Question

有两张表格超过 2000万的记录。数据库版本为Oracle 11.2.0.3.13标准版

SQL> select count(*) from atsd_tra_trade_print;

  COUNT(*)
----------
  20000000

SQL> select count(*) from atsd_mob_allocation_pos;

  COUNT(*)
----------
  20551780

我们对表运行以下查询。 查询结构类似。查询用于分页。 两个查询的排序列都已编入索引，并且不包含任何NULL值。查询和相应的autotrace输出/计划如下所示。

索引 INDX_ATTP_6 在ATSD_TRA_TRADE_PRINT（ TRANSACT_TIME ）上创建

索引 INDX_AMAP_6 是在ATSD_MOB_SETTLEMENT_POS上创建的（ DATE_TIME ）

索引的基数如下

INDX_ATTP_6 - 5972

INDX_AMAP_6 - 5974

上述两列的数据类型为 varchar2 。

在刷新buffer_cache 后，两个查询都运行。还收集了模式统计数据。

SELECT * FROM ( SELECT T2.*, ROWNUM AS RN FROM ( SELECT * FROM ATSDN_TRA_TRADE_PRINT T WHERE LATEST = 1 ORDER BY TRANSACT_TIME) T2) WHERE RN > 0 and ROWNUM <= 28;

输出：

28 rows selected. Elapsed: 00:00:00.16 Execution Plan ---------------------------------------------------------- Plan hash value: 3823692003 ------------------------------------------------------------------------- ------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------- ------------------------------- | 0 | SELECT STATEMENT | | 28 | 60060 | 8 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | |* 2 | VIEW | | 28 | 60060 | 8 (0)| 00:00:01 | | 3 | COUNT | | | | | | | 4 | VIEW | | 28 | 59696 | 8 (0)| 00:00:01 | |* 5 | TABLE ACCESS BY INDEX ROWID| ATSD_TRA_TRADE_PRINT | 7488K| 3234M| 8 (0)| 00:00:01 | | 6 | INDEX FULL SCAN | INDX_ATTP_6 | 57 | | 3 (0)| 00:00:01 | ------------------------------------------------------------------------- ------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM<=28) 2 - filter("RN">0) 5 - filter("LATEST"=1) Statistics ---------------------------------------------------------- 571 recursive calls 0 db block gets 948 consistent gets 49 physical reads 0 redo size 18586 bytes sent via SQL*Net to client 535 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 29 sorts (memory) 0 sorts (disk) 28 rows processed

上述查询按预期快速执行。但是对另一个表的查询会导致执行时间过长。我能看到的唯一观察是有大量的物理读取。两个查询执行计划看起来都很相似。

SELECT * FROM ( SELECT T2.*, ROWNUM as RN FROM ( SELECT * FROM ATSD_MOB_ALLOCATION_POS AL WHERE LATEST = 1 ORDER BY DATE_TIME) T2) WHERE RN > 0 and ROWNUM <= 28;

输出：

28 rows selected. Elapsed: 00:01:52.28 Execution Plan ---------------------------------------------------------- Plan hash value: 884170602 ------------------------------------------------------------------------- ---------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------- ---------------------------------- | 0 | SELECT STATEMENT | | 28 | 38892 | 7 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | |* 2 | VIEW | | 28 | 38892 | 7 (0)| 00:00:01 | | 3 | COUNT | | | | | | | 4 | VIEW | | 28 | 38528 | 7 (0)| 00:00:01 | |* 5 | TABLE ACCESS BY INDEX ROWID| ATSD_MOB_ALLOCATION_POS | 14M| 5863M| 7 (0)| 00:00:01 | | 6 | INDEX FULL SCAN | INDX_AMAP_6 | 39 | | 4 (0)| 00:00:01 | ------------------------------------------------------------------------- ---------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM<=28) 2 - filter("RN">0) 5 - filter("LATEST"=1) Statistics ---------------------------------------------------------- 281 recursive calls 0 db block gets 335395 consistent gets 332763 physical reads 116 redo size 13582 bytes sent via SQL*Net to client 535 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 17 sorts (memory) 0 sorts (disk) 28 rows processed

有人可以告诉我为什么会这样。在此先感谢... :)

Answer 1

问题在于索引结构......当它以asc方式遍历索引INDX_AMAP_6时，它必须在满足过滤条件的行之前读取500万条记录＆＃34; LATEST = 1＆＃34 ;。排序列和文件管理器列上的复合索引解决了这个问题。

在atsd_mob_allocation_pos上创建索引AMAP_TEST1（最新，date_time）;

索引扫描导致大量物理读取（Oracle）

1 个答案: