I have this query for load user stream in my app , is it too hard if we have 10000 matched row in 'follow' ?
SELECT *
FROM post
WHERE user_id
IN (SELECT follow_id
FROM follow
WHERE id='$some_id')
AND type='accepted'
ORDER BY id DESC LIMIT $page , 20
Syntactically your code looks correct.. I don't see any errors so then if you're talking about efficiency I would join the tables and include the second filter on the JOIN
SELECT p.*
FROM post p
JOIN follow f
ON f.follow_id = p.user_id
AND f.id = '$some_id'
WHERE p.type = 'accepted'
ORDER BY p.id DESC LIMIT $page , 20
MySQL handles large sets of data a lot better through a join than with an IN()...
Think of it this way.. because the IN() can have pretty much anything inside of it, MySQL has to check it with everything returned for each row... instead of checking once when you JOIN..
With that many returning, I have a feeling a Join might be more efficient
SELECT *
FROM post p Join
follow f On p.user_id = f.follow_id
WHERE f.id='$some_id'
AND p.type='accepted'
ORDER BY p.id DESC LIMIT $page , 20
Related
I've got my tables posts and user_unread_posts.
In posts, all the posts of a forum are saved, and in user_unread_posts all posts are saved which are read by a user.
user_undread_postslooks like this:
id uid pid
Now I want to allow users to open a random post which they haven't read. I've tried something like
SELECT * FROM posts
LEFT JOIN user_unread_posts uup
ON uup.pid=posts.id
WHERE uup.uid<>1
ORDER BY RAND()
LIMIT 1
(Whilst 1 is a placeholder UID)
But it doesn't work, like it should work, it return posts too, which are read... How can I fix that?
SELECT *
FROM posts
WHERE id NOT IN
(
SELECT pid
FROM user_unread_posts uup
WHERE uid = $myuserid
)
ORDER BY
RAND()
LIMIT 1
You wanted to use IS NULL with the LEFT JOIN. Using <> turns the LEFT JOIN into an INNER JOIN because NULL can never match the <> operator.
Here is a corrected query:
SELECT * FROM posts
LEFT JOIN user_unread_posts uup
ON uup.pid=posts.id
WHERE uup.uid IS NULL
ORDER BY RAND()
LIMIT 1
I'm trying to LIMIT the database output with 5, but it's only selecting 4. And yes, I'm sure there's more then 4.
$sql = "SELECT * FROM lookup
INNER JOIN lookupCategories
ON lookup.FK_lookupCategory=lookupCategories.lookupCategoriesId
INNER JOIN lookupType
ON lookup.FK_lookupType=lookupType.lookupTypeId
INNER JOIN authentication
ON lookup.FK_lookupUserId=authentication.userId
WHERE FK_lookupUserId='$_GET[id]'
ORDER BY lookupId DESC
LIMIT 5";
My code looks like this. What could be wrong? :)
Thanks in advance.
I would suggest that there are only 4 entries that match - to prove or disprove this, try your query without the LIMIT and see how many results are returned:
"SELECT COUNT(*) FROM lookup
INNER JOIN lookupCategories
ON lookup.FK_lookupCategory=lookupCategories.lookupCategoriesId
INNER JOIN lookupType
ON lookup.FK_lookupType=lookupType.lookupTypeId
INNER JOIN authentication
ON lookup.FK_lookupUserId=authentication.userId
WHERE FK_lookupUserId='$_GET[id]';
I cannot figure out why this is not working. Basically, I am running a subquery to count all rows of p.songid WHERE trackDeleted=0. The subquery works fine when I execute it by itself, but when I implement I get "subquery returned more than 1 row".
SELECT u.username, u.id, u.score, s.genre, s.songid, s.songTitle, s.timeSubmitted, s.userid, s.insWanted, s.bounty,
(SELECT COUNT(p.songid)
FROM songs s
LEFT JOIN users u
ON u.id = s.userid
LEFT JOIN posttracks p
ON s.songid = p.songid
WHERE p.trackDeleted=0
GROUP BY s.timeSubmitted ASC
LIMIT 25)
AS trackCount
FROM songs s
LEFT JOIN users u
ON u.id = s.userid
LEFT JOIN posttracks p
ON s.songid = p.songid
WHERE paid=1 AND s.timeSubmitted >= ( CURDATE() - INTERVAL 60 DAY )
GROUP BY s.timeSubmitted ASC
LIMIT 25
Obviously, a sub-query can't return more than one row, as this makes no sense. You only expect one value to be returned - COUNT(p.songid) - yet you GROUP BY s.timeSubmitted, which will make it return multiple rows, and multiple counts of p.songid.
