有关SQLite的substr函数的笔记

官方参考文档：SQLite Query Language: Core Functions http://www.sqlite.org/lang_corefunc.html

测试SQL语句：

SELECT substr('2012-12-12 10:11:12 fff',0,20) AS substr_0_20,
substr('2012-12-12 10:11:12 fff',0,19) AS substr_0_19,
substr('2012-12-12 10:11:12 fff',1,19) AS substr_1_19,
substr('2012-12-12 10:11:12 fff',19) AS substr_19

WINDOW上的Navicat、Mac上的MesaSQLite、Android上SQLite上测试语句结果：substr_0_20 和 substr_1_19结果保持一致；substr_0_19不知道为啥会少一位？

substr_0_20	substr_0_19	substr_1_19	substr_19
2012-12-12 10:11:12	2012-12-12 10:11:1	2012-12-12 10:11:12	2 fff

IOS的FMDB上测试结果：substr_0_20不知道为啥会多一位？

substr_0_20	substr_0_19	substr_1_19	substr_19
2012-12-12 10:11:12	2012-12-12 10:11:12	2012-12-12 10:11:12	2 fff

到底哪个是正常的呢？