SELECT 
  `sg_vector` 
FROM 
  cscart_sg_product_embeddings 
WHERE 
  product_id = 399 
  AND lang_code = 'en'

Query time 0.00021

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_sg_product_embeddings",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "12",
          "used_key_parts": ["product_id", "lang_code"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}