Re: BRIN is missing in multicolumn indexes documentation

> Good catch!
>
> In addtion to your patch, we should add the description for
> the multicolumn brin index, like the following one in the doc?
>
> ---------------
> A multicolumn GIN index can be used with query conditions that
> involve any subset of the index's columns. Unlike B-tree or GiST,
> index search effectiveness is the same regardless of which index
> column(s) the query conditions use.
> ---------------

I guess multicolumn BRIN behaves similarly to B-tree or GiST. But I'm
no expert, so I need someone knowledgeable to confirm this. If the
following wording is OK, I will update the patch.

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A multicolumn BRIN index can be used with query conditions that
involve any subset of the index's columns. Conditions on additional
columns restrict the entries returned by the index, but the condition
on the first column is the most important one for determining how much
of the index needs to be scanned.
-----

Regards,

PJ