Mysql – Does the order of keys in compound Primary key make any difference

MySQLprimary-key

Package

Package
-------
PackageID
PackageName
CityID
PRIMARY KEY (PackageID)
UNIQUE KEY (PackageName)
FOREIGN KEY (CityID)
  REFERENCES City (CityID)

Package versions:

PackageVersion
--------------
PackageID
VersionNumber
PackageType
PRIMARY KEY (PackageID, VersionNumber)
UNIQUE KEY (PackageID, VersionNumber, PackageType)
FOREIGN KEY (PackageID)
  REFERENCES Package (PackageID)
and subtypes (of versions):

LandPackageVersion
------------------
PackageID
VersionNumber
PackageType  DEFAULT 'L'
PRIMARY KEY (PackageID, VersionNumber, PackageType)
FOREIGN KEY (PackageID, VersionNumber, PackageType)
  REFERENCES PackageVersion (PackageID, VersionNumber, PackageType)

FlightPackageVersion
------------------
PackageID
VersionNumber
PackageType  DEFAULT 'F'
OriginID
PRIMARY KEY (PackageID, VersionNumber, PackageType)
FOREIGN KEY (PackageID, VersionNumber, PackageType)
  REFERENCES PackageVersion (PackageID, VersionNumber, PackageType)
FOREIGN KEY (OriginID)
  REFERENCES City (CityID)

When I define PackageVersion (read PackageVariant) table, I mark VersionNumber (aka idPackageVariant) as Auto Increment and a part of Primary Key in the order defined above:

PRIMARY KEY (idHolidayPackage, idPackageVariant)

CREATE  TABLE IF NOT EXISTS `wah_schema`.`HolidayPackageVariant` (
  `idHolidayPackage` INT UNSIGNED NOT NULL ,
  `idPackageVariant` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
  `packageType` VARCHAR(45) NOT NULL ,

  **PRIMARY KEY (`idHolidayPackage`, `idPackageVariant`) ,**

  UNIQUE INDEX `idHolidayPackage_UNIQUE` (`idHolidayPackage` ASC, `idPackageVariant` ASC, `packageType` ASC) ,

  INDEX `fk_HolidayPackageVariant_HolidayPackage` (`idHolidayPackage` ASC) ,

  CONSTRAINT `fk_HolidayPackageVariant_HolidayPackage`
    FOREIGN KEY (`idHolidayPackage` )
    REFERENCES `wah_schema`.`HolidayPackage` (`idHolidayPackage` )
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;

However, due to this bug in mySql, I need to have AutoIncrement Key as the first part of the Primary Key in a compound key.

So, does the order of keys in compound Primary key make any difference ?

Just for reference, the above design comes from here.

Best Answer

The order of keys can make a difference depending on your access pattern.

Example...

You want to find MAX(idPackageVariant) for a singleidHolidayPackage

With idHolidayPackage as the second column, then the index will be scanned. With idHolidayPackage as the first, it can be seeked which is more efficient.

This is a specifiy MySQL problem of course. IIRCl you'll need to do this. InnoDB uses the first unique key as the clustered index which may or may not be the primary key. However, this may not be the case

CREATE  TABLE IF NOT EXISTS `wah_schema`.`HolidayPackageVariant` (
  `idHolidayPackage` INT UNSIGNED NOT NULL ,
  `idPackageVariant` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
  `packageType` VARCHAR(45) NOT NULL ,

  UNIQUE INDEX `idHolidayPackage_UNIQUE` (`idHolidayPackage` ASC, `idPackageVariant` ASC, `packageType` ASC) ,

  PRIMARY KEY (`idHolidayPackage`, `idPackageVariant`) ,

  -- not needed INDEX `fk_HolidayPackageVariant_HolidayPackage` (`idHolidayPackage` ASC) ,

  CONSTRAINT `fk_HolidayPackageVariant_HolidayPackage`
    FOREIGN KEY (`idHolidayPackage` )
    REFERENCES `wah_schema`.`HolidayPackage` (`idHolidayPackage` )
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;

Alternatively, you can just have idPackageVariant as the primary and clustered key for efficiency.

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Mysql – Finding rows for a specified date range

Another way to get the result is this. It finds first all groups that the teacher has surely taught (or is going to) by checking that she has started within the month and then in another subquery it finds - for every group - the last teacher that started at the first day of the month or earlier.

With the unique index you have on the table, the second subquery should be quite efficient. The first subquery would benefit from an index on (teacherid, startdate, groupid):

SELECT groupid
FROM pupilgroupteacher
WHERE teacherid = @teacher 
  AND startdate >= @month + INTERVAL 1 DAY
  AND startdate < @month + INTERVAL 1 MONTH

UNION DISTINCT

SELECT gg.groupid
FROM 
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      FROM pupilgroupteacher
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The activity of altering big tables are done in phases:

Create a new table with required fields and indexes say in test DB (just structure)
Dump the data from the existing table and load the same to the newly created table in test DB
Now announce your downtime :)
Swap the tables by renaming - RENAME table ur_db.table_name to test.temp, test.table_name to ur_db.table_name, test.temp to test.table_name; This is an atomic operation taking fraction of a second.
Load the extra records to the newly created table ( the records that came after dumping and then loading). This step can be done before Step: 3 also once to reduce your downtime.
And your system is back

Few notes:

You need not hit information schema directly like this, try using SHOW TABLE STATUS from db like 'table_name'
The speed of alter table is more or less linked with the I/O speed. Last time when we ran a direct alter table(without the above steps), we had 40GB+ table size it took around 4hours. If your 20GB data is taking years, you are working on some outdated machine.
Also drop of the unwanted indexes like index_bslt_ondate, index_base_schedule_line_items_loan_base_schedule as some other indexes has the left most column as the indexed column

Please let me know if you need any clarification on any of these steps.

Edit : A simple python script to automate the process https://github.com/georgecj11/hotswap_mysql_table.git

Best Answer

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