Think about it this way, a subquery in the SELECT statement like you have needs to return a single value since it is going to act like just another column in your select list. Since you have a LIMIT 25 on yours, you're obviously expecting more than one value back, which is inocrrect for this usage.
OK, your query is a mess. Not only is the subquery broken, but I'm pretty sure the GROUP BY s.timeSubmitted ASC isn't doing what you think think it does. (Did you mean ORDER BY instead?) It might help if you explained in words what you're trying to accomplish.
Anyway, I'm going to take a wild guess and suggest that this might be what you want:
SELECT
u.username, u.id, u.score, s.genre, s.songid, s.songTitle,
s.timeSubmitted, s.userid, s.insWanted, s.bounty,
COUNT(p.songid) AS trackCount
FROM songs s
LEFT JOIN users u ON u.id = s.userid
LEFT JOIN posttracks p ON p.songid = s.songid AND p.trackDeleted = 0
WHERE paid = 1 AND s.timeSubmitted >= ( CURDATE() - INTERVAL 60 DAY )
GROUP BY s.songid
ORDER BY s.timeSubmitted ASC
LIMIT 25
Edit: Fixed the COUNT() so that it will correctly return 0 if there are no matching tracks.
I have the following MySQL. I want to pull the data in random order.
Could anyonet teach me how to do it please.
$Q = $this->db->query('SELECT P.*, C.Name AS CatName
FROM products AS P
LEFT JOIN categories C
ON C.id = P.category_id
WHERE C.Name = "Front bottom"
AND p.status = "active"
');
$Q = $this->db->query('SELECT P.*, C.Name AS CatName
FROM products AS P
LEFT JOIN categories C
ON C.id = P.category_id
WHERE C.Name = "Front bottom"
AND p.status = "active"
ORDER BY RAND()
');
you can use the RAND function of MySQL to do that, to be noted that it would perform really slowly on huge dataset (~ about 10k). MySQL would pickup a random number for each row of the table which could lead to problem if the table is huge.
A safer method would be to do a SELECT count(*) as n FROM table and to pickup a random number and do a query with LIMIT 1,n to pickup the nth row. That would work if you need only 1, or you don't care having the result in same order.
After if you really need a complete random set better to do it on server side in my opinion.
You can try
ORDER BY RAND()
The easiest way is using ORDER BY RAND(), but it's performance is miserable, especially for larger datasets (requires a random number for all matching rows).
Another way is randomly creating ids(Either in your code or using RAND() again: WHERE id in (RAND(), RAND(), RAND(), RAND()) should work, but no guarantee). This gets problematic as soon as some IDs don't exist.
Here is an interesting article on the topic.
SELECT i.*, i.id IN (
SELECT id
FROM w
WHERE w.status='active') AS wish
FROM i
INNER JOIN r ON i.id=r.id
WHERE r.member_id=1 && r.status='active'
ORDER BY wish DESC
LIMIT 0,50
That's a query that I'm trying to run. It doesn't scale well, and I'm wondering if someone here can tell me where I could improve things. I don't join w to r and i because I need to show rows from i that are unrepresented in w. I tried a left join, but it didn't perform too well. This is better, but not ideal yet. All three tables are very large. All three are indexed on the fields I'm joining and selecting on.
Any comments, pointers, or constructive criticisms would be greatly appreciated.
EDIT Addition:
I should have put this in my original question. It's the EXPLAIN as return from SQLYog.
id|select_type |table|type |possible_keys|key |key_len|ref |rows|Extra|
1 |PRIMARY |r |ref |member_id,id |member_id|3 |const|3120|Using where; Using temporary; Using filesort
1 |PRIMARY |i |eq_ref |id |id |8 |r.id |1 |
2 |DEPENDENT SUBQUERY|w |index_subquery|id,status |id |8 |func |8 |Using where
EDIT le dorfier - more comments ...
I should mention that the key for w is (member_id, id). So each id can exist multiple times in w, and I only want to know if it exists.
