Support indexes on columnstore tables

[paurora17]

You can't create columnstore tables with the following constraints:
UNIQUE PRIMARY KEY FOREIGN KEY

[pantonis]

It would be really really really good to have this. One of the 2 things that keep us from using pg_mooncake

[ritwizsinha]

Hey @dpxcc is someone looking at this ? Can I start with this one?

[dpxcc]

Hey @ritwizsinha, we still need to finalize the design for this one. Our initial idea is to use a 2-layer index approach: build index for each data file, and then build speedy lookup layer on top across multiple data files, similar to what we did back in SingleStore
[1] https://dl.acm.org/doi/pdf/10.1145/3514221.3526055
[2] https://cs.purdue.edu/homes/csjgwang/pubs/VLDB24_SingleStoreVec.pdf