Cost 和 Row 路径

以下 EXPLAIN 输出显示 Cost 运算符：

 Access Path: +-SELECT  LIMIT 10 [Cost: 370, Rows: 10] (PATH ID: 0)
 |  Output Only: 10 tuples
 |  Execute on: Query Initiator
 | +---> SORT [Cost: 370, Rows: 544] (PATH ID: 1)
 | |      Order: customer_dimension.customer_name ASC
 | |      Output Only: 10 tuples
 | |      Execute on: Query Initiator
 | | +---> STORAGE ACCESS for customer_dimension [Cost: 331, Rows: 544] (PATH ID: 2)
 | | |      Projection: public.customer_dimension_DBD_1_rep_vmartdb_design_vmartdb_design_node0001
 | | |      Materialize: customer_dimension.customer_state, customer_dimension.customer_name
 | | |      Filter: (customer_dimension.customer_gender = 'Male')
 | | |      Filter: (customer_dimension.customer_state = ANY (ARRAY['MA', 'NH']))
 | | |      Execute on: Query Initiator

Row 运算符是优化器估计查询将要返回的行数。数字后面的字母表示度量单位（K=千、M=百万、B=十亿、T=万亿），因此，以下查询的输出指示要返回的行数为 50 K。

=> EXPLAIN SELECT customer_gender FROM customer_dimension;
 Access Path:
 +-STORAGE ACCESS for customer_dimension [Cost: 17, Rows: 50K (3 RLE)] (PATH ID: 1)
 |  Projection: public.customer_dimension_DBD_1_rep_vmartdb_design_vmartdb_design_node0001
 |  Materialize: customer_dimension.customer_gender
 |  Execute on: Query Initiator

STORAGE ACCESS 路径中对 (3 RLE) 的引用表明优化器估计存储访问运算符返回的行数为 50K。由于列为运行长度编码 (RLE)，因此返回的 RLE 实际行数仅为三行：

  • 1 行用于女性

  • 1 行用于男性

  • 1 行用于表示未知 (NULL) 性别