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SELECT 
  cscart_power_labels.label_id, 
  cscart_power_labels_products.type 
FROM 
  cscart_power_labels 
  LEFT JOIN cscart_power_labels_products ON cscart_power_labels_products.label_id = cscart_power_labels.label_id 
  AND cscart_power_labels_products.product_id = 91902 
WHERE 
  cscart_power_labels_products.type != ''

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "Impossible WHERE noticed after reading const tables"
  }
}