SELECT 
  link_target 
FROM 
  cscart_image_gallery 
WHERE 
  pair_id = 4539 
  AND lang_code = 'ru'

Query time 0.00055

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_image_gallery",
      "access_type": "const",
      "possible_keys": ["pair_id"],
      "key": "pair_id",
      "key_length": "9",
      "used_key_parts": ["pair_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}