Vldb Statistics Gathering Strategy

VLDB Statistics Gathering StrategySomething You Need to Know But Maybe You Don’t

About Me • Technical Director @ Enmo Tech • ACOUG Co-founder, President • Interests: – My wife and my son – Database technology (all related) – World of Warcraft (online game) • http://www.enmotech.com • http://www.acoug.org • http://www.dbform.com云和恩墨成就所托

Statistics Gathering• Manually – dbms_stats.gather_table_stats• Automatically – 10g: DBMS_SCHEDULER – 11g: DBMS_AUTO_TASK_ADMIN云和恩墨成就所托

What is Histogram1210 8 Value 1 Value 2 6 Value 3 4 Value 4 Value 5 2 0 Column 1 Column 2 Column 3 Column 4云和恩墨成就所托

What is Histogram• frequency 800 700 700 600 500 500 Distinct Value 1 400 Distinct Value 2 400 Distinct Value 3 300 200 Distinct Value 4 200 Distinct Value 5 100 100 0 Records#云和恩墨成就所托

What is Histogram• height balanced 120 1 2 3 4 5 5 5 6 7 8 Bucket 1 100 Bucket 2 80 Bucket 3 Bucket 4 60 Bucket 5 Bucket 6 40 Bucket 7 20 Bucket 8 Bucket 9 0 Bucket 10 Records#云和恩墨成就所托

When We DON’T Need Histogram?• 此列不用于查询• 列值分布平均• 不需要多个执行计划云和恩墨成就所托

Histogram Impact COST!!云和恩墨成就所托

How Many Choices dba_tab_modific• method_opt=> ations – for columns size skewonly [column_name] – for columns size auto [column_name] COL_USAGE$ – for columns size repeat [column_name] – for columns size 1 [column_name] – for all columns – for all indexed columns云和恩墨成就所托

How Should We Do?• Step 1 – method_opt=>FOR ALL COLUMNS SIZE 1• Step 2 (repeat) – method_opt=>FOR COLUMNS SIZE AUTO [COLUMN_NAME]• Step 3 – Using atomatic statistics gathering job – (10g) exec DBMS_STATS.SET_PARAM (METHOD_OPT, FOR ALL COLUMNS SIZE REPEAT); – (11g) exec DBMS_STATS.SET_GLOBAL_PREFS(METHOD_OPT,FOR ALL COLUMNS SIZE REPEAT);云和恩墨成就所托

What is Granularity• 只针对分区表• ALL• AUTO (默认值)• DEFAULT = GLOBAL AND PARTITION• GLOBAL• GLOBAL AND PARTITION• PARTITION• SUBPARTITION云和恩墨成就所托

Global Statistics• 收集统计信息 exec dbms_stats.gather_table_stats(KAMUS, TAB_PART’);• 聚合统计信息 – 降低开销 exec dbms_stats.gather_table_stats(‘KAMUS’, ‘TAB_PART’, GRANU LARITY => SUBPARTITION); TAB_PART表，20万记录收集统计信息：Consistent Read = 23432 聚合统计信息：Consistent Read = 12036云和恩墨成就所托

坏的情况• 新加载数据• 收集有数据变化的子分区统计信息 exec dbms_stats.gather_table_stats(KAMUS,TAB_PART, GRANU LARITY => SUBPARTITION, PARTNAME => P_20111206_BEIJING);• 聚合统计信息正确• 列统计信息呢？NDV云和恩墨成就所托

还有坏的情况• 增加子分区 ALTER TABLE TAB_PART ADD PARTITION P_20111208 VALUES LESS THAN (20111209);• 新加载数据• 收集有数据变化的子分区统计信息 exec dbms_stats.gather_table_stats(KAMUS,TAB_PART, GRANULAR ITY => SUBPARTITION, PARTNAME => P_20111208_BEIJING);• 聚合统计信息呢？• 列统计信息呢？云和恩墨成就所托

还有更坏的情况• 新创建的分区表没有任何数据• 收集分区统计信息 exec dbms_stats.gather_table_stats(KAMUS,TAB_PART’);• 新加载数据• 聚合？ • 收集子分区的统计信息？ • 收集全部子分区的统计信息？ • 收集全部分区的统计信息？云和恩墨成就所托

Conclusion• 如果只选择收集SUBPARTITION统计信息，那么要确认聚合统计信息会正确生成。云和恩墨成就所托

问答云和恩墨成就所托

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Vldb Statistics Gathering Strategy

by Leyi Zhang on Dec 16, 2011

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