多次从一个相当大的表中提取数据的速度很慢

Question

I currently am offering a free test taking feature on my site that each user may take one of the tests and see their high score or whatever.

I have a foreach that goes through each test and checks to see what the highest grade is each student has gotten. The table holding the saved tests is pushing towards 15,000 rows of data.

here is roughly what I have:

foreach(testList as $test){

   SELECT saved_grade FROM system_saved 
   WHERE account_id = {{account_id}}
   AND test_id = {{test_id}}
   Order By saved_grade DESC
   LIMIT 0, 1

}

Its requiring each test to have to scan the whole table to find the highest grade everytime.

• Is there a better way of pulling the highest grade for a user?
• Would Indexing the table help this (I don't really understand this + new data is being put in everyday, I'm not sure if indexing is relevant)
• Is there anything else you can think of to help this run faster? (It currently takes about 20 seconds to load)

Answer 1

You dont need to select every test individually. Assuming account_id is unique for every person doing the test, and that saved_grade is not a string you should be able to do this

SELECT MAX(saved_grade) 
FROM system_saved 
WHERE account_id = {{account_id}}
AND test_id = {{test_id}}

Answer 2

Don't loop; use one query. This will retrieve the best grade for each student, ordered by student, then by test.

SELECT account_id, test_id, MAX(saved_grade) as `best_grade`
FROM system_saved
GROUP BY account_id DESC, test_id DESC

To get the best grade on any test by that student, use this instead:

SELECT account_id, MAX(saved_grade) as `best_grade`
FROM system_saved
GROUP BY account_id DESC

EDIT: If you want to know the test on which they got the highest score, you can do this:

SELECT account_id, test_id, saved_grade
FROM system_saved WHERE (account_id, saved_grade) IN (
    SELECT account_id, MAX(saved_grade) as `best_grade`
    FROM system_saved
    GROUP BY account_id DESC)

Answer 3

You maybe should use max() :

foreach(testList as $test){

   SELECT max(saved_grade) FROM system_saved 
    WHERE account_id = {{account_id}}
      AND test_id = {{test_id}}
}

Answer 4

The solution you are looking for is GROUP BY and agggregation function:

SELECT account_id, test_id, max( saved_grade ) as highest_grade
FROM system_saved 
GROUP BY account_id, test_id

Also create index for (account_id, test_id, saved_grade) to gain better performance than on unindexed data.