SELECT 
  id, 
  placeholder, 
  type, 
  no_pagination 
FROM 
  cscart_image_gallery_settings 
WHERE 
  placeholder <> ''

Query time 0.00023

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_image_gallery_settings",
      "access_type": "ALL",
      "rows": 310,
      "filtered": 100,
      "attached_condition": "cscart_image_gallery_settings.placeholder <> ''"
    }
  }
}