If segments were written in the default-value mode, but cluster is now in sql compatibility mode (new defaults in 28.0), would the query now contains null values? Basically, would the default-value mode written segments actually contains null vs empty string/0 values?

I believe the query now should see data in both modes ... prior ingestion data will have ''/0 in those fields, and newly ingested data will have NULLs in those fields. Your queries should see exactly what's in there ...

i mean if data was ingested in default-value mode and now the cluster is switched to sql compatibility mode, the query to the existing segments will still show ‘’/0 right?

I just tried this out ... • turns out that in legacy mode empty string and null string are synonymous and are stored in a form that is recognized as NULL in SQL NULL mode, so if you ingest NULL strings in legacy mode, then when your upgrade to SQL NULL mode they will still show up as NULL strings. • However numbers are different ... NULL numeric values ingested in legacy mode are stored as zeroes, and in SQL NULL mode they show up as zeroes, not NULLs.