Why does MySQL not always use index merge here?

Question

Consider this table:

CREATE TABLE `Alarms` (
  `AlarmId` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `DeviceId` BINARY(16) NOT NULL,
  `Code` BIGINT(20) UNSIGNED NOT NULL,
  `Ended` TINYINT(1) NOT NULL DEFAULT '0',
  `NaturalEnd` TINYINT(1) NOT NULL DEFAULT '0',
  `Pinned` TINYINT(1) NOT NULL DEFAULT '0',
  `Acknowledged` TINYINT(1) NOT NULL DEFAULT '0',
  `StartedAt` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00',
  `EndedAt` TIMESTAMP NULL DEFAULT NULL,
  `MarkedForDeletion` TINYINT(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`AlarmId`),
  KEY `Key1` (`Ended`,`Acknowledged`),
  KEY `Key2` (`Pinned`),
  KEY `Key3` (`DeviceId`,`Pinned`),
  KEY `Key4` (`DeviceId`,`StartedAt`,`EndedAt`),
  KEY `Key5` (`DeviceId`,`Ended`,`EndedAt`),
  KEY `Key6` (`MarkedForDeletion`)
) ENGINE=INNODB;

And, for this test, populate it like so:

-- Populate some dummy data; 500 alarms for each
-- of 1000 one-second periods
SET @testDevice = UNHEX('00030000000000000000000000000000');

DROP PROCEDURE IF EXISTS `injectAlarms`;
DELIMITER ;;
CREATE PROCEDURE injectAlarms()
BEGIN
    SET @fromdate  = '2018-02-18 00:00:00';
    SET @numdates  = 1000;
    SET @todate    = DATE_ADD(@fromdate, INTERVAL @numdates SECOND);

    -- Create table of alarm codes to join on
    DROP TABLE IF EXISTS `__codes`;
    CREATE TEMPORARY TABLE `__codes` (
        `Code` BIGINT NOT NULL PRIMARY KEY
    );

    SET @startcode = 0;
    SET @endcode   = 499;

    REPEAT
       INSERT INTO `__codes` VALUES(@startcode);
       SET @startcode = @startcode + 1;
    UNTIL @startcode > @endcode END REPEAT;

    -- Add an alarm for each code, for each second in range
    REPEAT
        INSERT INTO `Alarms`
            (`DeviceId`, `Code`, `Ended`, `NaturalEnd`, `Pinned`, `Acknowledged`, `StartedAt`, `EndedAt`)
            SELECT
                @testDevice,
                `Code`,
                TRUE, FALSE, FALSE, FALSE,
                @fromdate, @fromdate
            FROM `__codes`;

        SET @fromdate = DATE_ADD(@fromdate, INTERVAL 1 SECOND);
    UNTIL @fromdate > @todate END REPEAT;
END;;
DELIMITER ;

CALL injectAlarms();

Now, for some datasets the following query works quite well:

SELECT * FROM `Alarms`
WHERE
   ((`Alarms`.`Ended` = FALSE AND `Alarms`.`Acknowledged` = FALSE) OR `Alarms`.`Pinned` = TRUE) AND
   `MarkedForDeletion` = FALSE AND
   `DeviceId` = @testDevice
;

This is because MariaDB is clever enough to use index merges, e.g.:

id    select_type    table    type         possible_keys                 
1     SIMPLE         Alarms   index_merge  Key1,Key2,Key3,Key4,Key5,Key6 

key             key_len  ref     rows     Extra
Key1,Key2,Key3  2,1,17   (NULL)  2        Using union(Key1,intersect(Key2,Key3)); Using where

However if I use the dataset as populated by the procedure above, and flip the query around a bit (which is another view I need, but in this case will return many more rows):

SELECT * FROM `Alarms`
WHERE
  ((`Alarms`.`Ended` = TRUE OR `Alarms`.`Acknowledged` = TRUE) AND `Alarms`.`Pinned` = FALSE) AND
   `MarkedForDeletion` = FALSE AND
   `DeviceId` = @testDevice
;

… it doesn't:

id    select_type    table    type   possible_keys
1     SIMPLE         Alarms   ref    Key1,Key2,Key3,Key4,Key5,Key6

key   key_len  ref     rows     Extra
Key2  1        const  144706    Using where

I would rather like the index merges to happen more often. As it is, given the ref=const, this query plan doesn't look too scary … however, the query takes almost a second to run. That in itself isn't the end of the world, but the poorly-scaling nature of my design shows when trying a more exotic query, which takes a very long time:

-- Create a temporary table that we'll join against in a mo
DROP TABLE IF EXISTS `_ranges`;
CREATE TEMPORARY TABLE `_ranges` (
    `Start` TIMESTAMP NOT NULL DEFAULT 0,
    `End`   TIMESTAMP NOT NULL DEFAULT 0,
    PRIMARY KEY(`Start`, `End`)
);

-- Populate it (in reality this is performed by my application layer)
SET @endtime = 1518992216;
SET @starttime = @endtime - 86400;
SET @inter = 900;
DROP PROCEDURE IF EXISTS `populateRanges`;
DELIMITER ;;
CREATE PROCEDURE populateRanges()
BEGIN
REPEAT
    INSERT IGNORE INTO `_ranges` VALUES(FROM_UNIXTIME(@starttime),FROM_UNIXTIME(@starttime + @inter));
    SET @starttime = @starttime + @inter;
UNTIL @starttime > @endtime END REPEAT;
END;;
DELIMITER ;
CALL populateRanges();

-- Actual query
SELECT UNIX_TIMESTAMP(`_ranges`.`Start`) AS `Start_TS`,
COUNT(`Alarms`.`AlarmId`) AS `n`
FROM `_ranges`
LEFT JOIN `Alarms`
ON `Alarms`.`StartedAt` < `_ranges`.`End`
  AND (`Alarms`.`EndedAt` IS NULL OR `Alarms`.`EndedAt` >= `_ranges`.`Start`)

  AND ((`Alarms`.`EndedAt` IS NULL AND `Alarms`.`Acknowledged` = FALSE) OR `Alarms`.`Pinned` = TRUE)
-- Again, the above condition is sometimes replaced by:
-- AND ((`Alarms`.`EndedAt` IS NOT NULL OR `Alarms`.`Acknowledged` = TRUE) AND `Alarms`.`Pinned` = FALSE)

 AND `DeviceId` = @testDevice
 AND `MarkedForDeletion` = FALSE
 GROUP BY `_ranges`.`Start`

(This query is supposed to gather a list of counts per time slice, each count indicating how many alarms' [StartedAt,EndedAt] range intersects that time slice. The result populates a line graph.)

Again, when I designed these tables and there weren't many rows in them, index merges seemed to make everything whiz along. But now not so: with the dataset as given in injectAlarms(), this takes 40 seconds to complete!

I noticed this when adding the MarkedForDeletion column and performing some of my first large-dataset scale tests. This is why my choice of indexes doesn't make a big deal out of the presence of MarkedForDeletion, though the results described above are the same if I remove AND MarkedForDeletion = FALSE from my queries; however, I've kept the condition in, as ultimately I will need it to be there.

I've tried a few USE INDEX/FORCE INDEX combinations, but it never seems to use index merge as a result.

What indexes can I define to make this table behave quickly in the given cases? Or how can I restructure my queries to achieve the same goal?

(Above query plans obtained on MariaDB 5.5.56/CentOS 7, but solution must also work on MySQL 5.1.73/CentOS 6.)

Why does MySQL not always use index merge here?

Answers (1)

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