我们有一个非常简单的select语句访问具有唯一索引字段的数据。然而，优化器决定使用坏索引，选择大约需要4秒而不是0.0x秒。
oracle 11g不存在这个问题，但是使用oracle 12c。
表格统计数据是最新的。
看来，糟糕计划的估计是错误的（见下文），我们怎样才能避免这种情况？
我想通过添加字段统计信息或基线条目，但我希望还有另一种解决方案。
提前致谢。
表格定义
create table PS_CS_AKT_PROD_TB(business_unit VARCHAR2(5) not null,
ra_cmpgn_wave_id VARCHAR2(15) not null,
product_id VARCHAR2(18) not null,
cs_aboart_cd VARCHAR2(20) not null,
cs_einweis_id VARCHAR2(20) not null,
row_added_dttm TIMESTAMP(6),
row_added_oprid VARCHAR2(30) not null,
row_lastmant_dttm TIMESTAMP(6),
row_lastmant_oprid VARCHAR2(30) not null,
cs_recstat_xl VARCHAR2(4) not null,
/* ... further fields ... */
cs_kondition VARCHAR2(20) not null)
tablespace CS_APP pctfree 10 initrans 1
maxtrans 255 storage(initial 40K next 104K
minextents 1 maxextents unlimited);
索引
create unique index PS_CS_AKT_PROD_TB on PS_CS_AKT_PROD_TB(
BUSINESS_UNIT,RA_CMPGN_WAVE_ID,PRODUCT_ID,CS_ABOART_CD)
tablespace PSINDEX pctfree 10 initrans 2 maxtrans 255
storage(initial 40K next 104K minextents 1 maxextents unlimited);
create index PSBCS_AKT_PROD_TB on PS_CS_AKT_PROD_TB(
BUSINESS_UNIT,PRODUCT_ID)
tablespace PSINDEX pctfree 10 initrans 2 maxtrans 255
storage(initial 40K next 104K minextents 1 maxextents unlimited);
表格大小
select count(*) from PS_CS_AKT_PROD_TB;
--> 6372395
选择声明
给出了唯一索引的所有必填字段：
SELECT CS_STEUERUNG_XL, CS_EWF2EVT
FROM PS_CS_AKT_PROD_TB
WHERE BUSINESS_UNIT = :1
AND RA_CMPGN_WAVE_ID = :2
AND PRODUCT_ID = :3
AND CS_ABOART_CD = :4;
执行细则和解释计划
select v.CHILD_NUMBER,
elapsed_time / 1000000 elapsed_time,
executions,
round((elapsed_time / decode(executions, 0, 1, executions)) /
1000000,
4) elapsed_time_per_exec,
disk_reads,
buffer_gets,
rows_processed,
cpu_time
from v$sql v
where v.SQL_ID = 'dqrktmcraprvp';
/*
CHILD_NUMBER ELAPSED_TIME EXECUTIONS ELAPSED_TIME_PER_EXEC DISK_READS BUFFER_GETS ROWS_PROCESSED CPU_TIME
0 400,874709 100 4,0087 98457 495295 86 5929096
1 0,017217 8 0,0022 2 36 4 2108
2 0,002038 2 0,001 0 9 1 0
*/
select plan_table_output
from table(dbms_xplan.display_cursor('dqrktmcraprvp', 0)) t;
/*
SQL_ID dqrktmcraprvp, child number 0
-------------------------------------
SELECT CS_STEUERUNG_XL, CS_EWF2EVT FROM PS_CS_AKT_PROD_TB WHERE
BUSINESS_UNIT = :1 AND RA_CMPGN_WAVE_ID = :2 AND PRODUCT_ID = :3 AND
CS_ABOART_CD = :4
Plan hash value: 1118713352
---------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| PS_CS_AKT_PROD_TB | 1 | 46 | 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | PSBCS_AKT_PROD_TB | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(("RA_CMPGN_WAVE_ID"=:2 AND "CS_ABOART_CD"=:4))
2 - access("BUSINESS_UNIT"=:1 AND "PRODUCT_ID"=:3) */
select plan_table_output
from table(dbms_xplan.display_cursor('dqrktmcraprvp', 1)) t;
/*
SQL_ID dqrktmcraprvp, child number 1
-------------------------------------
SELECT CS_STEUERUNG_XL, CS_EWF2EVT FROM PS_CS_AKT_PROD_TB WHERE
BUSINESS_UNIT = :1 AND RA_CMPGN_WAVE_ID = :2 AND PRODUCT_ID = :3 AND
CS_ABOART_CD = :4
Plan hash value: 619225732
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| PS_CS_AKT_PROD_TB | 1 | 46 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PS_CS_AKT_PROD_TB | 1 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("BUSINESS_UNIT"=:1 AND "RA_CMPGN_WAVE_ID"=:2 AND "PRODUCT_ID"=:3 AND
"CS_ABOART_CD"=:4)
*/
修正案（见评论）
PROCUCT_ID的字段值分布不均
OCCURENCE_OF_PRODUCT_ID TOTAL
