DM 收集统计信息后内存中的SQL 执行计划无效测试

1. 创建测试表并插入有特殊数据分布的数据

-- 创建测试表

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CREATE TABLE DM_STATS_TEST (
    id INT,
    category VARCHAR(10),
    value INT);

-- 插入1000行数据，制造特殊分布：

category='A'：990行（占99%）
category='B'：10行（占1%）

INSERT INTO DM_STATS_TEST
SELECT LEVEL, 'A', LEVEL FROM DUAL CONNECT BY LEVEL <= 990
UNION ALL
SELECT 990+LEVEL, 'B', 1000+LEVEL FROM DUAL CONNECT BY LEVEL <= 10;
COMMIT;

在 CATEGORY 字段上创建一个索引

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CREATE OR REPLACE INDEX "EXAM"."IDX_CATEGORY" ON "EXAM"."DM_STATS_TEST"("CATEGORY" ASC) STORAGE(ON "MAIN", CLUSTERBTR) ;

2. 执行一个查询并查看执行计划

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SELECT * FROM DM_STATS_TEST WHERE CATEGORY =:VAR;
<===查询 B

1   #NSET2: [1, 25, 68] 
2     #PRJT2: [1, 25, 68]; exp_num(4), is_atom(FALSE) 
3       #BLKUP2: [1, 25, 68]; IDX_CATEGORY(DM_STATS_TEST)
4         #SSEK2: [1, 25, 68]; scan_type(ASC), IDX_CATEGORY(DM_STATS_TEST), scan_range[exp_param(no:0),exp_param(no:0)], is_global(0)·

SELECT PLN_ADDR,HASH_VALUE,SQLSTR,RT_METHOD,N_SUBPLNS,PHD_TIME FROM V$SQL_PLAN WHERE SQLSTR LIKE '%SELECT * FROM DM_STATS_TEST WHERE CATEGORY%' 
and lower(SQLSTR) NOT LIKE '%v$%';

PLN_ADDR			HASH_VALUE	SQLSTR												RT_METHOD			N_SUBPLNS	PHD_TIME
0x00000000797F7810	315894334	SELECT * FROM DM_STATS_TEST WHERE CATEGORY =:VAR;	0x00000000797F7E28	0	2026-01-06 15:37:00

3 - 使用常用方式收集的统计信息

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DBMS_STATS.GATHER_TABLE_STATS('EXAM','DM_STATS_TEST',null,100,TRUE,'FOR ALL COLUMNS SIZE AUTO',2);

4 - 再次执行相同查询并查看执行计划

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SELECT * FROM DM_STATS_TEST WHERE CATEGORY =:VAR;
<===查询 B

1   #NSET2: [1, 500, 68] 
2     #PRJT2: [1, 500, 68]; exp_num(4), is_atom(FALSE) 
3       #SLCT2: [1, 500, 68]; DM_STATS_TEST.category = exp_param(no:0) SLCT_PUSHDOWN(TRUE)
4         #CSCN2: [1, 1000, 68]; INDEX33555640(DM_STATS_TEST) NEED_SLCT(TRUE); btr_scan(1)

SELECT PLN_ADDR,HASH_VALUE,SQLSTR,RT_METHOD,N_SUBPLNS,PHD_TIME FROM V$SQL_PLAN WHERE SQLSTR LIKE '%SELECT * FROM DM_STATS_TEST WHERE CATEGORY%' 
and lower(SQLSTR) NOT LIKE '%v$%';

PLN_ADDR			HASH_VALUE	SQLSTR												RT_METHOD			N_SUBPLNS	PHD_TIME
0x00000000797F7810	315894334	SELECT * FROM DM_STATS_TEST WHERE CATEGORY =:VAR;	0x00000000797F7E28	0	2026-01-06 15:37:00

5 - 指定执行计划失效的统计信息

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begin
DBMS_STATS.GATHER_TABLE_STATS (
ownname => 'EXAM',
tabname => 'DM_STATS_TEST',
method_opt =>'FOR ALL COLUMNS SIZE AUTO',
granularity => 'ALL',
no_invalidate=>FALSE,
cascade => true,
force => true);
end;
/

6 - 直接检查内存中的执行计划

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SELECT PLN_ADDR,HASH_VALUE,SQLSTR,RT_METHOD,N_SUBPLNS,PHD_TIME FROM V$SQL_PLAN WHERE SQLSTR LIKE '%SELECT * FROM DM_STATS_TEST WHERE CATEGORY%' 
and lower(SQLSTR) NOT LIKE '%v$%';

空   <===其实到这就说明执行计划已经并清除了。

7 - 再次执行相同查询并查看执行计划

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SELECT * FROM DM_STATS_TEST WHERE CATEGORY =:VAR;
<===查询 B

1   #NSET2: [1, 500, 68] 
2     #PRJT2: [1, 500, 68]; exp_num(4), is_atom(FALSE) 
3       #SLCT2: [1, 500, 68]; DM_STATS_TEST.category = exp_param(no:0) SLCT_PUSHDOWN(TRUE)
4         #CSCN2: [1, 1000, 68]; INDEX33555640(DM_STATS_TEST) NEED_SLCT(TRUE); btr_scan(1)

SELECT PLN_ADDR,HASH_VALUE,SQLSTR,RT_METHOD,N_SUBPLNS,PHD_TIME FROM V$SQL_PLAN WHERE SQLSTR LIKE '%SELECT * FROM DM_STATS_TEST WHERE CATEGORY%' 
and lower(SQLSTR) NOT LIKE '%v$%';

PLN_ADDR			HASH_VALUE	SQLSTR												RT_METHOD			N_SUBPLNS	PHD_TIME
0x0000000079720A10	315894334	SELECT * FROM DM_STATS_TEST WHERE CATEGORY =:VAR;	0x0000000079721028	0	2026-01-06 15:46:54

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