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组合索引如何选择前导列的几点考虑

时间:2021-07-01 10:21:17 帮助过:21人阅读

? 选择组合索引的前导列,必须根据具体的业务(SQL)写法和列的数据分布不同而不同,很多书或网上都说,前导列要选择高选择性的,但是,脱离具体的业务,这些是没有意义的, 本文就举一些常见的例子来分析下如何正确选择前导列,以抛砖引玉,实际应用中,有更

?选择组合索引的前导列,必须根据具体的业务(SQL)写法和列的数据分布不同而不同,很多书或网上都说,前导列要选择高选择性的,但是,脱离具体的业务,这些是没有意义的,本文就举一些常见的例子来分析下如何正确选择前导列,以抛砖引玉,实际应用中,有更多复杂的情况需要具体分析。

1.都是等值条件的列,谁做前导列都一样

DROP TABLE t;
CREATE TABLE t?
AS
SELECT * FROM dba_objects;
CREATE INDEX idx1_t ON t(owner,object_id);
CREATE INDEX idx2_t ON t(object_id,owner);
BEGIN
? dbms_stats.gather_table_stats(ownname => USER,tabname => ‘T’,estimate_percent => 100,cascade => TRUE);
END;
?/
dingjun123@ORADB> ?SELECT COUNT(DISTINCT owner),COUNT(DISTINCT object_id),COUNT(*) FROM t;
COUNT(DISTINCTOWNER) COUNT(DISTINCTOBJECT_ID) ? COUNT(*)
——————– ———————— ———-
? ? ? ? ? ? ? ? ? 33 ? ? ? ? ? ? ? ? ? ?75250 ? ? ?75251
1 row selected.

owner有33个不同的值,object_id有75250,显然object_id的选择性更好。但是下面的查询,应用idx1_t与idx2_t的性能一样(COST与CONSISTENT GETS一样)。

dingjun123@ORADB> ?SELECT/*+index(t idx1_t)*/ * FROM t
? 2 ? WHERE owner=’DINGJUN123′ AND object_id=75677;
1 row selected.
Elapsed: 00:00:00.00
Execution Plan
———————————————————-
Plan hash value: 2071967826
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ? ? 1 | ? ?97 | ? ??2?? (0)| 00:00:01 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ? ? 1 | ? ?97 | ? ? 2 ? (0)| 00:00:01 |
|* ?2 | ? INDEX RANGE SCAN ? ? ? ? ?| IDX1_T | ? ? 1 | ? ? ? | ? ? 1 ? (0)| 00:00:01 |
————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
? ?2 – access(“OWNER”=’DINGJUN123′ AND “OBJECT_ID”=75677)
Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ? ? ??4 ?consistent gets
? ? ? ? ? 0 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ? ?1403 ?bytes sent via SQL*Net to client
? ? ? ? 416 ?bytes received via SQL*Net from client
? ? ? ? ? 2 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ? ? ? 1 ?rows processeddingjun123@ORADB> ? SELECT/*+index(t idx2_t)*/ * FROM t
? 2 ? WHERE owner=’DINGJUN123′ AND object_id=75677;
1 row selected.
Elapsed: 00:00:00.01
Execution Plan
———————————————————-
Plan hash value: 3787301248
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ? ? 1 | ? ?97 | ??? 2?? (0)| 00:00:01 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ? ? 1 | ? ?97 | ? ? 2 ? (0)| 00:00:01 |
|* ?2 | ? INDEX RANGE SCAN ? ? ? ? ?| IDX2_T | ? ? 1 | ? ? ? | ? ? 1 ? (0)| 00:00:01 |
————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
? ?2 – access(“OBJECT_ID”=75677 AND “OWNER”=’DINGJUN123′)
Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ? ??? 4 ?consistent gets
? ? ? ? ? 0 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ? ?1403 ?bytes sent via SQL*Net to client
? ? ? ? 416 ?bytes received via SQL*Net from client
? ? ? ? ? 2 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ? ? ? 1 ?rows processed

? ? ??虽然如此,但是要记住,这个索引不是仅仅给这2条SQL使用的,事实上可能我们有的查询谓词只有owner或object_id,这时候得考虑使用owner作为前导列还是使用object_id作为前导列。
? ??还有其他引用owner,object_id的情况,比如GROUP BY ,ORDER BY,甚至SELECT…都需要进行整体的分析,这样才能建立最佳的索引。
?
2.有的列是大于(等于)或小于(等于)或者是like 模糊匹配等不等条件,有的列是等值的条件,等值的一般作为前导列更好
–做5次,增加几十万行SYS的进去
INSERT INTO t SELECT * FROM t WHERE owner=’SYS’;
COMMIT;
–重新收集统计信息(省略)

