[GEOT-7248] fix query for multiple columns

[NielsCharlier]

test: geoserver/geoserver#6400

Checklist

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[taba90]

Looks good to me

[roelarents]

Coincidentally (or not), this increases count performance by putting that distinct in a subquery. I was looking into this today (coincidentally again) because we were updating geoserver until we hit a performance issue on a large postgres table (24M) while using the schema mapping extension.

So thank you! I hope this finds its way to geoserver soon.

See e.g. this article about postgres counts by Joe Nelson that mentions postgres only using an index for distinct select, not count.

[NielsCharlier]

I did not know that, but great to hear

[aaime]

Since it's a fix, adding backport labels, hopefully the bot will do the rest.