Union as sub query MySQL

Question

I'm wanting to optimize a query using a union as a sub query. Im not really sure how to construct the query though. I'm using MYSQL 5

Here is the original query:

SELECT  Parts.id 
FROM    Parts_Category, Parts 
    LEFT JOIN Image ON Parts.image_id = Image.id 
WHERE 
( 
    (
     Parts_Category.category_id = '508' OR 
     Parts_Category.main_category_id ='508'
    ) AND 
    Parts.id = Parts_Category.Parts_id 
) AND 
Parts.status = 'A' 
GROUP BY 
    Parts.id

What I want to do is replace this ( (Parts_Category.category_id = '508' OR Parts_Category.main_category_id ='508' ) part with the union below. This way I can drop the GROUP BY clause and use straight col indexes which should improve performance. Parts and parts category tables contains half a million records each so any gain would be great.

(
    SELECT * FROM
    (
     (SELECT Parts_id FROM Parts_Category WHERE category_id = '508') 
     UNION 
     (SELECT Parts_id FROM Parts_Category WHERE main_category_id = '508')
    )
    as Parts_id
)

Can anybody give me a clue on how to re-write it? I've tried for hours but can't get it as I'm only fairly new to MySQL.

Answer 1

SELECT  Parts.id
FROM    (
        SELECT  parts_id
        FROM    Parts_Category
        WHERE   Parts_Category.category_id = '508'
        UNION
        SELECT  parts_id
        FROM    Parts_Category
        WHERE   Parts_Category.main_category_id = '508'
        ) pc
JOIN    Parts
ON      parts.id = pc.parts_id
        AND Parts.status = 'A'
LEFT JOIN
        Image
ON      image.id = parts.image_id

Note that MySQL can use Index Merge and you can rewrite your query as this:

SELECT  Parts.id
FROM    (
        SELECT  DISTINCT parts_id
        FROM    Parts_Category
        WHERE   Parts_Category.category_id = '508'
                OR Parts_Category.main_category_id = '508'
        ) pc
JOIN    Parts
ON      parts.id = pc.parts_id
        AND Parts.status = 'A'
LEFT JOIN
        Image
ON      image.id = parts.image_id

, which will be more efficient if you have the following indexes:

Parts_Category (category_id, parts_id)
Parts_Category (main_category_id, parts_id)