Query optimization in MYSQL for a query with sub queries?
In an assignment, I was given a query and asked to optimize it. The query is:
SELECT
C.yearID as year,
name as teamName,
C.lgID as league,
D.cnt as totalBatters,
C.cnt as aboveAverageBatters
FROM
(SELECT
count(masterID) as cnt, A.yearID, A.teamID, A.lgID
FROM
(select
masterID,
teamID,
yearID,
lgID,
sum(AB),
sum(H),
sum(H) / sum(AB) as avg
FROM
batting
GROUP BY teamID , yearID , lgID , masterID) B, (select
teamID,
yearID,
lgID,
sum(AB),
sum(H),
sum(H) / sum(AB) as avg
FROM
batting
WHERE ab is not null
GROUP BY teamID , yearID , lgID) A
WHERE
A.avg >= B.avg AND A.teamID = B.teamID
AND A.yearID = B.yearID
AND A.lgID = B.lgID
GROUP BY teamID , yearID , lgID) C,
(SELECT
count(masterID) as cnt, yearID, teamID, lgID
FROM
batting
WHERE ab is not null
GROUP BY yearID , teamID , lgID) D,
teams
WHERE
C.cnt / D.cnt >= 0.75
AND C.yearID = D.yearID
AND C.teamID = D.teamID
AND C.lgID = D.lgID
AND teams.yearID = C.yearID
AND teams.lgID = C.lgID
AND teams.teamID = C.teamID
I was wondering what can be done to optimize this? I am new to this concept and a little confused on how to proceed. In General, how to optimize sub queries which have select statements in it?
In General, how to optimize sub queries which have select statements in it?
Here are some ideas to get you started. I will respect the fact that it is an assignment and in the end, you will gain much better understanding of the SQL queries by going through it yourself and learning along the way.
I would hope that the assignment included a sort of data set that you could import into MySQL so that you can run the query as you make changes and notice the impact on the execution plan and overall performance.
Aliases
Before even thinking of optimizing, perhaps you can look at how to make the code easier to read, understand and maintain. The subqueries behave in a manner similar to regular tables, and as such, they should be given aliases/names that make sense as to what the data set means.
They are aliased B , A , C and D , which look almost intentionally named to obfuscate, but actually you would be surprised how often you see poor naming/aliasing in real-life, production SQL code.
Try looking (and if you can, running) each subquery on its own, look at the fields and what they mean, then replace the alias with a good name, and update the references in the different columns accordingly. This will optimize the query for better clarity, and ultimately, maintainability.
JOIN s
Hopefully by the time of this assignment, the various types of JOIN operations have already been covered. If not, here is a good summary from a StackOverflow answer. There is a large number of other resources covering the ins and outs of JOIN , including a good post on TechOnTheNet.
Let's strip down the subqueries and look at how the query as a whole is structured. I replaced the logic by comments to make it more apparent:
SELECT
--columns
FROM
(
SELECT
--columns
FROM
(
select
--columns
FROM batting
) B,
(
select
--columns
) A
WHERE
--some comparisons of averages
) C,
(
SELECT
--columns
FROM batting
) D,
teams
WHERE
--a filter based on a calculation
C.cnt / D.cnt >= 0.75
--um... what is all this stuff doing down here?
--shouldn't those be in a JOIN?
AND C.yearID = D.yearID
AND C.teamID = D.teamID
AND C.lgID = D.lgID
AND teams.yearID = C.yearID
AND teams.lgID = C.lgID
AND teams.teamID = C.teamID
Do you notice anything peculiar or anything that looks strange? If you have not read this before, I strongly recommend reading Bad habits to kick : using old-style JOINs by Aaron Bertrand.
After reading it, look at the skeleton of this query again, and the improvements you can make by using modern JOIN should stand out. This will make the query that much more optimal in clarity and maintainability.
Consistency in keywords case
Another way you can make it more readable is by using consistent capitalization of keywords. As it is, it's about 50/50 using CAPITAL CASE and small case . It may seem insignificant for just one script, but when this inconsistency is spread over a whole code base, it can get really irritating for the next person who will have to develop in it and maintain it.
Performance
So, by now if you've applied everything so far, the code should be a lot easier to decipher. As far as performance is concerned, there are two things that stand out to me as detrimental. There are lots of aggregates, and consequently, lots of GROUP BY s.
Look at each subquery by itself first, and at each aggregation. Look at how each of the fields is used in the context of the whole query. See which ones you might be able to remove, that perhaps the person who wrote the query originally thought they were going to need, but ended up not using and forgot to remove them.
Try the same tactic for the GROUP BY fields, which are every field you are not aggregating that is included in a query that has one or more aggregate operations. GROUP BY can get pretty expensive, and that is compounded by the fact the derived subqueries also have GROUP BY .
There are a few other tricks you can try, that are more advanced and may improve execution at the compromise of I/O, for example extracting one or more subqueries' result sets into temporary tables, which would free up locks on the primary tables.
Optimizations like these might not always necessarily improve the execution speed per se, but in a production environment with database servers under load, often speed is not the primary concern for optimization, but rather "lightness" (or as small of a load footprint on the servers as possible) is often much more valuable than raw speed which ends up using more resources.
I hope this helps!
链接地址: http://www.djcxy.com/p/62458.html上一篇: 排名前20位的查询
下一篇: 使用子查询查询MYSQL中的查询优化？