SELECT 
  gallery_id, 
  placeholder 
FROM 
  cscart_cp_image_galleries 
WHERE 
  status = 'A' 
  AND dependent != 'Y' 
  AND placeholder != ""

Query time 0.00007

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_cp_image_galleries",
          "access_type": "ALL",
          "rows": 12,
          "filtered": 100,
          "attached_condition": "cscart_cp_image_galleries.`status` = 'A' and cscart_cp_image_galleries.dependent <> 'Y' and cscart_cp_image_galleries.placeholder <> ''"
        }
      }
    ]
  }
}