3
  • MariaDB Version 10.5.15
  • simple Table, two columns with an index

If I make a query on a table and use two columns in the where conndition that have an index, no index is used. Why?

CREATE TABLE `logs` (
  `log_id` int(10) NOT NULL AUTO_INCREMENT,
  `date` datetime DEFAULT NULL,
  `status` int(10) DEFAULT 0,
  `text` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`log_id`),
  KEY `status` (`status`),
  KEY `date` (`date`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Where date

analyze select count(*)
from logs 
where `date` < "2022-01-01 00:00:00"

uses index => 1.4s

id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra
1 SIMPLE logs range date date 6 5416235 7969250.00 100 100 Using where; Using index

Where status

analyze select count(*)
from logs 
where status = 860

uses index => ~0.9s

id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra
1 SIMPLE logs ref status status 5 const 5416235 5536942.00 100 100 Using index

Where date AND status

analyze select count(*)
from logs 
where status = 860
and `date` < "2022-01-01 00:00:00"

uses no index => ~7s

id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra
1 SIMPLE logs ALL status,date 10832494 11430473.00 25 43.85 Using where

1 Answer 1

4
ALTER TABLE logs
    DROP INDEX(status)
    ADD INDEX(status, date)

The added "composite" index will help both

WHERE status = ...

and

WHERE status = ... AND date < ...

and also

WHERE date < ... AND status = ...
1

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