MySQL 8.0.15 Dam slow select query - mysql

Question

Motto of the query is very simple, to find out the last entry on a foreign key column.
the pseudo code I can say is
select vehicleid , last_journey_point , last_journey_time from journeyTable.
here is my SQL statement
-- loconumber is a indexed column
-- journeyserla is a autonumber primary key int(11)
-- the table locojourney contains 400,000 records
-- the below block of code executes in 19 secs
with LocomotiveLastRun AS(
-- this block of code runs in 0.016 sec
SELECT locojourney.loconumber , MAX(locojourney.journeyserla) as lastrunid
FROM locojourney GROUP BY loconumber)
SELECT locojourney.CurrentCombiners , locojourney.JourneySerla ,
locojourney.From_RunPoint , locojourney.NEXT_RunPoint
FROM LocomotiveLastRun FORCE INDEX(lastrunid)
JOIN locojourney FORCE INDEX(PRIMARY) ON x.lastrunid = locojourney.journeyserla
WHERE locojourney.ishoc = 'n'
the EXPLAIN command shows a derived table which is using no index and using where and type ALL
This is the table definition:
-- SHOW CREATE TABLE locojourney
CREATE TABLE `locojourney` (
`trainID` smallint(5) NOT NULL,
`LocoNumber` varchar(5) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
`CurrentLocoBase` varchar(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT NULL,
`CurrentDuedate` date DEFAULT NULL,
`LocoConsist` varchar(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
`CurrentLocoDomain` varchar(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT NULL,
`DomainChange` varchar(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
`FEDR` enum('N','Y') CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT 'N',
`LADR` enum('N','Y') CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT 'N',
`ISBANKER` enum('N','Y') CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT 'N',
`TrainName` varchar(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
`WithOutLoad` enum('N','Y') CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT 'N',
`runRoute` varchar(50) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
`From_RunPoint` varchar(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
`From_RunTime` datetime NOT NULL,
`NEXT_RunPoint` varchar(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
`NEXT_RunTime` datetime NOT NULL,
`Affects_Outage` enum('N','Y') CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT 'N',
`Affects_Mileage` enum('N','Y') CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT 'N',
`GroundDistance` double(5,2) DEFAULT '0.00',
`SHGallowance` int(11) DEFAULT '0',
`Outage` double(5,4) DEFAULT '0.0000',
`UnderServiceType` enum('FHT','CHG','DEP','MIX','DETN') CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT 'FHT',
`SubServiceHead` varchar(25) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT 'RUN',
`IShoc` enum('N','Y') CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT 'N',
`CurrentCombiners` varchar(28) CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT NULL,
`RunSetSerla` varchar(25) CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT NULL,
`JourneySerla` int(11) NOT NULL AUTO_INCREMENT,
`NominationSerla` varchar(50) CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT NULL,
`Traction` enum('DSL','AC') CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT 'DSL',
`Trainload` smallint(4) NOT NULL DEFAULT '0',
`LeadAssist` enum('Y','N') CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT 'N',
`DEO` varchar(100) CHARACTER SET latin1 COLLATE latin1_swedish_ci DEFAULT NULL,
`DEOtime` timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`JourneySerla`),
KEY `trainID` (`trainID`) USING BTREE,
KEY `routesection_idx` (`runRoute`) USING BTREE,
KEY `loconumber_idx` (`LocoNumber`) USING BTREE,
KEY `runsetserla_idx` (`RunSetSerla`) USING BTREE,
KEY `subservicehead_idx` (`SubServiceHead`) USING BTREE,
CONSTRAINT `locojourney_ibfk_1` FOREIGN KEY (`SubServiceHead`) REFERENCES `ineffective` (`IneffectiveHead`) ON UPDATE CASCADE,
CONSTRAINT `locojourney_ibfk_3` FOREIGN KEY (`runRoute`) REFERENCES `routesections` (`Sectionname`) ON DELETE RESTRICT ON UPDATE CASCADE,
CONSTRAINT `loconumber_fk` FOREIGN KEY (`LocoNumber`) REFERENCES `lococontainer` (`LocoNumber`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=345719 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

Answer 1

with LocomotiveLastRun AS(
-- this block of code runs in 0.016 sec
SELECT locojourney.loconumber , MAX(locojourney.journeyserla) as lastrunid
FROM locojourney
GROUP BY loconumber)
Why is this CTE subquery fast? Because your table already has an index on (loconumber, journeyserla). (InnoDb automatically appends the primary key to every index.) This query can be satisfied with a loose index scan on that index, and those are fast.
Now for your main query:
Get rid of FORCE INDEX(). Don't even dream of using that unless you have at least a decade of SQL experience or you have read the source code for the InnoDB indexing stuff in MySQL. Notably, it's completely useless on the CTE because CTEs don't have indexes.
For clarity put your main (detail) table first and your CTE second.
For clarity recast the JOIN as a WHERE...IN...
Those three suggestions give us this:
WITH LocomotiveLastRun AS (...)
SELECT locojourney.CurrentCombiners , locojourney.JourneySerla ,
locojourney.From_RunPoint , locojourney.NEXT_RunPoint
FROM locojourney
WHERE journeyserla IN (SELECT lastrunid FROM LocomotiveLastRun)
AND locojourney.ishoc = 'n'
Now, it's plain what index can help this query.
An index on (ishoc) will help a bit. (It's actually an index, because InnoDB, on (ishoc, journeyserla) so it helps with both WHERE conditions.) The query planner uses BTREE random access to find the first index row with the ishoc value 'n', then scans the values of the primary key to match them with the IN clause.
Instead of that index, a compound index that covers the query will help even more. Such a covering index helps especially because each row of your table is large, with many columns. That index mentions the columns in the WHERE clause and those you want to select, like this:
(ishoc, journeyserla, CurrentCombiners, From_RunPoint, NEXT_RunPoint)
The query planner can satisfy your query entirely from the index, which saves on disk reading time to satisfy the query. If you use your query a lot, this index is a good idea. But, it does consume disk space and slow down INSERT and UPDATE operations a bit.
Read https://use-the-index-luke.com/

Answer 2

Give this a try:
SELECT lj.CurrentCombiners , lj.JourneySerla , lj.From_RunPoint , lj.NEXT_RunPoint
FROM ( SELECT MAX(journeyserla) as lastrunid
FROM locojourney
GROUP BY loconumber
) AS llr
JOIN locojourney AS lj ON llr.lastrunid = lj.journeyserla
WHERE lj.ishoc = 'n'
(time it and provide EXPLAIN for it)