View the SQL execution plan according to explain and optimize the SQL

1. Use Explain Keyword view SQL Implementation plan of

Statement for ：EXPLAIN " Yours sql sentence "

  And then according to the above figure sql The implementation plan focuses on two main points 1.Type Whether to query all tables   ALL Represents full table query ,2. Focus on rows  See how many lines were scanned （ Estimate ）, Both can determine whether optimization is needed

2. Determine the sql It needs to be optimized

Use   show index from tableName( Table name )

See which indexes the table uses , To avoid index creation conflicts

3. Combine the previous two operations to determine which field to use the index on , The suggestion is in where Add an index to the field after the condition , But there is one thing that needs special attention ： It doesn't make sense to add an index to the status field or gender field （ Try to select the field with high discrimination as the index , The formula of discrimination is count(distinct col)/count(*), Express The proportion of fields that do not repeat , The larger the ratio, the fewer records we scan , The only key differentiator is 1, And some states 、 Gender fields may be differentiated in front of big data 0. Add an index to the gender field , Does not significantly speed up Retrieval Speed
Link to the original text ：https://blog.csdn.net/qq_43193797/article/details/88592605）

The statement to create the index is ：create index Custom index name on Table name （ Field name ）

4. Create the index and execute it again explain "sql sentence " Whether the two indicators mentioned above have been optimized , If there is no optimization effect , Maybe the index is invalid .

summary ; Although creating an index can greatly improve the speed of query , But it will reduce the speed of updating and inserting , Because the index also needs to be updated , Therefore, the index should be added according to the actual business needs