[TiDB docs, vector-search-index]: the two classic causes: ordering by the wrong distance function (the index is built on one function, for example cosine, and the query orders by L2), and using DESC order when the index only supports ascending. EXPLAIN then SHOW WARNINGS surfaces ANN index not used: not ordering by COSINE distance, or index can be used only when ordering by vec_cosine_distance() in ASC order. Match the ORDER BY function and direction to the index definition. Vector search needs TiDB 8.4.0+, 8.5.0+ recommended.

Context: A vector query runs as a full scan instead of using the vector index. EXPLAIN shows ANN index not used warnings.