Indexing rules - try to use covering indexes

===============

1 Prepare data

1.1 Create a table

DROP TABLE IF EXISTS staff;
CREATE TABLE IF NOT EXISTS staff (
    id INT PRIMARY KEY auto_increment,
    name VARCHAR(50),
    age INT,
    pos VARCHAR ( 50 ) COMMENT ' Position ' ,
    salary DECIMAL(10,2)
);

1.2 Insert data

INSERT INTO staff(name, age, pos, salary) VALUES('Alice', 22, 'HR', 5000);

2 Test & Explain Analysis

2.1 Create an index

CREATE INDEX idx_nameAgePos ON staff(name, age, pos);

2.2 Testing

Case#1：

EXPLAIN SELECT * FROM staff where name = 'Alice' AND age = 22 AND pos = 'HR';

Note: 'SELECT *' is used in SELECT, and 'Using index condition' is used in Extra

Case#2：

EXPLAIN SELECT name, age, pos FROM staff where name = 'Alice' AND age = 22 AND pos = 'HR';

Note: SELECT is 'SELECT name, age, pos', and Extra is 'Using where; Using indx'

Which is better 'Using where; Using indx' or 'Using index condition'? I haven't fully understood it yet, and I can't give a complete answer. The clues that can be provided are as follows.

An answer found on StackOverFlow, you can refer to the address: https://stackoverflow.com/questions/28759576/mysql-using-index-condition-vs-using-where-using-index

Meanwhile, the screenshot is as follows:

A simple translation is as follows:

'Using index condition': Where condition contains indexed and non-indexed columns, the query optimizer will first parse the indexed columns and query the rows of other conditions from the table
'Using where; Using indx': 'Using index' means that the entire table does not need to be scanned. 'Using where' may still do a full table scan on non-indexed columns, but it will use the index first, if there are any indexed columns in the Where condition, just like using the index condition
Which one is better? 'Using where; Using indx' would be better than 'Using index condition' if it was all covering indexes.

I feel that I am still confused, and the translation is too blunt,,,,,,,,,,,, and then I will add and improve it after I understand it thoroughly. . . . . TODO

But as you can see from the above example, 'SELECT name, age, pos' corresponds to 'Using where; Using indx', which is full index coverage; and 'SELECT *' corresponds to 'Using index condition', which The four fields of name, age, pos, and salary will be queried, and salary is not on the index. So, for sure, 'Using where; Using indx' would be better than 'Using index condition'.

3 Conclusion

Use covering indexes as much as possible - queries that only access the index (index columns and query columns are consistent), reduce SELECT *

Indexing rules - try to use covering indexes

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