GROUPBY HASH 查询计划

下面是一个关于 GROUPBY HASH 操作在 EXPLAIN 输出中的样式的示例。

=> EXPLAIN SELECT COUNT(DISTINCT annual_income)
     FROM customer_dimension
     WHERE customer_region='NorthWest';

该输出显示优化器选择了效率更低的 GROUPBY HASH 方法，这意味着投影未在 annual_income 列上预分类。如果此类投影可用，优化器将选择 GROUPBY PIPELINED 算法。

Access Path:
+-GROUPBY NOTHING [Cost: 256, Rows: 1 (NO STATISTICS)] (PATH ID: 1)
|  Aggregates: count(DISTINCT customer_dimension.annual_income)
| +---> GROUPBY HASH (LOCAL RESEGMENT GROUPS) [Cost: 253, Rows: 10K (NO STATISTICS)] (PATH ID: 2)
| |      Group By: customer_dimension.annual_income
| | +---> STORAGE ACCESS for customer_dimension [Cost: 227, Rows: 50K (NO STATISTICS)] (PATH ID: 3)
| | |      Projection: public.customer_dimension_super
| | |      Materialize: customer_dimension.annual_income
| | |      Filter: (customer_dimension.customer_region = 'NorthWest'
...