dingjun123@ORADB> SELECT * FROM t
? 2 ?WHERE owner=’DINGJUN123′
? 3 ?AND object_id>=107889;
1 row selected.
Elapsed: 00:00:00.01
Execution Plan
———————————————————-
Plan hash value: 2071967826
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ? ? 1 | ? ?96 | ? ? 4 ? (0)| 00:00:01 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ? ? 1 | ? ?96 | ? ? 4 ? (0)| 00:00:01 |
|*??2 | ? INDEX RANGE SCAN ? ? ? ? ?| IDX1_T | ? ? 1 | ? ? ? | ? ? 3 ? (0)| 00:00:01?|
————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
???2 – access(“OWNER”=’DINGJUN123′ AND “OBJECT_ID”>=107889 AND “OBJECT_ID” IS
? ? ? ? ? ? ? NOT NULL)
Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ? ??? 5 ?consistent gets
? ? ? ? ? 0 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ? ?1399 ?bytes sent via SQL*Net to client
? ? ? ? 416 ?bytes received via SQL*Net from client
? ? ? ? ? 2 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ? ? ? 1 ?rows processed

? ? ??上面的SQL走idx1_t,注意观察谓词,只有access,说明索引完全被利用上,很显然因为owner是前导列,而且是等值查询,按照前导列查询,然后只要分析索引的第2列object_id,当发现不满足条件object_id>=107889之后就停止了,索引扫描没有浪费。

dingjun123@ORADB> SELECT/*+index(t idx2_t)*/ * FROM t
? 2 ?WHERE owner=’DINGJUN123′
? 3 ?AND object_id>=107889;
1 row selected.
Elapsed: 00:00:00.01
Execution Plan
———————————————————-
Plan hash value: 3787301248
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ? ? 1 | ? ?96 | ? ? 4 ? (0)| 00:00:01 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ? ? 1 | ? ?96 | ? ? 4 ? (0)| 00:00:01 |
|* ?2 | ? INDEX RANGE SCAN ? ? ? ? ?| IDX2_T | ? ? 1 | ? ? ? | ? ? 3 ? (0)| 00:00:01 |
————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
? ?2 – access(“OBJECT_ID”>=107889 AND “OWNER”=’DINGJUN123′ AND “OBJECT_ID” IS
? ? ? ? ? ? ? NOT NULL)
? ??? ?filter(“OWNER”=’DINGJUN123′)
Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ? ? ??5 ?consistent gets
? ? ? ? ? 0 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ? ?1399 ?bytes sent via SQL*Net to client
? ? ? ? 416 ?bytes received via SQL*Net from client
? ? ? ? ? 2 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ? ? ? 1 ?rows processed

? ? ??强制使用idx2_t,object_id是前导列,谓词有access,还有filter,说明索引没有被完全利用上,这是因为object_id的不是等值查询,满足object_id>=107889的,按照顺序搜索
所以,中间可能有一些不满足owner=’DINGJUN123′的,还要filter掉。
这种查询和不等值条件作为前导列的查询,一旦object_id>=107889不满足owner=’DINGJUN123′的很多,那么必然造成过多不必要的索引搜索,COST与逻辑读会上升很快,
从而性能急剧下降,因为本例子基本都满足owner条件,所以没有啥浪费。但是下面的例子

dingjun123@ORADB> SELECT * FROM t
? 2 ?WHERE owner=’DINGJUN123′
? 3 ?AND object_id>=100;
2540 rows selected.
Elapsed: 00:00:00.15
Execution Plan
———————————————————-
Plan hash value: 2071967826
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ?2539 | ? 238K| ? 499 ? (0)| 00:00:06 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ?2539 | ? 238K| ? 499 ? (0)| 00:00:06 |
|* ?2 | ? INDEX RANGE SCAN ? ? ? ? ?| IDX1_T | ?2539 | ? ? ? | ? ?12 ? (0)| 00:00:01 |
————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
? ?2 – access(“OWNER”=’DINGJUN123′ AND “OBJECT_ID”>=100 AND “OBJECT_ID” IS
? ? ? ? ? ? ? NOT NULL)
Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ? ??527 ?consistent gets
? ? ? ? ?21 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ?268134 ?bytes sent via SQL*Net to client
? ? ? ?2275 ?bytes received via SQL*Net from client
? ? ? ? 171 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ???2540 ?rows processed

还是使用idx1_t,没有问题。看下面的强制使用idx2_t的。

dingjun123@ORADB> SELECT/*+index(t idx2_t)*/ * FROM t
? 2 ?WHERE owner=’DINGJUN123′
? 3 ?AND object_id>=100;2540 rows selected.
Elapsed: 00:00:00.33
Execution Plan
———————————————————-
Plan hash value: 3787301248
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ?2539 | ? 238K| ?3762 ? (1)| 00:00:46 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ?2539 | ? 238K| ?3762 ? (1)| 00:00:46 |
|* ?2 | ? INDEX RANGE SCAN ? ? ? ? ?| IDX2_T | ?2539 | ? ? ? | ?3274 ? (1)| 00:00:40 |
————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
? ?2 – access(“OBJECT_ID”>=100 AND “OWNER”=’DINGJUN123′ AND “OBJECT_ID” IS
? ? ? ? ? ? ? NOT NULL)
? ? ???filter(“OWNER”=’DINGJUN123′)
Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ???3763 ?consistent gets
? ? ? ? ? 0 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ?268134 ?bytes sent via SQL*Net to client
? ? ? ?2275 ?bytes received via SQL*Net from client
? ? ? ? 171 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ? ?2540 ?rows processed

dingjun123@ORADB> SELECT COUNT(*) FROM t WHERE object_id >= 100;
? COUNT(*)
———-
? ?1032649

