Re: Restrictions for a specific situation in my DB

On Mon, 11 May 2020 17:33:25 -0500
JORGE MALDONADO <jorgemal1960@gmail.com> wrote:

> Hi,

Hi,

> I have a table with the following structure:
> 
> ------------------------------------------------------------------------------
> 
> FIELD       TYPE                         COMMENTS
> ------------------------------------------------------------------------------
>     id            serial                 primary key
>     m1          integer              id of record in another table
>     m2          integer              id of record in another table
> 
> So, "m1" and "m2" are foreign keys.
> I have read that it is a good practice to define an index for each
> foreign key so "m1" and "m2" are also regular and independent indexes.
> 
> Now, the combination of "m1" and "m2" together cannot be duplicated.
> So for example, the following entries
> 
> ---------------------------------------------------------------------
> id             m1            m2           COMMENTS
> ---------------------------------------------------------------------
>  1              2               10
>  2              8                3
>  3             18               1
>  4              2               10           This is invalid.
>  5             13               8            This is invalid.
>  6             18              18           This is invalid.
> 
> Maybe the case for records 4 and 5 can be achieved by setting 2
> restrictions:
> Restriction 1: "m1 + m2" fields
> Restriction 2: "m2 + m1" fields
> 
> But, is this a good approach?

No, you don't want to do that.

Imagine #2 is 8/4, 8+4=12 == 10+2=12…

The only possibility is to use 2 unique indexes with your 2 FK :
	m1, m2
	m2, m1
this way, as you can't have a doublon either ways, you'll fulfill your
double condition (if I understand it correctly).

Jean-Yves