我正在尝试编写一个只在满足条件时才提取第一个（随机）行的查询。
-- Create table
create table TRANSACTIONS_SAMPLE
(
institution_id NUMBER(5) not null,
id NUMBER(10) not null,
partitionkey NUMBER(10) default 0 not null,
cardid NUMBER(10),
accountid NUMBER(10),
batchid NUMBER(10) not null,
amt_bill NUMBER(16,3),
load_date DATE not null,
trxn_date DATE not null,
single_msg_flag NUMBER(5),
authaccounttype VARCHAR2(2 BYTE),
originator VARCHAR2(50),
amount NUMBER(16,3) default 0.000 not null,
embeddedfee NUMBER(16,3) default 0.000 not null,,
valuedate DATE,
startofinterest DATE,
minduevaluedate DATE,
postdate DATE,
posttimestamp DATE,
Status CHAR(4 BYTE) default 'NEW' not null,
)
partition by list (PARTITIONKEY)
(
partition 0002913151 values (1234567)
tablespace LIVE
pctfree 10
initrans 16
maxtrans 255
storage
(
initial 8M
next 1M
minextents 1
maxextents unlimited
)
);
-- Create/Recreate indexes
create index TRANSACTIONS_SAMPLEI01 on TRANSACTIONS_SAMPLE (ACCOUNTID)
local;
create index TRANSACTIONS_SAMPLEI02 on TRANSACTIONS_SAMPLE (LOAD_DATE)
local;
create index TRANSACTIONS_SAMPLEI03 on TRANSACTIONS_SAMPLE (BATCHID)
local;
create index TRANSACTIONS_SAMPLEI04 on TRANSACTIONS_SAMPLE (POSTDATE)
local;
create index TRANSACTIONS_SAMPLEI05 on TRANSACTIONS_SAMPLE (POSTTIMESTAMP)
local;
create index TRANSACTIONS_SAMPLEI06 on TRANSACTIONS_SAMPLE (STATUS, PARTITIONKEY)
local;
create index TRANSACTIONS_SAMPLEI07 on TRANSACTIONS_SAMPLE (CARDID, TRXN_DATE)
local;
create unique index TRANSACTIONS_SAMPLEUI01 on TRANSACTIONS_SAMPLE (ID, PARTITIONKEY)
local;
-- Create/Recreate primary, unique and foreign key constraints
alter table TRANSACTIONS_SAMPLE
add constraint TRANSACTIONS_SAMPLEPK primary key (ID, PARTITIONKEY);
--QUERY
Select * From (
Select t.AccountId From Transactions_sample t Group by t.Accountid Having Count(t.AccountId) > 10 order by dbms_random.random)
Where Rownum = 1
此查询的问题是全表扫描。我希望在不必完全访问表的情况下获得相同的结果。有什么想法吗？
由于
答案 0 :(得分：3)
如果您为TRANSACTIONS_SAMPLEI01添加过滤器，则可以使用where AccountId is not null将其转换为完整索引扫描。但是，当然，只有当你不想计算空值时才会这样做。
该列可以为空，但索引不包含空值。要包含空值计数，它必须执行全表扫描，因为它无法从索引中获取该计数。如果您有该过滤器，则优化器知道所有帐户ID值必须在索引中，因此它只需要引用它，而不是表本身。
explain plan for
Select * From (
Select t.AccountId From Transactions_sample t where AccountId is not null Group by t.Accountid Having Count(t.AccountId) > 10 order by dbms_random.random)
Where Rownum = 1;
select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------
Plan hash value: 381125580
---------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
---------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 0 (0)| 00:00:01 | | |
|* 1 | COUNT STOPKEY | | | | | | | |
| 2 | VIEW | | 1 | 13 | 0 (0)| 00:00:01 | | |
|* 3 | SORT ORDER BY STOPKEY | | 1 | 13 | 0 (0)| 00:00:01 | | |
|* 4 | FILTER | | | | | | | |
| 5 | SORT GROUP BY NOSORT | | 1 | 13 | 0 (0)| 00:00:01 | | |
| 6 | PARTITION LIST SINGLE| | 1 | 13 | 0 (0)| 00:00:01 | 1 | 1 |
|* 7 | INDEX FULL SCAN | TRANSACTIONS_SAMPLEI01 | 1 | 13 | 0 (0)| 00:00:01 | 1 | 1 |
---------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM=1)
3 - filter(ROWNUM=1)
4 - filter(COUNT("T"."ACCOUNTID")>10)
7 - filter("ACCOUNTID" IS NOT NULL)
Note
-----
- dynamic sampling used for this statement (level=2)
或者，如果可以使列不可为空，那么就不需要过滤器。