Postgres：Levenshtein错误

Question

当我尝试对postgres数据库运行levenshtein函数时出现错误。

    ERROR: function levenshtein(text, text) does not exist
  Hint: No function matches the given name and argument types. You might need to add explicit type casts.
  Position: 152
ERROR: function levenshtein(text, text) does not exist
  Hint: No function matches the given name and argument types. You might need to add explicit type casts.
  Position: 152

这是使用postgres website中的示例：

SELECT levenshtein('GUMBO', 'GAMBOL');

有什么理由？