DBMS_STATS.AUTO_CASCADE 是否有效？

Question

根据文档，“使用常量 DBMS_STATS.AUTO_CASCADE 让 Oracle 确定是否要收集索引统计信息。”

但看起来 AUTO_CASCADE 总是会收集索引统计信息，即使它显然不需要。

这个测试用例演示了这个问题：

SQL> --Create table
SQL> create table statistics_test(a number, b number, c number, d number, e number);

Table created.

SQL> --Add 10 million rows (about 320 MB)
SQL> begin
  2     for i in 1 .. 100 loop
  3             insert into statistics_test
  4             select level, level, level, level, level from dual
  5             connect by level <= 100000;
  6     end loop;
  7     commit;
  8  end;
  9  /

PL/SQL procedure successfully completed.

SQL> --Create large index (AFTER the data load)
SQL> create index statistics_test_index on statistics_test(a,b,c,d,e);

Index created.

SQL> --Statistics are automatically gathered on index creation or rebuild.
SQL> select to_char(last_analyzed, 'HH24:MI:SS') last_analyzed
  2  from user_indexes
  3  where index_name = 'STATISTICS_TEST_INDEX';

LAST_ANA
--------
16:11:43

SQL> --Gather stats, use AUTO_CASCADE.
SQL> exec dbms_stats.gather_table_stats(user, 'STATISTICS_TEST', cascade => DBMS_STATS.AUTO_CASCADE);

PL/SQL procedure successfully completed.

SQL> --FAIL: Time changed - index was re-analyzed even though no data changed.
SQL> select to_char(last_analyzed, 'HH24:MI:SS') last_analyzed
  2  from user_indexes
  3  where index_name = 'STATISTICS_TEST_INDEX';

LAST_ANA
--------
16:12:12

SQL> --Let's assume that LAST_ANALYZED is lying.
SQL> --What I'm really worried about is the extra time spent analyzing indexes.
SQL> --Compare performance of AUTO_CASCADE, TRUE, and FALSE.
SQL> --FAIL: AUTO_CASCADE is as slow as TRUE.  It should be as fast as FALSE.
SQL> set timing on
SQL> exec dbms_stats.gather_table_stats(user, 'STATISTICS_TEST', cascade => DBMS_STATS.AUTO_CASCADE);

PL/SQL procedure successfully completed.

Elapsed: 00:00:20.80
SQL> exec dbms_stats.gather_table_stats(user, 'STATISTICS_TEST', cascade => true);

PL/SQL procedure successfully completed.

Elapsed: 00:00:19.58
SQL> exec dbms_stats.gather_table_stats(user, 'STATISTICS_TEST', cascade => false);

PL/SQL procedure successfully completed.

Elapsed: 00:00:08.27

我认为这并不重要，但我的系统使用默认值 ESTIMATE_PERCENT、METHOD_OPT、DEGREE、GRANULARITY、CASCADE 和 NO_INVALIDATE。我可以在 10.2.0.4、11.2.0.1、11.2.0.3 和 12.2.0.1 上重现这个问题。

这看起来像是一个错误，但我在 My Oracle Support 上找不到任何内容。奇怪的是，具有重大性能影响的默认设置会被完全破坏这么长时间。

我在这里错过了什么吗？

Answer 1

您确定索引已被分析 - 还是只是DBMS_STATS更新了LAST_ANALYZED时间戳？找出它的最佳方法是跟踪执行分析的过程。