Question

I am creating a database model with Workbench and create the following table:

CREATE  TABLE IF NOT EXISTS `Database`.`table1` (
  `idtable1` INT NOT NULL ,
  `uniquecolumn` INT NOT NULL ,
  PRIMARY KEY (`idtable1`) ,
  UNIQUE INDEX `UniqueIndex` (`uniquecolumn` ASC) )
ENGINE = InnoDB

It has a primary key, and a unique key on my second column.

When I create foreign key constraints on them, Workbench automatically adds two indexes:

CREATE  TABLE IF NOT EXISTS `Database`.`table1` (
  `idtable1` INT NOT NULL ,
  `uniquecolumn` INT NOT NULL ,
  PRIMARY KEY (`idtable1`) ,
  UNIQUE INDEX `UniqueIndex` (`uniquecolumn` ASC) ,
  INDEX `FKOne` (`idtable1` ASC) ,                   //here
  INDEX `FKTwo` (`uniquecolumn` ASC) ,               //(I don't want this!)
  CONSTRAINT `FKOne`
    FOREIGN KEY (`idtable1` )
    REFERENCES `Database`.`table2` (`idtable2` )
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  CONSTRAINT `FKTwo`
    FOREIGN KEY (`uniquecolumn` )
    REFERENCES `Database`.`table2` (`idtable2` )
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB

(The above is the forward-engineered script after adding the foreign keys to my model)

I have four indexes now.

This is what the MySQL Reference Manual says:

In the referencing table, there must be an index where the foreign key
columns are listed as the first columns in the same order. Such an
index is created on the referencing table automatically if it does not
exist.

So I understand there is no need to create indexes FKOne and FKTwo, since there are already a Primary Key and a Unique index, on the same columns, in the same order. Yet MySQL Workbench doesn’t allow me to delete indexes FKOne and FKTwo. And I think I should be able to do this:

CREATE  TABLE IF NOT EXISTS `Database`.`table1` (
  `idtable1` INT NOT NULL ,
  `uniquecolumn` INT NOT NULL ,
  PRIMARY KEY (`idtable1`) ,
  UNIQUE INDEX `UniqueIndex` (`uniquecolumn` ASC) ,
  CONSTRAINT `FKOne`
    FOREIGN KEY (`idtable1` )
    REFERENCES `Database`.`table2` (`idtable2` )
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  CONSTRAINT `FKTwo`
    FOREIGN KEY (`uniquecolumn` )
    REFERENCES `Database`.`table2` (`idtable2` )
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB

Am I right? Would this code work? Is there some way to do it with Workbench? (Apart from deleting those two lines at the last moment before forward-engineering).

Or maybe MySQL is smart enough to refrain from creating totally redundant indexes and I don’t have to worry about it…?

Answer 1

(I’m assuming this is when defining a model.)

See Bug 53277, where I mention the following obscure workaround:

You start with a foreign key and its corresponding generated index that you want to get rid of. Make sure the key is (at least temporarily) on a single non-unique column. In the Indexes tab, change the Type to UNIQUE. Then go to the Columns tab, where UQ is now checked, and uncheck it. The unwanted index is eliminated!