I couldn't find documentation about what subset of SQL you can use. I saw mention of "all supported Postgres data types", but not anything about what features work. Any links?
If you use cstore_fdw with PostgreSQL you can use the full PostgreSQL SQL dialect for querying your table. However, we don't currently support writing to your cstore_fdw table using SQL commands like INSERT/UPDATE/DELETE. PostgreSQL added foreign data wrapper support for these commands with their 9.3 release and it is something we're considering adding to cstore_fdw in the future.