why select a more column make mysql use index condition pushdown? - mysql

Question

in the test database employees, employees has 300,000+ rows.
CREATE TABLE employees (
emp_no INT NOT NULL,
birth_date DATE NOT NULL,
first_name VARCHAR(14) NOT NULL,
last_name VARCHAR(16) NOT NULL,
gender ENUM ('M','F') NOT NULL,
hire_date DATE NOT NULL,
PRIMARY KEY (emp_no)
);
alter table employees add index IDX_LAST_NAME(last_name);
explain
select last_name, emp_no
from employees
where last_name in ('Aamodt', 'Aaaa', 'Ab', 'Ac')
order by last_name asc, emp_no asc
limit 100;
shows
It uses covering index for where filter and order by.
explain format=JSON
{
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "43.97"
},
"ordering_operation": {
"using_filesort": false,
"table": {
"table_name": "employees",
"access_type": "range",
"possible_keys": [
"IDX_LAST_NAME"
],
"key": "IDX_LAST_NAME",
"used_key_parts": [
"last_name"
],
"key_length": "66",
"rows_examined_per_scan": 208,
"rows_produced_per_join": 208,
"filtered": "100.00",
"using_index": true,
"cost_info": {
"read_cost": "23.17",
"eval_cost": "20.80",
"prefix_cost": "43.97",
"data_read_per_join": "27K"
},
"used_columns": [
"emp_no",
"last_name"
],
"attached_condition": "(`employees`.`employees`.`last_name` in ('Aamodt','Aaaa','Ab','Ac'))"
}
}
}
}
While
explain
select last_name, emp_no, first_name
from employees
where last_name in ('Aamodt', 'Aaaa', 'Ab', 'Ac')
order by last_name asc, emp_no asc
limit 100;
shows
explain format=JSON
{
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "168.10"
},
"ordering_operation": {
"using_filesort": false,
"table": {
"table_name": "employees",
"access_type": "range",
"possible_keys": [
"IDX_LAST_NAME"
],
"key": "IDX_LAST_NAME",
"used_key_parts": [
"last_name"
],
"key_length": "66",
"rows_examined_per_scan": 208,
"rows_produced_per_join": 208,
"filtered": "100.00",
"index_condition": "(`employees`.`employees`.`last_name` in ('Aamodt','Aaaa','Ab','Ac'))",
"cost_info": {
"read_cost": "147.30",
"eval_cost": "20.80",
"prefix_cost": "168.10",
"data_read_per_join": "27K"
},
"used_columns": [
"emp_no",
"first_name",
"last_name"
]
}
}
}
}
I think it use IDX_LAST_NAME index for where filer and order by, but also need to retrieve full table rows.
But why the second SQL use index condition pushdown? Considering performance, does the
second SQL only take more cost for retrieving the full rows than the first SQL?

Answer 1

These must be InnoDB tables. Indexes in that storage engine always have the PK appended to them, so your first query can be covered by the index on last_name.
Your second query probably uses your index on last_name for two things: finding the right rows and handling the ORDER BY last_name ... LIMIT 100 operation without sorting the entire table. But it still must hit the clustered index implementing the primary key to retrieve the first_name.
To see more details use EXPLAIN ANALYZE FORMAT=TREE on MySQL or ANALYZE FORMAT=JSON on MariaDB to retrieve your execution plan.
To know whether your query is unacceptably slow, you need to test it. If your second query is unacceptably slow you can create an index to cover it.
CREATE INDEX l_e_f ON employees (last_name, emp_id, first_name);