除了使用相关查询来解决以下问题之外，还有更有效的方法吗？

Question

I have three database tables:

• product
• product_manufacturer
• product_manufacturer_warranties .

The product table has a one-to-one mapping with product_manufacturer and the product_id is stored in the product_manufacturer table. The product_manufactuer table has a one-to-many mapping with the product_manufacturer_warranties table.

I need to write a query that retrieves all columns from the product table and two other columns that can be used to determine if a valid join exists for product and product_manufacturer and product_manufacturer and product_manufactuer_warranties respectively.

I have written the following co-related query that can handle the above scenario:

select product.*, pm.web_id,
        (   SELECT count(*)
            FROM product_manufacturer_warranty pmw
            WHERE pm.web_id = pmw.product_manufacturer_id)
            AS total_warranties
from product
left join product_manufacturer pm on product.web_id = pm.product_id

I wonder if there is a better or more efficient way of achieving this using SQL on the PostgreSQL server.

Answer 1

Do the aggregation once, and join to the result:

select product.*, 
       pm.web_id,
       pmw.total_warranties
from product
  left join product_manufacturer pm on product.web_id = pm.product_id
  left join (   
     SELECT product_manufacturer_id, count(*) as total_warranties
     FROM product_manufacturer_warranty pmw
     group by product_manufacturer_id
  ) as pmw on pm.web_id = pmw.product_manufacturer_id)

Answer 2

Since you are only interested in bare existence , don't run a potentially much more expensive count() .
And while paging through the (potentially big) table, don't compute counts for all products, which would be much more expensive, yet.

This should give you optimal performance:

Paging up:

SELECT p.*
     , m.product_id IS NOT NULL AS has_maufacturer
     , EXISTS (
         SELECT FROM product_manufacturer_warranty w
         WHERE  w.product_manufacturer_id = m.web_id
         ) AS has_warranties
FROM  (
   SELECT *
   FROM   product
   WHERE  product_id > $max_product_id_of_last_page
   -- more filters here?
   ORDER  BY product_id
   LIMIT  $page_size
   ) p
LEFT   JOIN product_manufacturer m ON p.web_id = m.product_id

The query returns exactly what you asked for:

all columns from the product table and two other columns that can be used to determine if a valid join exists for product and product_manufacturer and product_manufacturer and product_manufactuer_warranties respectively.

• First select products of interest in the subquery p
• Then left-join to product_manufacturer . Cannot multiply rows, defined as one-to-one relationship!
• Then check for bare existence with EXISTS . Substantially cheaper if there can be many related rows. See:
  • PL/pgSQL checking if a row exists

Paging down accordingly:

SELECT p.*
     , m.product_id IS NOT NULL AS has_maufacturer
     , EXISTS (
         SELECT FROM product_manufacturer_warranty w
         WHERE  w.product_manufacturer_id = m.web_id
         ) AS has_warranties
FROM  (
   SELECT *
   FROM   product
   WHERE  product_id < $min_product_id_of_last_page
   -- more filters here?
   ORDER  BY product_id DESC
   LIMIT  $page_size
   ) p
LEFT   JOIN product_manufacturer m ON p.web_id = m.product_id
ORDER  BY product_id;

Adapt to your actual sort order. This is more tricky for multiple ORDER BY expressions. Either way, don't fall for LIMIT / OFFSET unless your table is trivially small. See:

• Optimize query with OFFSET on large table