Magneto - Could pull them together, gather the pieces, but not reassemble.
Thor - Could maybe contain the pieces, but not reassemble it.
Green Lantern (Hal Jordan) -
Silver Surfer - Matter manipulation
Dr Strange - Could maybe do a localized time reversal?
Green Lantern (Alan Scott) (Pre Flashpoint)
Thanos - Matter manipulation
None of them can physically re-assemble it and I'm assuming he put all the pieces back in the right places and it's mass was the same.It probably has the same topography.
Now imagine the pieces are spread all over your city.
You don't actually know how many pieces are there.
You don't have a guide to putting it back together, just a fuzzy picture/memory. There might be a good picture somewhere of sufficient detail, but you'll have to find it first.
It's impressive as speed, coordination, and focus feats, but it doesn't make any goddamned sense. What held the moon together after he gathered all of the pieces and particles and fit them all together? Was that ever explained?
Carver,Rage, Bran and I combined our wills. Even abhi chipped in. With momentum like that, it's hard to stand against. Like one of those fatbergs which clog sewers up.
The same thing that holds every large astrological body together. Gravity.
All Superman would have to do is bring all the pieces back together and gravity would hold it together. He'd have to do it quickly enough for it to be near-simultaneous which is what makes the feat so crazy though, I believe.
Have you read it yet? I am only asking because you said you were going to read it (and I'm not being a dick about it).
But to answer the question, to the best of my knowledge, no, it's not a dream, he's on his own, he stops to fight some creatures which drain solar energy right after that scene.