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表P（父）和C（子）都有 10 个分区cat和 316 个子分区effective_date。表P有以下索引create index ix_p_cat on p (cat);。

与执行完整分区访问相比，对优化器而言，使用分区列上的索引进行索引范围扫描怎么可能更可取（成本更低）？

我的想法是，在任何一种情况下都需要来自 P 的相同数量的数据块，因此不妨避免读取额外的索引块。但是，优化器不同意。

以下是两个解释计划。第一个是显示优化器想要使用索引来构建哈希表，第二个是提示不使用索引。

分析了表格和索引。

Oracle 企业版 19c。提前致谢。

select C.some_col
  from "P"
  join "C"
    on P.code = C.code
       and P.cat = :cat                                           
       and C.cat = :cat 
;

------------------------------------------------------------------------------------------------------------------------
| id  | Operation                                   | name     | rows  | Bytes | cost (%CPU)| time     | Pstart| Pstop |
------------------------------------------------------------------------------------------------------------------------
|   0 | select statement                            |          |   275G|  5127G|  1280K (58)| 00:00:51 |       |       |
|*  1 |  hash join                                  |          |   275G|  5127G|  1280K (58)| 00:00:51 |       |       |
|   2 |   table access by global index ROWID BATCHED| P        | 60363 |   412K|  1642   (1)| 00:00:01 | ROWID | ROWID |
|*  3 |    index range scan                         | IX_P_CAT | 60363 |       |   231   (0)| 00:00:01 |       |       |
|   4 |   partition LIST single                     |          |    41M|   510M|   539K  (1)| 00:00:22 |   key |   key |
|   5 |    partition range all                      |          |    41M|   510M|   539K  (1)| 00:00:22 |     1 |   316 |
|   6 |     table access full                       | C        |    41M|   510M|   539K  (1)| 00:00:22 |       |       |
------------------------------------------------------------------------------------------------------------------------
 
query block name / object alias (identified by operation id):
-------------------------------------------------------------
 
   1 - SEL$58A6D7F6
   2 - SEL$58A6D7F6 / p@SEL$1
   3 - SEL$58A6D7F6 / p@SEL$1
   6 - SEL$58A6D7F6 / C@SEL$1
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("P"."CODE"="C"."CODE")
   3 - access("P"."CAT"=:CAT)
 
Column Projection Information (identified by operation id):
-----------------------------------------------------------
 
   1 - (#keys=1) "C"."SOME_COL"[NUMBER,22], "C"."SOME_COL"[NUMBER,22]
   2 - "P"."CODE"[CHARACTER,2]
   3 - "P".ROWID[ROWID,10]
   4 - "C"."CODE"[CHARACTER,2], "C"."SOME_COL"[NUMBER,22]
   5 - "C"."CODE"[CHARACTER,2], "C"."SOME_COL"[NUMBER,22]
   6 - "C"."CODE"[CHARACTER,2], "C"."SOME_COL"[NUMBER,22]
 
Note
-----
   - this is an adaptive plan

无索引

select /*+ no_index(P) */ 
       C.some_col
  from "P"
  join "C"
    on P.code = C.code
       and P.cat = :cat                                           
       and C.cat = :cat 
;

-----------------------------------------------------------------------------------------------
| id  | Operation              | name | rows  | Bytes | cost (%CPU)| time     | Pstart| Pstop |
-----------------------------------------------------------------------------------------------
|   0 | select statement       |      |   275G|  5127G|  1287K (58)| 00:00:51 |       |       |
|*  1 |  hash join             |      |   275G|  5127G|  1287K (58)| 00:00:51 |       |       |
|   2 |   partition LIST single|      | 60363 |   412K|  8152   (1)| 00:00:01 |   key |   key |
|   3 |    partition range all |      | 60363 |   412K|  8152   (1)| 00:00:01 |     1 |   316 |
|   4 |     table access full  | P    | 60363 |   412K|  8152   (1)| 00:00:01 |       |       |
|   5 |   partition LIST single|      |    41M|   510M|   539K  (1)| 00:00:22 |   key |   key |
|   6 |    partition range all |      |    41M|   510M|   539K  (1)| 00:00:22 |     1 |   316 |
|   7 |     table access full  | C    |    41M|   510M|   539K  (1)| 00:00:22 |       |       |
-----------------------------------------------------------------------------------------------
 
query block name / object alias (identified by operation id):
-------------------------------------------------------------
 
   1 - SEL$58A6D7F6
   4 - SEL$58A6D7F6 / p@SEL$1
   7 - SEL$58A6D7F6 / C@SEL$1
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("P"."CODE"="C"."CODE")
 
Column Projection Information (identified by operation id):
-----------------------------------------------------------
 
   1 - (#keys=1) "C"."SOME_COL"[NUMBER,22], "C"."SOME_COL"[NUMBER,22]
   2 - "P"."CODE"[CHARACTER,2]
   3 - "P"."CODE"[CHARACTER,2]
   4 - "P"."CODE"[CHARACTER,2]
   5 - "C"."CODE"[CHARACTER,2], "C"."SOME_COL"[NUMBER,22]
   6 - "C"."CODE"[CHARACTER,2], "C"."SOME_COL"[NUMBER,22]
   7 - "C"."CODE"[CHARACTER,2], "C"."SOME_COL"[NUMBER,22]
 
Hint Report (identified by operation id / query block name / object alias):
Total hints for statement: 1
---------------------------------------------------------------------------
 
   4 -  SEL$58A6D7F6 / p@SEL$1
           -  no_index(p)
 
Note
-----
   - this is an adaptive plan
4

1 回答 1

2

具有数千个子分区但少于一百万行的表可能有大量的空段空间，这将导致奇怪的优化器决策。运行以下查询以查看表和索引使用了多少空间：

select segment_name, sum(bytes)/1024/1024 mb, count(*) segment_count
from dba_segments
where segment_name in ('P', 'C', 'IX_P_CAT')
group by segment_name;

我试图在我的 19c 数据库上重新创建您的表，并且表“P”的每个分区消耗了 2.5 GB 的空间，尽管实际数据应该只需要几兆字节。每个系统的确切值都会有所不同，但我猜大多数系统都会有很大的值。Oracle 段通常是用于容纳超过一千行的重型数据结构；如果 Oracle 一次分配一个字节，性能会很差，因此它通常一次分配兆字节。但是如果你有 316 个子分区，那么这些兆字节加起来。

通常，选择大部分数据的最佳方式是使用全表扫描或全（子）分区扫描。但是，如果表有这么多浪费的空间，那么使用小索引并按 ROWID 查找每一行比完全扫描所有大部分为空的段更有效。

您可以通过使用更少的子分区、调整段分配设置或缩小表来解决问题，如下所示：

alter table p enable row movement;
alter table p shrink space;
begin
    dbms_stats.gather_table_stats(user, 'P');
end;
/
于 2021-10-28T05:56:42.587 回答