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Why is the number of rows 0 in my PostgreSQL plan?

I have a query that equally joins two tables, TableA and TableB, using a nested loop. Because of the "equi" -join contraint, all rows returned as a result will therefore correspond to at least one row from each of these two tables. However, according to the plan (EXPLAIN ANALYZE), the actual row count is 0 from TableB, although the row is returned in the final result. How can real strings be considered equal to zero here?

Here is the execution plan:

=> 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

And the definitions of the two tables ... First, the product table:

=> \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)

Stock table:

=> \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)
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http://www.postgresql.org/docs/current/static/using-explain.html:

node . , . node, . , . , , node.

, ( int ), , products stock.

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