WHERE x IN () is identical to an INNER JOIN to a SELECT DISTINCT subquery, and in general, a join to a subquery will typically perform better if the optimizer doesn't turn the IN into a JOIN - which it should:
SELECT i.*
FROM i
INNER JOIN (
SELECT DISTINCT id
FROM w
WHERE w.status = 'active'
) AS wish
ON i.id = wish.id
INNER JOIN r
ON i.id = r.id
WHERE r.member_id = 1 && r.status = 'active'
ORDER BY wish.id DESC
LIMIT 0,50
Which, would probably be equivalent to this if you don't need the DISTINCT:
SELECT i.*
FROM i
INNER JOIN w
ON w.status = 'active'
AND i.id = wish.id
INNER JOIN r
ON i.id = r.id
AND r.member_id = 1 && r.status = 'active'
ORDER BY i.id DESC
LIMIT 0,50
Please post your schema.
If you are using wish as an existence flag, try:
SELECT i.*, CASE WHEN w.id IS NOT NULL THEN 1 ELSE 0 END AS wish
FROM i
INNER JOIN r
ON i.id = r.id
AND r.member_id = 1 && r.status = 'active'
LEFT JOIN w
ON w.status = 'active'
AND i.id = w.id
ORDER BY wish DESC
LIMIT 0,50
You can use the same technique with a LEFT JOIN to a SELECT DISTINCT subquery. I assume you aren't specifying the w.member_id because you want to know if any members have this? In this case, definitely use the SELECT DISTINCT. You should have an index with id as the first column on w as well in order for that to perform:
SELECT i.*, CASE WHEN w.id IS NOT NULL THEN 1 ELSE 0 END AS wish
FROM i
INNER JOIN r
ON i.id = r.id
AND r.member_id = 1 && r.status = 'active'
LEFT JOIN (
SELECT DISTINCT w.id
FROM w
WHERE w.status = 'active'
) AS w
ON i.id = w.id
ORDER BY wish DESC
LIMIT 0,50
I should have put this in my original question. It's the EXPLAIN as return from SQLYog.
id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|
1|PRIMARY|r|ref|member_id,id|member_id|3|const|3120|Using where; Using temporary; Using filesort
1|PRIMARY|i|eq_ref|id|id|8|r.id|1|
2|DEPENDENT SUBQUERY|w|index_subquery|id,status|id|8|func|8|Using where
Please post the EXPLAIN listing. And explain what the tables and columns mean.
wish appears to be a boolean - and you're ORDERing by it?
EDIT: Well, it looks like it's doing what it's being instructed to do. Cade seems to be thinking expansively on what this all could possibly mean (he probably deserves a vote just for effort.) But I'd really rather you tell us.
Wild guessing just confuses everyone (including you, I'm sure.)
OK, based on new info, here's my (slightly less wild) guess.
SELECT i.*,
CASE WHEN EXISTS (SELECT 1 FROM w WHERE id = i.id AND w.status = 'active' THEN 1 ELSE 0 END) AS wish
FROM i
INNER JOIN r ON i.id = r.id AND r.status = 'active'
WHERE r.member_id = 1
Do you want a row for each match in w? Or just to know for i.id , whether there is an active w record? I assumed the second answer, so you don't need to ORDER BY - it's for only one ID anyway. And since you're only returning columns from i, if there are multiple rows in r, you'll just get duplicate rows.
How about posting what you expect to get for a proper answer?
...
ORDER BY wish DESC
LIMIT 0,50
This appears to be the big expense. You're sorting by a computed column "wish" which cannot benefit from an index. This forces it to use a filesort (as indicated by the EXPLAIN) output, which means it writes the whole result set to disk and sorts it using disk I/O which is very slow.
When you post questions like this, you should not expect people to guess how you have defined your tables and indexes. It's very simple to get the full definitions:
mysql> SHOW CREATE TABLE w;
mysql> SHOW CREATE TABLE i;
mysql> SHOW CREATE TABLE r;
Then paste the output into your question.
It's not clear what your purpose is for the "wish" column. The "IN" predicate is a boolean expression, so it always results in 0 or 1. But I'm guessing you're trying to use "IN" in hopes of accomplishing a join without doing a join. It would help if you describe what you're trying to accomplish.
Try this:
SELECT i.*
FROM i
INNER JOIN r ON i.id=r.id
LEFT OUTER JOIN w ON i.id=w.id AND w.status='active'
WHERE r.member_id=1 AND r.status='active'
AND w.id IS NULL
LIMIT 0,50;
It uses an additional outer join, but it doesn't incur a filesort according to my test with EXPLAIN.
Have you tried this?
SELECT i.*, w.id as wish FROM i
LEFT OUTER JOIN w ON i.id = w.id
AND w.status = 'active'
WHERE i.id in (SELECT id FROM r WHERE r.member_id = 1 AND r.status = 'active')
ORDER BY wish DESC
LIMIT 0,50