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当你在成立直方图的时辰，MySQL server会将全部数据读到内存中，然后在内存中举办操纵，包罗排序。假如对一个很大的表成立直方图，也许会有风险将几百M的数据都读到内存中，但这是不明智的。为了规避这个风险，MySQL会按照给定的histogram_generation_max_mem_size的值计较该将几多行数据读到内存中。假如按照当前histogram_generation_max_mem_size的限定，MySQL以为只能读一部门数据，那么MySQL会举办取样。通过“sampling-rate”属性，可以调查到取样比率。
- mysql> SET histogram_generation_max_mem_size = 1000000;
- Query OK, 0 rows affected (0.00 sec)
- mysql> ANALYZE TABLE customer UPDATE HISTOGRAM ON c_birth_country WITH 16 BUCKETS;
- +----------------+-----------+----------+------------------------------------------------------------+
- | Table | Op | Msg_type | Msg_text |
- +----------------+-----------+----------+------------------------------------------------------------+
- | tpcds.customer | histogram | status | Histogram statistics created for column 'c_birth_country'. |
- +----------------+-----------+----------+------------------------------------------------------------+
- 1 row in set (0.22 sec)
- mysql> SELECT histogram->>'$."sampling-rate"'
- -> FROM information_schema.column_statistics
- -> WHERE table_name = "customer"
- -> AND column_name = "c_birth_country";
- +---------------------------------+
- | histogram->>'$."sampling-rate"' |
- +---------------------------------+
- | 0.048743243211626014 |
- +---------------------------------+
- 1 row in set (0.00 sec)
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