Half the catalogue has attributes the other half does not, and merchants add new ones weekly. How would you store them?

Short answer

Decide by which attributes are filtered or constrained, not by how many exist. Anything a query filters, sorts or validates on becomes a real typed column; the open-ended remainder goes in a JSONB document with a GIN index, and entity-attribute-value is the fallback that costs you every guarantee.

Keep data modelling explicit in the answer because that is the concept the interviewer is actually trying to test. A good data modelling explanation names the trade-off, the failure mode, and the evidence you would use before choosing. Use data modelling once more at the decision point so the answer reads as judgement rather than a detached example.

Keep data modelling explicit in the answer because that is the concept the interviewer is actually trying to test. A good data modelling explanation names the trade-off, the failure mode, and the evidence you would use before choosing.