Oracle获取LOB长度的两种方法效率对比
前两天写了一篇文章，介绍LENGTH等一系列的常规函数一样可以处理LOB类型，有人在BLOG回复中提到，使用LENGTH的效率要比DBMS_LOB.GETLENGTH低，这里进行简单的测试。
建立一个测试表：
SQL> CREATE TABLE T_LOB (ID NUMBER, CONTENTS CLOB);
Table created.
SQL> DECLARE
2 V_LOB CLOB;
3 BEGIN
4 INSERT INTO T_LOB
5 VALUES (1, EMPTY_CLOB())
6 RETURN CONTENTS INTO V_LOB;
7 FOR I IN 1..100 LOOP
8 DBMS_LOB.WRITEAPPEND(V_LOB, 32767, LPAD('A', 32767, 'A'));
9 END LOOP;
10 END;
11 /
PL/SQL procedure successfully completed.
表中有一条数据，下面在PL/SQL中分别使用LENGTH和DBMS_LOB.GETLENGTH循环获取长度对比二者的效率：
SQL> SET TIMING ON
SQL> DECLARE
2 V_NUM NUMBER;
3 BEGIN
4 FOR I IN 1..10000 LOOP
5 SELECT LENGTH(CONTENTS) INTO V_NUM FROM T_LOB;
6 END LOOP;
7 END;
8 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.38
SQL> DECLARE
2 V_NUM NUMBER;
3 BEGIN
4 FOR I IN 1..10000 LOOP
5 SELECT DBMS_LOB.GETLENGTH(CONTENTS) INTO V_NUM FROM T_LOB;
6 END LOOP;
7 END;
8 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.60
SQL> DECLARE
2 V_NUM NUMBER;
3 BEGIN
4 FOR I IN 1..100000 LOOP
5 SELECT LENGTH(CONTENTS) INTO V_NUM FROM T_LOB;
6 END LOOP;
7 END;
8 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:03.82
SQL> DECLARE
2 V_NUM NUMBER;
3 BEGIN
4 FOR I IN 1..100000 LOOP
5 SELECT DBMS_LOB.GETLENGTH(CONTENTS) INTO V_NUM FROM T_LOB;
6 END LOOP;
7 END;
8 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:06.06
可以看到，使用LENGTH所需的时间不到DBMS_LOB.GETLENGTH的2/3。
这是PL/SQL代码，下面测试一下SQL语句中二者的效率区别：
SQL> INSERT INTO T_LOB
2 SELECT A.*
3 FROM T_LOB A, ALL_OBJECTS B
4 WHERE ROWNUM < 1000;
999 rows created.
Elapsed: 00:01:45.69
SQL> SELECT COUNT(*) FROM T_LOB;
COUNT(*)
----------
1000
Elapsed: 00:00:00.00
SQL> COMMIT;
Commit complete.
Elapsed: 00:00:00.00
由于1000条记录太少，不足以看出二者的需求，因此查询中包含了T_LOB表两次，为了避免大量数据返回客户端带来的影响，设置AUTOTRACE为TRACEONLY STATISTICS：
SQL> SET AUTOT TRACE STAT
SQL> SELECT LENGTH(A.CONTENTS) FROM T_LOB A, T_LOB;
1000000 rows selected.
Elapsed: 00:00:06.03
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
1017 consistent gets
0 physical reads
0 redo size
17200447 bytes sent via SQL*Net to client
733818 bytes received via SQL*Net from client
66668 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1000000 rows processed
SQL> SELECT DBMS_LOB.GETLENGTH(A.CONTENTS) FROM T_LOB A, T_LOB;
1000000 rows selected.
Elapsed: 00:00:15.12
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
1017 consistent gets
0 physical reads
0 redo size
17200459 bytes sent via SQL*Net to client
733818 bytes received via SQL*Net from client
66668 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1000000 rows processed
这次区别更加明显，LENGTH所用时间只有DBMS_LENGTH.GETLENGTH的2/5。显然LENGTH的效应要比DBMS_LENGTH.GETLENGTH更高。
其实这也不难理解，LENGTH的实现是Oracle通过外部C程序实现的。而DBMS_LOB包是通过PL/SQL程序实现的。所以二者效率上有所区别是正常的，而且对于数据量不大的情况，这种区别并不明显。
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