Mysql - i want to reorder a 100k row database every hour. I have a field called 'order' that i sort by. how can i best reorder it?
I currently do this (pseudo):
mainpage.php : select * from table order by `order` desc limit 100;
and hourly:
cronjob.php : select * from table order by rand();
$i=0;
foreach($row) {
$i++;
update table set order = $i where id = $row['id']
}
but it takes ages.
If i just do 'update table set order = rand()' there will be duplicates and i don't want order to have duplicates (but it isn't set to a UNIQUE index because as it is updating there will be duplicates.
whats the best way to go about doing this?
(i do it this way because just doing "select * from table order by rand() limit 100" was really slow, but having it on an index is much faster. it just takes quite a while to reorder it)
(mysql 5)
Why not use an auto increment field and then generate a list of random values (between the min and max stored) to use when selecting?
MySQL isn't a sequential-on-disk storage anyway. This doesn't get you anything. Doing this might make rows show up in your management client in a certain order, but it won't actually add any speed to anything. Please just add an ORDER BY clause to your select statement.
how about creating another table - my_wierd_sort_table.
then delete it entirely, and insert the PK and your rand() column - and use that to order by in your selects...
Related
I'm thinking of implement a view history for my wordpress blog, where users can view their previously viewed articles as a list in their account page.
I would like to limit this to 24 unique page history per user at any point of time, meaning, if the number of articles exceeds 24, the oldest article row would be deleted, and the new article added to the table.
I'm using PHP and MySQL.
Here's my current thoughts on implementation:
Create a table with user_id and post_id columns
When user views an article, insert new row into the table
Select the rows with the current user_id, and if number of rows is more than 24,
Delete the oldest row
I'm not sure if this is the best method, since it's 3 additional database queries per user page view which is pretty heavy.
Is there a better way to do this?
The ideea is good. Improve it by updating the oldest row instead of deleting it and then adding a new one. ;)
Also make a single read query and a single write query.
make a query that is like this one
SELECT (SELECT COUNT(*) FROM recentArticles WHERE userID = 23)
AS NoOfArticles, articleID, timestamp
FROM recentArticles
WHERE userID = 23
ORDER BY timestamp ASC
LIMIT 1;
If NoOfArticles < 24 then execute an insert query, else execute an update query to articleID
You could always implement this using cookies to key on which pages have been visited and keeping a running list on the users PC. This will reduce the traffic to the site, put the processing scripts on the user-side, and could be easier to implement.
That being said, I agree with the other answer about updating the oldest entry if the table is full. This eliminates the need to delete and add a row. Key off a time-date stamp to sort the entries when you're displaying them and to figure out which page was the oldest (and needs to be updated if >= 24 pages.
You can merge two queries in one. In this way, You will save execution time of your script. So, basically, DELETE row if the post count is more than 24. You can modify this query according to your exact need. But yes, you can think on this way.
DELETE FROM `table_name`
WHERE id=(SELECT (CASE WHEN COUNT(id)>24 then id END)
AS last_id
FROM `table_name`
WHERE user_id='XX'
ORDER BY id DESC LIMIT 1);
I have issuse with displaying random item from table, is there way to display just one random item, but not the item i preffered to not to be displayed?
I have table PRODUCT
and rows
ID
TITLE
URL_TITLE
and
$link = "coffee";
I want to display random just one product from table PRODUCT, but not the the same i have in $link
What i want to say i just want random item from table, but not when in $link="coffee" to get that random element
p.s
$link in in URL_TITLE row :)
This should help:
SELECT ID, TITLE, URL_TITLE
FROM PRODUCT
WHERE URL_TITLE != "coffee"
ORDER BY RAND()
LIMIT 1;
Note that in some versions of SQL, != is written <>.
Obviously, you want to select all rows or a different subset, just use SELECT * or whatever you need.
Edit: as per HamZa's comment and James' answer, using ORDER BY RAND() is bad practice on large tables. You could potentially generate a random ID and select that, checking that it's not your illegal one, but if you have a whole bunch that you can't select, you could potentially call this query a ton of times (which is bad).
