I have the following requirement. I have 4 MySQL databases and an application in which the user needs to get the count of number of records in tables of each of these databases. The issue is that count may change in every minute or second. So whenever the user mouse-hovering the particular UI area, I need to have a call to all these databases and get the count. I don’t think it is a best approach, as these tables contain millions of records and every time on mouse over, a dB call is going to all these databases.
Trigger is the one approach I found. Rather than we are pulling data from the database, I feel like whenever any insert/update/delete happening to these tables, a trigger will execute and that will increment/decrement the count in another table (which contain only the count of these tables). But I have read like triggers will affect database performance, but also read some situation trigger is the only solution.
So please guide me in my situation triggers are the solution? If it affects the database performance I don’t need that. Is there any other better approach for this problem?
Thanks
What I understood is you have 4 databases and n number of tables in each of them and when the user hovers over a particular area in your application the user should see the number of rows in that table.
I would suggest you to use count(*) to return the number of rows in each table in the database.Triggers are used to do something when a particular event like update,delete or insert occurs in a database.It's not a good idea to invoke triggers to react to user interactions like hovering.If you can tell me in which language you are designing the front end I can be more specific.
Example:
SELECT COUNT(*) FROM tablename where condition
OR
SELECT SQL_CALC_FOUND_ROWS * FROM tablename
WHERE condition
LIMIT 5;
SELECT FOUND_ROWS();
The second one is used when you want to limit the results but still return total number of rows found.Hope it helps.
Please don't use count(*). This is inefficient, possibly to the point of causing a table scan. If you can get to the information schema, this should return the result you need sub-second:
select table_rows from information_schema.tables where table_name = 'tablename'
If you can't for some reason, and your table has a primary key, try:
SELECT COUNT(field) FROM tablename
...where field is part of the primary key. This will be slower, especially on large tables, but still better than count(*).
Definitely don't use trigger.
Related
Is it OK to use the number of rows in result returned by query (COUNT() function in MySQL) for any checks and so on?
For example if I want to check how many posts have the user made today to check if he can create another (in another words, have the user reached his daily limit), is it a good practice to just send a query like this
SELECT COUNT(post_text) FROM posts WHERE (date_published = CURDATE() AND userId = 115);
or is there a better approach. I faced this a couple of times (I don't write database logic often) and it always kinda confused me if I get this wrong or not. So hope you will just clarify this for me once and for all, thanks.
If a user has daily limits, then you probably want to do this check in the database.
In that case, you would implement this restriction using a trigger rather than at the application level. This ensures that the restriction is always applied, regardless of competing threads, table locks, or who is doing the update.
If you do want to implement the restriction at the application level, then you would use a query, presumably with count(). I would expect the query to include the user id:
SELECT COUNT(*)
FROM posts p
WHERE p.date_published = CURDATE() AND p.user_id = ?;
We have a table where we limit the number of rows per user to a certain number. Before insert, we check if user has exceeded the storage capacity (number of rows) and then insert as appropriate. For example,
select count(id) from items where user=123;
say, if count < 10,
insert into items set user=123, a=xyx;
However, this approach requires two queries. Is there a way in which is can be done in a single query.
Thanks
Pretty much any approach you take will require two queries. Whether you do this in a trigger or in application code is a matter of choice.
If you use your method, then add an index on items(user). This will make it cheap to count the number of rows.
An alternative is to increment a value in the user table for each item. So, when a user inserts an item, then increment users.itemcount. Remember, though, to reduce the count when you delete items.
If you just want to have 1 query in a code you can try to use conditional insert with approach described here MySQL Conditional Insert
I would like to store random numbers in one MySql table, randomly retrieve one and insert it into another table column each time a new record is created. I want to delete the retrieved number from the random number table as it is used.
The random numbers are 3 digit, there are 900 of them.
I have read several posts here that describe the problems using unique random numbers and triggering their insertion. I want to use this method as it seems to be reliable while generating few problems.
Can anyone here give me an example of a sql query that will accomplish the above? (If sql query is not the recommended way to do this please feel free to recommend a better method.)
Thank you for any help you can give.
