MERGE 路径

如果 MERGE 语句及其表满足 MERGE 优化中所述的标准，则 Vertica 会为该语句准备优化的查询计划。

使用 EXPLAIN 关键字可以确定 Vertica 是否可以为给定的 MERGE 语句生成优化的查询计划。如果可以进行优化，EXPLAIN 生成的输出将包含 [Semi] 路径，如以下示例片段所示：

...
Access Path:
+-DML DELETE [Cost: 0, Rows: 0]
|  Target Projection: public.A_b1 (DELETE ON CONTAINER)
|  Target Prep:
|  Execute on: All Nodes
| +---> JOIN MERGEJOIN(inputs presorted) [Semi] [Cost: 6, Rows: 1 (NO STATISTICS)] (PATH ID: 1)
         Inner (RESEGMENT)
| |      Join Cond: (A.a1 = VAL(2))
| |      Execute on: All Nodes
| | +-- Outer -> STORAGE ACCESS for A [Cost: 2, Rows: 2 (NO STATISTICS)] (PATH ID: 2)
...

相反，如果 Vertica 无法创建优化计划，EXPLAIN 生成的输出将包含 RightOuter 路径：

...
Access Path: +-DML MERGE
 |  Target Projection: public.locations_b1
 |  Target Projection: public.locations_b0
 |  Target Prep:
 |  Execute on: All Nodes
 | +---> JOIN MERGEJOIN(inputs presorted) [RightOuter] [Cost: 28, Rows: 3 (NO STATISTICS)] (PATH ID: 1) Outer (RESEGMENT) Inner (RESEGMENT)
 | |      Join Cond: (locations.user_id = VAL(2)) AND (locations.location_x = VAL(2)) AND (locations.location_y = VAL(2))
 | |      Execute on: All Nodes
 | | +-- Outer -> STORAGE ACCESS for <No Alias> [Cost: 15, Rows: 2 (NO STATISTICS)] (PATH ID: 2)
...