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如何分析思考oralce的一些问题的一点感悟！

在pub闲逛时看到这么一个问题：

开发人员反应，用pl/sql developer查询mchnt这张IOT表特别慢，但其它表都很快，具体情况如下：
1、表数据量很少
select count(1) from ap07.mchnt;
  COUNT(1)
----------
      12

2、mchnt表的pk index的段信息：
------------------Basic Info---------------------
segment_name............................PK_MCHNT
segment_type............................INDEX
segment_owner...........................AP07
tablespace_name.........................TS_VASP
segment_management......................AUTO
initial_blocks..........................8
initial_bytes...........................65536
header_file_id..........................6
header_block_id.........................571
------------------Used Info---------------------
total_blocks............................8
total_bytes.............................65536
HWM_blocks..............................4
HWM_bytes...............................32768
used_sys_blocks.........................3
used_data_blocks........................1
Full Blocks.............................0
100%~75% Used Blocks....................0
75%~50% Used Blocks.....................1
50%~25% Used Blocks.....................0
25%~0% Used/Empty Blocks................0
last_used_blocks........................4
------------------Unused Info---------------------
unused_blocks...........................4
unused_bytes............................32768
Unformatted Blocks......................0

3、表mchnt上的索引信息
select index_name,index_type,table_name from dba_indexes where owner='AP07' and table_name='MCHNT';
INDEX_NAME                   INDEX_TYPE                TABLE_NAME
------------------------------ --------------------------- ------------------------------
IDX_MCHNT_NO                NORMAL                   MCHNT
SYS_IL0000116890C00016$$    LOB                      MCHNT
PK_MCHNT                IOT - TOP                MCHNT

4、表结构
desc ap07.mchnt;
Name                                                                   Null? Type
------------------------------------------------------------------------ -------- -------------------------------------------------
N_USR_ID                                                                NOT NULL NUMBER(8)
C_MCHNT_NO                                                             NOT NULL CHAR(8)
C_TSC_PWD                                                                      VARCHAR2(64)
N_MCHNT_TYPE                                                                   CHAR(2)
N_MCHNT_GRP_ID                                                          NOT NULL NUMBER(8)
N_MCHNT_PROV_ID                                                                CHAR(6)
C_MCHNT_PROV_NAME                                                                VARCHAR2(50)
N_MCHNT_CITY_ID                                                                CHAR(6)
C_MCHNT_CITY_NAME                                                                VARCHAR2(50)
C_MCHNT_ADDR                                                                   VARCHAR2(200)
N_DBIZ_AREA                                                                      NUMBER(5,2)
C_DBIZ_MODE                                                                      CHAR(1)
C_ZIPCODE                                                                      CHAR(6)
C_CERT_TYPE                                                                      CHAR(1)
C_CERT_NO                                                                      VARCHAR2(30)
B_CERT_IMG                                                                      BLOB
C_CORP_NAME                                                                      VARCHAR2(20)
C_CORP_CRED_TYPE                                                                CHAR(2)
C_CORP_CRED_NO                                                                   VARCHAR2(50)
C_CTT_NAME                                                                      VARCHAR2(20)
C_CTT_SEX                                                                      CHAR(1)
C_CTT_CRED_TYPE                                                                CHAR(2)
C_CTT_CRED_NO                                                                   VARCHAR2(50)
C_TEL                                                                            VARCHAR2(20)
C_MOB                                                                            CHAR(11)
C_EMAIL                                                                         VARCHAR2(60)
C_FAX_NO                                                                         VARCHAR2(15)
N_SELL_POS_USR_ID                                                                NUMBER(8)
C_SELL_POS_USR_NAME                                                             VARCHAR2(60)
C_AUTH_BIZ_TYPE                                                                CHAR(1)
N_PRI_ID                                                                NOT NULL NUMBER(8)
C_PACT_NO                                                                      VARCHAR2(50)
C_PACT_EFF_TIME                                                                DATE
C_PACT_EXP_TIME                                                                DATE
C_CLOSE_USR_SUGG                                                                VARCHAR2(600)
C_IS_SYNC_POSP                                                                   CHAR(1)
C_IS_SYNC_OWS                                                                   CHAR(1)
D_UP_TIME                                                                      DATE
N_VERSION                                                             NOT NULL NUMBER(4)
N_MCHNT_RET_PROFIT_FLAG                                                          CHAR(1)
N_MCHNT_FEE_FLAG                                                          CHAR(1)

请大家帮分析，该如何处理？