upto 10^1-1 1134
upto 10^2-1 1607
upto 10^3-1 1649
upto 10^4-1 455
upto 10^5-1 279
dbms_xplan.display_cursor的输出（null，null，＆＃39; + OUTLINE＆＃39;）
最多37秒：
SQL_ID ga79yhh54r5bu, child number 0
-------------------------------------
select a.cs_ewf2evt, a.cs_steuerung_xl from ps_cs_akt_prod_tb a where
a.business_unit = :1 and a.ra_cmpgn_wave_id = :2 and a.product_id = :3
and a.cs_aboart_cd = :4
Plan hash value: 1118713352
---------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| PS_CS_AKT_PROD_TB | 1 | 46 | 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | PSBCS_AKT_PROD_TB | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------------
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('12.1.0.2')
DB_VERSION('12.1.0.2')
OPT_PARAM('_optimizer_max_permutations' 50)
OPT_PARAM('_unnest_subquery' 'false')
OPT_PARAM('optimizer_dynamic_sampling' 4)
OPT_PARAM('_gby_hash_aggregation_enabled' 'false')
OPT_PARAM('_optimizer_use_feedback' 'false')
OPT_PARAM('_px_adaptive_dist_method' 'off')
OPT_PARAM('_optimizer_dsdir_usage_control' 0)
OPT_PARAM('_optimizer_adaptive_plans' 'false')
OPT_PARAM('_optimizer_strans_adaptive_pruning' 'false')
OPT_PARAM('_optimizer_gather_feedback' 'false')
OPT_PARAM('_optimizer_nlj_hj_adaptive_join' 'false')
OPT_PARAM('optimizer_index_cost_adj' 20)
ALL_ROWS
OUTLINE_LEAF(@"SEL$1")
INDEX_RS_ASC(@"SEL$1" "A"@"SEL$1" ("PS_CS_AKT_PROD_TB"."BUSINESS_UNIT"
"PS_CS_AKT_PROD_TB"."PRODUCT_ID"))
BATCH_TABLE_ACCESS_BY_ROWID(@"SEL$1" "A"@"SEL$1")
END_OUTLINE_DATA
*/
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(("A"."RA_CMPGN_WAVE_ID"=:2 AND "A"."CS_ABOART_CD"=:4))
2 - access("A"."BUSINESS_UNIT"=:1 AND "A"."PRODUCT_ID"=:3)
~0.06秒
SQL_ID ga79yhh54r5bu, child number 0
-------------------------------------
select a.cs_ewf2evt, a.cs_steuerung_xl from ps_cs_akt_prod_tb a where
a.business_unit = :1 and a.ra_cmpgn_wave_id = :2 and a.product_id = :3
and a.cs_aboart_cd = :4
Plan hash value: 619225732
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| PS_CS_AKT_PROD_TB | 1 | 46 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PS_CS_AKT_PROD_TB | 1 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('12.1.0.2')
DB_VERSION('12.1.0.2')
OPT_PARAM('_optimizer_max_permutations' 50)
OPT_PARAM('_unnest_subquery' 'false')
OPT_PARAM('optimizer_dynamic_sampling' 4)
OPT_PARAM('_gby_hash_aggregation_enabled' 'false')
OPT_PARAM('_optimizer_use_feedback' 'false')
OPT_PARAM('_px_adaptive_dist_method' 'off')
OPT_PARAM('_optimizer_dsdir_usage_control' 0)
OPT_PARAM('_optimizer_adaptive_plans' 'false')
OPT_PARAM('_optimizer_strans_adaptive_pruning' 'false')
OPT_PARAM('_optimizer_gather_feedback' 'false')
OPT_PARAM('_optimizer_nlj_hj_adaptive_join' 'false')
OPT_PARAM('optimizer_index_cost_adj' 20)
ALL_ROWS
OUTLINE_LEAF(@"SEL$1")
INDEX_RS_ASC(@"SEL$1" "A"@"SEL$1" ("PS_CS_AKT_PROD_TB"."BUSINESS_UNIT"
"PS_CS_AKT_PROD_TB"."RA_CMPGN_WAVE_ID" "PS_CS_AKT_PROD_TB"."PRODUCT_ID"
"PS_CS_AKT_PROD_TB"."CS_ABOART_CD"))
END_OUTLINE_DATA
*/
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("A"."BUSINESS_UNIT"=:1 AND "A"."RA_CMPGN_WAVE_ID"=:2 AND
"A"."PRODUCT_ID"=:3 AND "A"."CS_ABOART_CD"=:4)
答案 0 :(得分：1)
PRODUCT_ID字段的值不均匀分布（请参阅上述问题中的分发列表）。
因此，在极少数情况下，这两个计划几乎相同。因为使用绑定变量调用语句，所以仅在第一次执行时处理估计。
用
alter session set "_optim_peek_user_binds"=false;
优化器被迫在第二次执行时重新评估绑定值。