My question is this: From where does Oracle get the estimated cardinality of 406 in this case, and how can I improve its accuracy, so that the estimate is more in line of what really happens during query execution? Update: Here is a snippet of a 10053 trace I ran on the ...
oracle histograms Purpose of Histograms When Oracle Database Creates Histograms Cardinality Algorithms When Using Histograms Frequency Histograms HeightBalanced Histograms (Legacy) Hybrid Histograms 上传者:xblvesting时间:2018-11-09 oracle explain plan
What an astonishing result: Not only Oracle will try now to access all rows of the fact table by single-block random I/O, which by itself can be a disaster for larger real-life fact tables, in particular when dealing with Exadata features like Smart Scans which are only possible with mul...
The result, along with a few others, appears in slide 11. Oracle’s estimate at this point is one row; but the actual number of rows is 42, which means that the big index range scan required by the plan would be executed 42 times, rather than once – and that’s a pretty big ...
《PostgreSQL 统计信息pg_statistic格式及导入导出dump_stat - 兼容Oracle》 4、再次查看执行计划,选择性正确了。 postgres=# explain select * from a where f1(id)=1; QUERY PLAN --- Index Scan using idx_a_1 on a (cost=0.43..2.65 rows=1 width=4) Index Cond: (f1(id) = 1) (2 rows) pos...
What an astonishing result: Not only Oracle will try now to access all rows of the fact table by single-block random I/O, which by itself can be a disaster for larger real-life fact tables, in particular when dealing with Exadata features like Smart Scans which are only possible with mul...
《PostgreSQL 11 preview - 强制auto prepared statment开关(自动化plan cache)(类似Oracle cursor_sharing force)》 《[未完待续] PostgreSQL PRO 特性 - AQO(机器学习执行计划优化器)》 《PostgreSQL pg_stat_reset清除track_counts的隐患》 《PostgreSQL 统计信息pg_statistic格式及导入导出dump_stat - 兼容Oracle》...
文章被收录于专栏:Oracle数据库技术 概述 本文将介绍在11gR2的版本上推出了基数反馈(Cardinality Feedback 以后简称CFB)功能,通过这个特性, 对于某些查询在第一次执行时,如果CBO发现根据统计信息估算出的基数(Computed cardinality)和SQL执行时的实际值差距很大的情况发生时, 在SQL下次执行时,会根据实际值调整基数,重新...