Using rand() in queries is not ideal, especially in large tables, but even small ones as you never know when they'll grow (ie site or service exceeds your expectations etc).
You can use it like this:
select item from product where item != '$link' ORDER BY rand() LIMIT 1
But you should use something better, like temp tables, or you could select all the IDs from the database in one query, use PHP to select a random one from them all, then use the ID selected by PHP and grab the data from the database.
SELECT id
FROM PRODUCT
WHERE URL_TITLE != "coffee"
ORDER BY RAND()
LIMIT 1;
Although I don't know PHP, I think a simple way to do it is to pass the $link value as part of a where condition.
Using 'pure' SQL:
select *
from product
where URL_TITLE <> 'coffee'
order by rand()
limit 1
I'm trying to get 4 random results from a table that holds approx 7 million records. Additionally, I also want to get 4 random records from the same table that are filtered by category.
Now, as you would imagine doing random sorting on a table this large causes the queries to take a few seconds, which is not ideal.
One other method I thought of for the non-filtered result set would be to just get PHP to select some random numbers between 1 - 7,000,000 or so and then do an IN(...) with the query to only grab those rows - and yes, I know that this method has a caveat in that you may get less than 4 if a record with that id no longer exists.
However, the above method obviously will not work with the category filtering as PHP doesn't know which record numbers belong to which category and hence cannot select the record numbers to select from.
Are there any better ways I can do this? Only way I can think of would be to store the record id's for each category in another table and then select random results from that and then select only those record ID's from the main table in a secondary query; but I'm sure there is a better way!?
You could of course use the RAND() function on a query using a LIMIT and WHERE (for the category). That however as you pointed out, entails a scan of the database which takes time, especially in your case due to the volume of data.
Your other alternative, again as you pointed out, to store id/category_id in another table might prove a bit faster but again there has to be a LIMIT and WHERE on that table which will also contain the same amount of records as the master table.
A different approach (if applicable) would be to have a table per category and store in that the IDs. If your categories are fixed or do not change that often, then you should be able to use that approach. In that case you will effectively remove the WHERE from the clause and getting a RAND() with a LIMIT on each category table would be faster since each category table will contain a subset of records from your main table.
Some other alternatives would be to use a key/value pair database just for that operation. MongoDb or Google AppEngine can help with that and are really fast.
You could also go towards the approach of a Master/Slave in your MySQL. The slave replicates content in real time but when you need to perform the expensive query you query the slave instead of the master, thus passing the load to a different machine.
Finally you could go with Sphinx which is a lot easier to install and maintain. You can then treat each of those category queries as a document search and let Sphinx randomize the results. This way you offset this expensive operation to a different layer and let MySQL continue with other operations.
Just some issues to consider.
Working off your random number approach
Get the max id in the database.
Create a temp table to store your matches.
Loop n times doing the following
Generate a random number between 1 and maxId
Get the first record with a record Id greater than the random number and insert it into your temp table
Your temp table now contains your random results.
Or you could dynamically generate sql with a union to do the query in one step.
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
UNION
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
UNION
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
UNION
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
Note: my sql may not be valid, as I'm not a mySql guy, but the theory should be sound
First you need to get number of rows ... something like this
select count(1) from tbl where category = ?
then select a random number
$offset = rand(1,$rowsNum);
and select a row with offset
select * FROM tbl LIMIT $offset, 1
in this way you avoid missing ids. The only problem is you need to run second query several times. Union may help in this case.
For MySQl you can use
RAND()
SELECT column FROM table
ORDER BY RAND()
LIMIT 4
I'm running a query to select an array of id's from one table, so that I can update another table with the data from the resulting dataset.
