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优化器建设了一个直方图，约莫读了c_birth_country列4.8%的数据。取样是不确定的，因此意义不大。同样的数据，同样的两条语句‘‘ANALYZE TABLE tbl UPDATE HISTOGRAM …’’，假如用了取样，获得的直方图也许就纷歧样。
查询案例
统计直方图可以带来些什么？我们可以看个例子，这个例子顶用了直方图，在执行时刻上会有很大的差异。
情形：
- TPC-DS Benchmark with scale factor of 1
- Intel Core i7-4770
- Debian Stretch
- MySQL 8.0 RC1
- innodb_buffer_pool_size = 2G
- optimizer_switch = "condition_fanout_filter=on"
Query 90
查询如下：上午售卖的数目与晚上售卖的数目的比率。
- mysql> SELECT CAST(amc AS DECIMAL(15, 4)) / CAST(pmc AS DECIMAL(15, 4)) am_pm_ratio
- -> FROM (SELECT COUNT(*) amc
- -> FROM web_sales,
- -> household_demographics,
- -> time_dim,
- -> web_page
- -> WHERE ws_sold_time_sk = time_dim.t_time_sk
- -> AND ws_ship_hdemo_sk = household_demographics.hd_demo_sk
- -> AND ws_web_page_sk = web_page.wp_web_page_sk
- -> AND time_dim.t_hour BETWEEN 9 AND 9 + 1
- -> AND household_demographics.hd_dep_count = 2
- -> AND web_page.wp_char_count BETWEEN 5000 AND 5200) at,
- -> (SELECT COUNT(*) pmc
- -> FROM web_sales,
- -> household_demographics,
- -> time_dim,
- -> web_page
- -> WHERE ws_sold_time_sk = time_dim.t_time_sk
- -> AND ws_ship_hdemo_sk = household_demographics.hd_demo_sk
- -> AND ws_web_page_sk = web_page.wp_web_page_sk
- -> AND time_dim.t_hour BETWEEN 15 AND 15 + 1
- -> AND household_demographics.hd_dep_count = 2
- -> AND web_page.wp_char_count BETWEEN 5000 AND 5200) pt
- -> ORDER BY am_pm_ratio
- -> LIMIT 100;
- +-------------+
- | am_pm_ratio |
- +-------------+
- | 1.27619048 |
- +-------------+
- 1 row in set (1.48 sec)
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