① a tree, no traversal→② score-based, both ways→③ LLM-guided instead→④ traversal learned→⑤ multimodal
| Method | Who writes the hierarchy | Who traverses it | LLM calls / query | Table · figure evidence |
|---|
| RAPTOR | machine · pyramid of cluster summaries | no traversal · flat similarity over all levels | 0 | ✗ |
| TreeRAG | the document's own hierarchy | similarity · bidirectional (up + down) | 0 | ✗ |
| MemWalker | machine · segment summaries | LLM reasoning + backtracking | per step | ✗ |
| PageIndex | the document's own hierarchy | LLM reasoning (no vectors) | iterative | ✗ |
| RDR2 | the document's own hierarchy | a trained LLM navigator | calls | ✗ |
| HiKEY | the document's own hierarchy (auto-parsed) | scores, two stages · pick docs → pick sections | 0 (reader only) | ✓ first-class |
- One blank remains · documents whose key evidence sits in tables and figures → Part 3