Primary vs Secondary Index:

Primary Index: Ordered file on ordering key field (primary key or ordering field), sparse or dense, one per file, determines physical order (clustering). Example: File ordered by rollno, primary index on rollno. Has index entry per block. Cannot have many.

Secondary Index: On non-ordering field (any attribute), dense, may be many secondary indexes per file, does not determine file order, needs extra level (bucket for duplicates). Example: Index on name or department where file not ordered by that. Secondary is non-clustering, requires more entries.

Differences: Primary is on ordered key, reflects block anchors, sparse, clustering; Secondary is on non-ordered, dense, non-clustering, allows duplicates via buckets. Primary has unique entries per block, secondary may have many entries per value.