与VARCHAR2相比，将'TinyString'字符串（只是一个示例字符串）插入到行内CLOB时，我遇到了显着的性能下降。我的理解是，当存储＆lt;在启用了STORAGE IN ROW的CLOB中有4000个字节，数据以与VARCHAR2相同的方式有效存储（除非它'溢出'4000bytes）并且应该没有显着的性能下降。但是，我的基准测试程序*表明，将相同的数据插入到CLOB中比插入VARCHAR2慢15倍。
看看下面的代码：
我有很多桌子，每个桌子都附有一个类似下面的复合触发器：
CREATE OR REPLACE TRIGGER mdhl_basic_trigger_compound
FOR INSERT OR UPDATE OR DELETE ON target_table
COMPOUND TRIGGER TYPE EVENTS_HIST IS TABLE OF log_table%ROWTYPE INDEX BY PLS_INTEGER;
coll_events_hist EVENTS_HIST;
ctr PLS_INTEGER := 0;
my_bgroup VARCHAR2(3);
BEFORE EACH ROW IS
BEGIN
IF INSERTING OR UPDATING THEN
my_bgroup := :NEW.BGROUP;
ELSE
my_bgroup := :OLD.BGROUP;
END IF;
ctr := ctr + 1;
coll_events_hist(ctr).BGROUP := my_bgroup;
coll_events_hist(ctr).TABLE_NAME := 'BASIC_MDHL';
coll_events_hist(ctr).EVENT_TS := current_timestamp;
coll_events_hist(ctr).EVENT_RAW := 'TinyString';
END BEFORE EACH ROW;
AFTER STATEMENT IS
BEGIN
FORALL counter IN 1 .. coll_events_hist.count()
INSERT INTO log_table VALUES coll_events_hist(counter);
END AFTER STATEMENT;
END mdhl_basic_trigger_compound;
在target_table上执行任何操作后，上述触发器会将coll_events_hist类型中填充的数据存储到log_table，其定义方式如下：
CREATE TABLE "USERNAME"."LOG_TABLE"
( "BGROUP" VARCHAR2(3) NOT NULL ENABLE,
"TABLE_NAME" VARCHAR2(255) NOT NULL ENABLE,
"EVENT_TS" TIMESTAMP (7) DEFAULT current_timestamp,
"EVENT_RAW" CLOB
)
SEGMENT CREATION IMMEDIATE
PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "USERS"
LOB ("EVENT_RAW") STORE AS BASICFILE "EV_RAW_SEG"(
TABLESPACE "USERS" ENABLE STORAGE IN ROW CHUNK 16384 PCTVERSION 5
CACHE
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT))
我的设置是： Windows 7 SP1， Oracle 11g
*我的benchamrking程序迭代10次，在每次迭代中更新target_table上的21k行。
答案 0 :(得分：3)
在你的情况下是“tinystring”总是＆lt; 32767？
你的时间将被浪费在FORALL部分，查找你所做的所有临时蠢事。
你会发现每个行部分的插入效果更好：
例如在我的测试系统上使用你的高音扳机：
SQL> insert into target_Table select 'ABC' from dual connect by level <= 10000;
10000 rows created.
Elapsed: 00:00:10.49
将触发器设为：
SQL> CREATE OR REPLACE TRIGGER mdhl_basic_trigger
2 before INSERT OR UPDATE OR DELETE ON target_table for each row
3 declare
4
5 my_bgroup VARCHAR2(3);
6
7 v_timer2 number := 0;
8 v_timer number;
9 BEGIN
10
11 IF INSERTING OR UPDATING THEN
12 my_bgroup := :NEW.BGROUP;
13 ELSE
14 my_bgroup := :OLD.BGROUP;
15 END IF;
16
17 INSERT INTO log_table VALUES(my_bgroup, 'BASIC_MDHL', current_timestamp, 'TinyString');
18
19 END mdhl_basic_trigger;
20 /
SQL> insert into target_Table select 'ABC' from dual connect by level <= 10000;
10000 rows created.
