Eight types matter in 2026: relational, document, key-value, wide-column, search, graph, time-series and vector.
DB-Engines lists 170 relational systems, 72 key-value, 58 document, 46 time-series, 44 graph, 27 vector and 13 wide-column (October 2026).
Oracle leads relational at 1,119.8, MongoDB leads document at 374.6, and Redis leads key-value at 156.1.
Types overlap: PostgreSQL lists relational, document, graph, spatial and vector models.
Start relational, and add a specialised store only when one access pattern dominates.