No database modality is “agent memory” by itself. Each preserves a different dimension of experience, while useful memory depends on the surrounding policy: what gets stored, consolidated, versioned, forgotten, retrieved, and trusted.
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Vector stores are strong at fuzzy semantic recall and candidate generation. They are weak at exact identity, relationships, causality, chronology, and staleness. Good implementations add metadata filters, entity signals, and reranking.
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Graph databases are strong at relationships, provenance, multi-hop reasoning, contradictions, and causal paths. Their costs are entity extraction, ontology maintenance, traversal complexity, and heavier operations.