Elapsed: 00:00:01.18
如果你知道你的字符串总是<32k，那么如果你创建你的触发器，你可以保持这个速度提升：
SQL> CREATE OR REPLACE TRIGGER mdhl_basic_trigger_compound
2 FOR INSERT OR UPDATE OR DELETE ON target_table
3
4 COMPOUND TRIGGER
5
6 type events_rec is record (BGROUP VARCHAR2(3),
7 TABLE_NAME VARCHAR2(255) ,
8 EVENT_TS TIMESTAMP (7),
9 EVENT_RAW varchar2(32767));
10 TYPE EVENTS_HIST IS TABLE OF events_rec INDEX BY PLS_INTEGER;
11 coll_events_hist EVENTS_HIST;
12 ctr PLS_INTEGER := 0;
13 my_bgroup VARCHAR2(3);
14
15 v_timer2 number := 0;
16 v_timer number;
17 BEFORE EACH ROW IS
18 BEGIN
19
20 IF INSERTING OR UPDATING THEN
21 my_bgroup := :NEW.BGROUP;
22 ELSE
23 my_bgroup := :OLD.BGROUP;
24 END IF;
25
26 ctr := ctr + 1;
27 coll_events_hist(ctr).BGROUP := my_bgroup;
28 coll_events_hist(ctr).TABLE_NAME := 'BASIC_MDHL';
29 coll_events_hist(ctr).EVENT_TS := current_timestamp;
30 coll_events_hist(ctr).EVENT_RAW := 'TinyString';
31
32 END BEFORE EACH ROW;
33
34 AFTER STATEMENT IS
35 BEGIN
36 v_timer := dbms_utility.get_time;
37 FORALL counter IN 1 .. coll_events_hist.count()
38 INSERT INTO log_table VALUES coll_events_hist(counter);
39 v_timer2 := v_timer2 + (dbms_utility.get_time - v_timer);
40 dbms_output.put_line(v_timer2/100);
41 END AFTER STATEMENT;
42 END mdhl_basic_trigger_compound;
43 /
SQL> insert into target_Table select 'ABC' from dual connect by level <= 10000;
10000 rows created.
Elapsed: 00:00:00.39
即。推迟操作直到插入。
答案 1 :(得分：1)
即使CLOB内联存储，与标准VARCHAR2相比也会产生一些开销，如appendix C of the LOB performance guideline中所述。
当LOB的长度小于3964字节时，它将以36字节的标头内联存储。长度为VARCHAR2的X将被存储为X字节的数据，并带有额外的一个或两个字节的开销。
我认为这种开销将带入内存，这意味着PLSQL CLOB对象的效率将低于同等大小的VARCHAR2。
34-35个额外字节将加起来，如以下脚本所示：
SQL> create table test_var(a varchar2(4000));
Table created
SQL> create table test_clob(a clob);
Table created
SQL> SET SERVEROUTPUT ON
SQL> DECLARE
2 l_time TIMESTAMP := systimestamp;
3 BEGIN
4 FOR i IN 1..100000 LOOP
5 INSERT INTO test_var VALUES (rpad('x', 1000, 'x'));
6 END LOOP;
7 dbms_output.put_line(systimestamp - l_time);
8 END;
9 /
+000000000 00:00:16.180299000
SQL> DECLARE
2 l_time TIMESTAMP := systimestamp;
3 BEGIN
4 FOR i IN 1..100000 LOOP
5 INSERT INTO test_clob VALUES (rpad('x', 1000, 'x'));
6 END LOOP;
7 dbms_output.put_line(systimestamp - l_time);
8 END;
9 /
+000000000 00:00:27.180716000
插入CLOB需要更多时间，这可以通过消耗的额外空间来解释：
SQL> EXEC dbms_stats.gather_table_stats(USER, 'TEST_VAR');
PL/SQL procedure successfully completed.
SQL> EXEC dbms_stats.gather_table_stats(USER, 'TEST_CLOB');
PL/SQL procedure successfully completed.
SQL> select blocks, table_name from user_tables where table_name like 'TEST_%';
BLOCKS TABLE_NAME
---------- ------------------------------
33335 TEST_CLOB
28572 TEST_VAR
当我们插入较小的字符串时问题会更加严重：
-- after TRUNCATE tables
SQL> DECLARE
2 l_time TIMESTAMP := systimestamp;
3 BEGIN
4 FOR i IN 1..1000000 LOOP
5 INSERT INTO test_var VALUES (rpad('x', 10, 'x'));
6 END LOOP;
7 dbms_output.put_line(systimestamp - l_time);
8 END;
9 /
+000000000 00:00:51.916675000
SQL> DECLARE
2 l_time TIMESTAMP := systimestamp;
3 BEGIN
4 FOR i IN 1..1000000 LOOP
5 INSERT INTO test_clob VALUES (rpad('x', 10, 'x'));
6 END LOOP;
7 dbms_output.put_line(systimestamp - l_time);
8 END;
9 /
+000000000 00:01:57.377676000
-- Gather statistics
SQL> select blocks, table_name from user_tables where table_name like 'TEST_%';
BLOCKS TABLE_NAME
---------- ------------------------------
7198 TEST_CLOB
2206 TEST_VAR