?SELECT COUNT(*) FROM t WHERE object_id >= 100;
返回1032649行,但是WHERE owner=’DINGJUN123′ AND object_id>=100 只返回2540行,要filter掉百万行,轮询索引,造成了极大的浪费。

3.如果都是比较,都是<,>之类的表达式
??这种情况,前导列,根据谓词,选择条件能够定位最接近处理结果的基数,并能够减少索引后filter的工作,因为必然有一列是要走access之后的filter,最好是filter能够过滤较少数据,不要做过多过滤。
??
例如:

dingjun123@ORADB> SELECT * FROM t
? 2 ?WHERE owner>=’DINGJUN123′
? 3 ?AND object_id>=107872;37 rows selected.
Elapsed: 00:00:00.00
Execution Plan
———————————————————-
Plan hash value: 3787301248
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ? 205 | 19680 | ???43?? (0)| 00:00:01 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ? 205 | 19680 | ? ?43 ? (0)| 00:00:01 |
|* ?2 | ??INDEX RANGE SCAN ? ? ? ? ?| IDX2_T?| ? 205 | ? ? ? | ? ? 3 ? (0)| 00:00:01 |
————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
? ?2 – access(“OBJECT_ID”>=107872 AND “OWNER”>=’DINGJUN123′ AND “OBJECT_ID”
? ? ? ? ? ? ? IS NOT NULL)
? ? ? ?filter(“OWNER”>=’DINGJUN123′)

Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ? ? ?14 ?consistent gets
? ? ? ? ? 0 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ? ?6468 ?bytes sent via SQL*Net to client
? ? ? ? 438 ?bytes received via SQL*Net from client
? ? ? ? ? 4 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ? ? ?37 ?rows processed

dingjun123@ORADB> SELECT COUNT(*) FROM t WHERE object_id>=107872;
? COUNT(*)
———-
? ? ? ? 37
1 row selected.

??关闭index SKIP SCAN,因为owner种类很少,oracle选择skip SCAN
?alter session set “_optimizer_skip_scan_enabled” = false;
?

dingjun123@ORADB> SELECT/*+index(t idx1_t)*/ * FROM t
? 2 ?WHERE owner>=’DINGJUN123′
? 3 ?AND object_id>=107872;
37 rows selected.
Elapsed: 00:00:00.23
Execution Plan
———————————————————-
Plan hash value: 2071967826
————————————————————————————–
| Id ?| Operation ? ? ? ? ? ? ? ? ? | Name ? | Rows ?| Bytes | Cost (%CPU)| Time ? ? |
————————————————————————————–
| ? 0 | SELECT STATEMENT ? ? ? ? ? ?| ? ? ? ?| ? 205 | 19680 | ?3740?? (1)| 00:00:45 |
| ? 1 | ?TABLE ACCESS BY INDEX ROWID| T ? ? ?| ? 205 | 19680 | ?3740 ? (1)| 00:00:45 |
|* ?2 | ? INDEX RANGE SCAN ? ? ? ? ?| IDX1_T | ? 205 | ? ? ? | ?3700 ? (1)| 00:00:45 |
————————————————————————————–Predicate Information (identified by operation id):
—————————————————
? ?2 – access(“OWNER”>=’DINGJUN123′ AND “OBJECT_ID”>=107872 AND “OWNER” IS
? ? ? ? ? ? ? NOT NULL)
? ? ? ?filter(“OBJECT_ID”>=107872)
Statistics
———————————————————-
? ? ? ? ? 1 ?recursive calls
? ? ? ? ? 0 ?db block gets
? ? ???3688 ?consistent gets
? ? ? ? ? 0 ?physical reads
? ? ? ? ? 0 ?redo size
? ? ? ?6468 ?bytes sent via SQL*Net to client
? ? ? ? 438 ?bytes received via SQL*Net from client
? ? ? ? ? 4 ?SQL*Net roundtrips to/from client
? ? ? ? ? 0 ?sorts (memory)
? ? ? ? ? 0 ?sorts (disk)
? ? ? ? ?37 ?rows processed

因为owner>=’DINGJUN123′返回大量行,但是事实结果很少,只有几十行,过滤object_id>=107872,需要做大量工作,逻辑读和COST增大千倍+,性能低下。

后记:
? ? ?当然如何选择前导列的顺序很复杂,得全盘考虑对应的谓词,SELECT的列等要素,还要考虑ORDER BY ,GROUP BY等列,比如3列组合索引,如何考虑顺序。
? ? ?后续会补充更多的组合索引如何创建的要点。

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