本文介绍Oracle生成全局唯一ID的函数SYS_GUID，后续会对SYS_GUID和Sequence两种方法进行比较。
SYS_GUID 函数生成并返回一个由 16 个字节组成的全局唯一标识符（RAW 值）。在大多数平台上，生成的标识符由主机标识符、调用该函数的进程或线程的进程或线程标识符以及该进程或线程的非重复值（字节序列）组成。
create table loc as select * from locations;
ALTER TABLE loc ADD (uid_col RAW(16));
UPDATE loc SET uid_col = SYS_GUID();
SELECT location_id, uid_col FROM loc
ORDER BY location_id, uid_col;
LOCATION_ID UID_COL
----------- --------------------------------
1000 27C94AC98E2B13C5E0634600000A203B
1100 27C94AC98E2C13C5E0634600000A203B
1200 27C94AC98E2D13C5E0634600000A203B
1300 27C94AC98E2E13C5E0634600000A203B
1400 27C94AC98E2F13C5E0634600000A203B
1500 27C94AC98E3013C5E0634600000A203B
1600 27C94AC98E3113C5E0634600000A203B
1700 27C94AC98E3213C5E0634600000A203B
1800 27C94AC98E3313C5E0634600000A203B
1900 27C94AC98E3413C5E0634600000A203B
2000 27C94AC98E3513C5E0634600000A203B
2100 27C94AC98E3613C5E0634600000A203B
2200 27C94AC98E3713C5E0634600000A203B
2300 27C94AC98E3813C5E0634600000A203B
2400 27C94AC98E3913C5E0634600000A203B
2500 27C94AC98E3A13C5E0634600000A203B
2600 27C94AC98E3B13C5E0634600000A203B
2700 27C94AC98E3C13C5E0634600000A203B
2800 27C94AC98E3D13C5E0634600000A203B
2900 27C94AC98E3E13C5E0634600000A203B
3000 27C94AC98E3F13C5E0634600000A203B
3100 27C94AC98E4013C5E0634600000A203B
3200 27C94AC98E4113C5E0634600000A203B
23 rows selected.
注意以上输出中的第12-13字节是有序的，从B1，C1，到40，41。这模拟了sequence的特点，但这不是必然的，例如在不同的节点上，如RAC。
查询时，直接使用uid_col和rawtohex(uid_col)显式转换的结果都是一样的，说明前者数据库进行了隐式转换。
SQL> desc loc
Name Null? Type
----------------------------------------- -------- ----------------------------
LOCATION_ID NUMBER(4)
STREET_ADDRESS VARCHAR2(40)
POSTAL_CODE VARCHAR2(12)
CITY NOT NULL VARCHAR2(30)
STATE_PROVINCE VARCHAR2(25)
COUNTRY_ID CHAR(2)
UID_COL RAW(16)
SQL> select rawtohex(uid_col) from loc;
RAWTOHEX(UID_COL)
--------------------------------
27C94AC98E2B13C5E0634600000A203B
27C94AC98E2C13C5E0634600000A203B
27C94AC98E2D13C5E0634600000A203B
27C94AC98E2E13C5E0634600000A203B
27C94AC98E2F13C5E0634600000A203B
27C94AC98E3013C5E0634600000A203B
27C94AC98E3113C5E0634600000A203B
27C94AC98E3213C5E0634600000A203B
27C94AC98E3313C5E0634600000A203B
27C94AC98E3413C5E0634600000A203B
27C94AC98E3513C5E0634600000A203B
27C94AC98E3613C5E0634600000A203B
27C94AC98E3713C5E0634600000A203B
27C94AC98E3813C5E0634600000A203B
27C94AC98E3913C5E0634600000A203B
27C94AC98E3A13C5E0634600000A203B
27C94AC98E3B13C5E0634600000A203B
27C94AC98E3C13C5E0634600000A203B
27C94AC98E3D13C5E0634600000A203B
27C94AC98E3E13C5E0634600000A203B
27C94AC98E3F13C5E0634600000A203B
27C94AC98E4013C5E0634600000A203B
27C94AC98E4113C5E0634600000A203B
23 rows selected.
SQL> select city from loc where uid_col='27C94AC98E2B13C5E0634600000A203B';
CITY
------------------------------
Roma
SQL> select city from loc where uid_col=hextoraw('27C94AC98E2B13C5E0634600000A203B');
CITY
------------------------------
Roma
SYS_GUID的高并发插入性能比sequence要好，引发的latch更少，也就是scalability更好。但缺点是：
- 占用空间较多
- 由于数据较分散，后续查询时会引发更多的I/O。
所以，选用Sequence还是SYS_GUID，实际上没有明确的答案。需要视具体情况而定。后续有时间再做详细讨论。