctly ,SQL The statement will still not go to the index when executing . In which cases SQL Statements do not go through the index ？

1. WHERE In the condition of 1=1, The query condition is always true , May lead to WHERE Condition invalidation , Perform a full table query ;
2. Using functions or implicit conversions will result in no indexing ;
3. LIKE Match when the wildcard symbol is in front , Don't walk index ;
4. Negative conditions are used ;
5. OR One of them has an index , Another situation without ;
6. Multi column indexes need to meet the leftmost matching principle ;
7. The character sets or codes of the two tables are different , When querying the associated table ;
8. IN Too much content , It's not going to be indexed ;

2.2 Disassembly batch

If the amount of data is large , You can try to disassemble batches to reduce slow queries ：

1. Multi level nested query is changed to multiple query ;
2. IN Subqueries affect query performance , use JOIN Way to replace ;
3. The number of queries is too large , Split into multiple queries 、 assemble .
4. Reverse query used （ such as NOT IN） perhaps IN Too many statement parameter sets , This may cause a full table scan , In this case, try to split the statement , Or filter in memory ;
5. take ⼤ Field 、 Fields with low access frequency are split into separate tables to store , Separate hot and cold data ;
6. Sub database and sub table ;
7. file ;