Re: all foreign key should have index?

>> No. But better SQL products construct a "PK=FK" index under the
>> covers for you.
>
> Do you have a list of those?
>
> Is it configurable? I'm not sure I want my rdbms generating a 5GB index on
> my foreign key when I might not require it.
>
> Backup bloat to start with.

If the index required will be 5GB in size, then you probably can't operate _without_ it because each FK check would require a hideous Seq Scan across a 10GB table to do its check.

If you want a case for *not* needing the index, use the one that's sensible, namely the case where the table referenced is only a few pages in size such that the sequential scan is expected to be faster than an index scan...

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