//db query result
$query = "SELECT image_id FROM jos_jxgallery_images ORDER BY jos_jxgallery_images.image_id DESC LIMIT 25";
$query_execute = mysql_query($query);
mysql_close($db_config);
while ($items = mysql_fetch_array($query_execute)) {
echo $items['image_id'];
echo '<br/>';
}
I think I need to do it in the while loop, I just have the echo there to see what's in items variable. That works ok. I think the thing to do is in the while loop..I'd like to replace the 'echoing' with an actual update SET query for my other table. Something like...
while ($items = mysql_fetch_array($query_execute)) {
$q = "UPDATE jx_gallery_images_ratings SET image_id ='".$items."";
mysql_query($q);
But the new table has no data. Is there just a better way to write this...maybe even as one query or something? Any help is appreciated.
EDIT: I should explain a little better. The table is empty, and I could go ahead and use an insert from one table to another just to get the id's there. However, after that...in a way it is somewhat a 'temp' table. But not really. Whatever ordering of image_id's I have created in my SELECT query from my first table (There are other rows to sort by other than image_id, like 'hits', for example)...so the second table needs to be updated with the same ordering of image_id's. Probably be running this several little snippet several times with a cron job. So, yeah, I'm trying to update the second table with the ordering of the SELECT query of the first table and just put the id's in my second table, again...according to the order of the first SELECT query.
If the table is empty, you should do an insert. You can do it in a single query like this:
INSERT INTO jx_gallery_images_ratings (image_id)
SELECT image_id FROM jos_jxgallery_images ORDER BY jos_jxgallery_images.image_id DESC LIMIT 25
Note that you probably wouldn't really need the ORDER BY, adn you could do it for all images at once by removing the LIMIT
Something like:
UPDATE tbl_updateme SET row_to_update = (SELECT row_you_need from tbl_target WHERE tbl_updateme.comparison_row = tbl_target.comparison_row)
INSERT INTO jx_gallery_images_ratings (image_id) (SELECT image_id FROM jos_jxgallery_images ORDER BY jos_jxgallery_images.image_id DESC LIMIT 25)
Or
Easy way is create trigger that updates table after selection with Dynamic SQL
To randomly select records from one table; do I have to always set a temporary variable in PHP? I need some help with selecting random rows within a CodeIgniter model, and then display three different ones in a view every time my homepage is viewed. Does anyone have any thoughts on how to solve this issue? Thanks in advance!
If you don't have a ton of rows, you can simply:
SELECT * FROM myTable ORDER BY RAND() LIMIT 3;
If you have many rows, this will get slow, but for smaller data sets it will work fine.
As Steve Michel mentions in his answer, this method can get very ugly for large tables. His suggestion is a good place to jump off from. If you know the approximate maximum integer PK on the table, you can do something like generating a random number between one and your max PK value, then grab random rows one at a time like:
$q="SELECT * FROM table WHERE id >= {$myRandomValue}";
$row = $db->fetchOne($q); //or whatever CI's interface to grab a single is like
Of course, if you need 3 random rows, you'll have three queries here, but as they're entirely on the PK, they'll be fast(er than randomizing the whole table).
I would do something like:
SELECT * FROM table ORDER BY RAND() LIMIT 1;
This will put the data in a random order and then return only the first row from that random order.
I have this piece of code in production to get a random quote. Using MySQL's RAND function was super slow. Even with 100 quotes in the database, I was noticing a lag time on the website. With this, there was no lag at all.
$result = mysql_query('SELECT COUNT(*) FROM quotes');
$count = mysql_fetch_row($result);
$id = rand(1, $count[0]);
$result = mysql_query("SELECT author, quote FROM quotes WHERE id=$id");
you need a query like this:
SELECT *
FROM tablename
WHERE somefield='something'
ORDER BY RAND() LIMIT 3
It is taken from the second result of
http://www.google.com/search?q=mysql+random
and it should work ;)
Ordering a big table by rand() can be very expensive if the table is very large. MySQL will need to build a temporary table and sort it. If you have primary key and you know how many rows are in the table, use LIMIT x,1 to grab a random row, where x is the number of the row you want to get.