database - 为什么我的PostgreSQL计划中的行计数为0？

我有一个查询，它使用嵌套循环将两个表TableA和TableB等价联接。由于存在“等价”联接约束，因此结果中返回的所有行将对应于这两个表中的至少一个行。但是，根据计划(EXPLAIN ANALYZE)，即使在最终结果中返回了一行，实际从TableB开始的行计数也为0。这里实际行数如何等于零？

这是执行计划:

=> explain analyze select p.id, p.title, s.count from products p, stock s where p.id = s.p_id and s.w_id = 6 and p.type = 9 and s.count > 0 order by p.title;
                                                          QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
 Sort  (cost=42.42..42.42 rows=2 width=36) (actual time=0.198..0.199 rows=1 loops=1)
   Sort Key: p.title
   Sort Method: quicksort  Memory: 25kB
   ->  Nested Loop  (cost=0.00..42.41 rows=2 width=36) (actual time=0.170..0.181 rows=1 loops=1)
         ->  Seq Scan on products p  (cost=0.00..9.25 rows=4 width=32) (actual time=0.068..0.106 rows=4 loops=1)
               Filter: (type = 9)
         ->  Index Scan using stock_pk on stock s  (cost=0.00..8.28 rows=1 width=8) (actual time=0.015..0.015 rows=0 loops=4)
               Index Cond: ((w_id = 6) AND (p_id = p.id))
               Filter: (count > 0)
 Total runtime: 0.290 ms

还有两个表定义...产品表首先:

=> \d products
           Table "public.products"
 Column |          Type          | Modifiers
--------+------------------------+-----------
 id     | integer                | not null
 title  | character varying(100) |
 type   | integer                |
 price  | double precision       |
 filler | character(500)         |
Indexes:
    "products_pkey" PRIMARY KEY, btree (id)
    "products_type_idx" btree (type)
Referenced by:
    TABLE "orderline" CONSTRAINT "orderline_p_id_fkey" FOREIGN KEY (p_id) REFERENCES products(id)
    TABLE "stock" CONSTRAINT "stock_p_id_fkey" FOREIGN KEY (p_id) REFERENCES products(id)

库存表:

=> \d stock
     Table "public.stock"
 Column |  Type   | Modifiers
--------+---------+-----------
 w_id   | integer | not null
 p_id   | integer | not null
 count  | integer |
Indexes:
    "stock_pk" PRIMARY KEY, btree (w_id, p_id)
    "stock_p_id_idx" btree (p_id)
Foreign-key constraints:
    "stock_p_id_fkey" FOREIGN KEY (p_id) REFERENCES products(id)
    "stock_w_id_fkey" FOREIGN KEY (w_id) REFERENCES warehouses(id)

最佳答案

内部索引扫描的实际行是每次调用返回的平均行数。

看http://www.postgresql.org/docs/current/static/using-explain.html:



我不确定它是如何四舍五入的(我在平均后猜测是最接近的整数)，但是可能是products中的大多数行在stock中没有相应的行。