Eliminating a Cross Join in SQL

Question

I am trying to optimize a query that contains a cross join. I have large query that I proceed to cross join with a derived table.

Would it improve the speed of the query by turning the derived table into a view? Or even capturing that information in a permanent table?

Here is my query

SELECT  VIEWER_ID, 
        QUESTION_ID, 
        ANSWER_ID, 
        sum(ANSWER_SCORE) AS ANSWER_SCORE_SUMMED
FROM(SELECT  cr.COMMUNICATIONS_ID AS ANSWER_ID, 
        cr.CONSUMER_ID as VIEWER_ID,
        nc.PARENT_COMMUNICATIONS_ID AS QUESTION_ID,
        case when  cr.CONSUMER_ID= nc.SENDER_CONSUMER_ID then 3*((24/(((UNIX_TIMESTAMP(NOW())-UNIX_TIMESTAMP(cal.LAST_MOD_TIME)+3600)/3600))*(ces.EXPERT_SCORE * cirm.CONSUMER_RATING) + (12.5 * scs.SIMILARITY)* (1 - EXP(-0.5 * (cal.TIPS_AMOUNT / ATV.AVG_TIPS)) + .15)))
            else ((24/(((UNIX_TIMESTAMP(NOW())-UNIX_TIMESTAMP(cal.LAST_MOD_TIME)+3600)/3600))*(ces.EXPERT_SCORE * cirm.CONSUMER_RATING) + (12.5 * scs.SIMILARITY)* (1 - EXP(-0.5 * (cal.TIPS_AMOUNT / ATV.AVG_TIPS)) + .15)))
        end as ANSWER_SCORE
FROM (SELECT 234 AS CONSUMER_ID, 
             ACTION_LOG_ID, 
            COMMUNICATIONS_ID 
     FROM consumer_action_log 
     WHERE COMM_TYPE_ID=4) AS cr
JOIN network_communications AS nc 
    ON cr.COMMUNICATIONS_ID=nc.COMMUNICATIONS_ID
JOIN consumer_action_log AS cal 
    ON cr.ACTION_LOG_ID=cal.ACTION_LOG_ID
JOIN communication_interest_mapping AS cim 
    ON nc.PARENT_COMMUNICATIONS_ID=cim.COMMUNICATION_ID
JOIN consumer_interest_rating_mapping AS cirm 
    ON cr.CONSUMER_ID=cirm.CONSUMER_ID
    AND cim.CONSUMER_INTEREST_EXPERT_ID=cirm.CONSUMER_INTEREST_ID
JOIN consumer_expert_score AS ces 
    ON nc.SENDER_CONSUMER_ID=ces.CONSUMER_ID
    AND cim.CONSUMER_INTEREST_EXPERT_ID=ces.CONSUMER_EXPERT_ID
JOIN survey_customer_similarity AS scs 
    ON cr.CONSUMER_ID=scs.CONSUMER_ID_2 
    AND cal.SENDER_CONSUMER_ID=scs.CONSUMER_ID_1 
    OR cr.CONSUMER_ID=scs.CONSUMER_ID_1 
    AND cal.SENDER_CONSUMER_ID=scs.CONSUMER_ID_2
CROSS JOIN
    (
        SELECT AVG(cal.TIPS_AMOUNT) AS AVG_TIPS
        FROM CONSUMER_ACTION_LOG AS cal
        JOIN (SELECT 234 AS CONSUMER_ID, 
                     ACTION_LOG_ID, 
                     COMMUNICATIONS_ID 
              FROM consumer_action_log 
              WHERE COMM_TYPE_ID=4) AS cr 
        ON cal.SENDER_CONSUMER_ID=cr.consumer_id
    ) ATV) AS ASM
GROUP BY ANSWER_ID
ORDER BY ANSWER_SCORE_SUMMED DESC;

It is a long query, so no need to read it all. The gist is simply that there is a cross join. I am new to sql, but I have been told that cross joins slow down speed.

Answer 1

As such, your cross join is not much of a deal, because the second inline query only returns one row.

The cross joins "slow down speed" in the same way as loading your car slows down speed.

It does of course, but if you need the things moved you load them into a car, and if you need a cartesian product, you do a cross join.

Answer 2

If your question is if Cross Joins slower ?(than what ? ;) ) , then the simplified answer is yes. Of course this is a very general answer to a general question. But for you, you need to quantify how slow.

For all practical purposes, your query could be as fast as it could be. So to validate, run explain plan on your query ( I am an oracle guy, but I assume running explain plan in mysql is not that different. Link http://dev.mysql.com/doc/refman/5.0/en/explain.html ).

Then create a temporary table and replace those in your query and check the plan again.

Note: If you find your explain plan for your query good enough, you do not have to waste time on further analysis.