I put together the two suggestions here and tried this trigger and query:
CREATE TRIGGER rand_num before
INSERT ON uau3h_users FOR EACH ROW
insert into uau3h_users (member_number)
select random_number from uau3h_rand900
where random_number not in (select member_number from uau3h_users)
order by random_number
limit 1
But it seems that there is already a trigger attached to that table so the new one cause a conflict, things stopped working until I removed it. Any ideas about how accomplish the same using another method?
You are only dealing with 900 records, so performance is not a major issue.
If you are doing a single insert into a table, you can do something like the following:
insert into t(rand)
select rand
from rand900
where rand not in (select rand from t)
order by rand()
limit 1
In other words, you don't have to continually delete from one table and move to the other. You can just choose to insert values that don't already exist. If performance is a concern, then indexes will help in this case.
More than likely you need to take a look into Triggers. You can do some stuff for instance after inserting a record in a table. Refer this link to more details.
http://dev.mysql.com/doc/refman/5.0/en/create-trigger.html
I have a very large number of rows in my table, table_1. Sometimes I just need to retrieve a particular row.
I assume, when I use SELECT query with WHERE clause, it loops through the very first row until it matches my requirement.
Is there any way to make the query jump to a particular row and then start from that row?
Example:
Suppose there are 50,000,000 rows and the id which I want to search for is 53750. What I need is: the search can start from 50000 so that it can save time for searching 49999 rows.
I don't know the exact term since I am not expert of SQL!
You need to create an index : http://dev.mysql.com/doc/refman/5.1/en/create-index.html
ALTER TABLE_1 ADD UNIQUE INDEX (ID);
The way I understand it, you want to select a row with id 53750. If you have a field named id you could do this:
SELECT * FROM table_1 WHERE id = 53750
Along with indexing the id field. That's the fastest way to do so. As far as I know.
ALTER table_1 ADD UNIQUE INDEX (<collumn>)
Would be a great first step if it has not been generated automatically. You can also use:
EXPLAIN <your query here>
To see which kind of query works best in this case. Note that if you want to change the where statement (anywhere in the future) but see a returning value in there it will be a good idea to put an index on that aswell.
Create an index on the column you want to do the SELECT on:
CREATE INDEX index_1 ON table_1 (id);
Then, select the row just like you would before.
But also, please read up on databases, database design and optimization. Your question is full of false assumptions. Don't just copy and paste our answers verbatim. Get educated!
There are several things to know about optimizing select queries like Range and Where clause Optimization, the documentation is pretty informative baout this issue, read the section: Optimizing SELECT Statements. Creating an index on the column you evaluate is very helpfull regarding performance too.
One possible solution You can create View then query from view. here is details of creating view and obtain data from view
http://www.w3schools.com/sql/sql_view.asp
now you just split that huge number of rows into many view (i. e row 1-10000 in one view then 10001-20000 another view )
then query from view.
I am pretty sure that any SQL database with a little respect for themselves does not start looping from the first row to get the desired row. But I am also not sure how they makes it work, so I can't give an exact answer.
You could check out what's in your WHERE-clause and how the table is indexed. Do you have a proper primary key? Like using a numeric data type for that. Do you have indexes on more columns, that is used in your queries?
There is also alot to concider when installing the database server, like where to put the data and log files, how much memory to give the server and setting the growth. There's a lot you can do to tune your server.
You could try and split your tables in partitions
More about alter tables to add partitions
Selecting from a specific partition
In your case you could create a partition on ID for every 50.000 rows and when you want to skip the first 50.000 you just select from partition 2. How to do this ies explained quite well in the MySQL documentation.
You may try simple as this one.
query = "SELECT * FROM tblname LIMIT 50000,0
i just tried it with phpmyadmin. WHERE the "50,000" is the starting row to look up.
EDIT :
But if i we're you i wouldn't use this one, because it will lapses the 1 - 49999 records to search.
I'm using mysql to update a field in a table when a condition is met...
Should I first do a SELECT to see if the condition is met or do I just try to use UPDATE every time, because if the condition is not met, nothing happens.
To be concrete, here is my SELECT:
SELECT * FROM forum_subscriptions
WHERE IDTopic=11111 AND IDUser=11111 and status=0
I am checking here if I am on forum topic 11111 and if if I (user ID 1) is subscribed to this topic and my status on the subscription is 0 (that means that he didn't yet get email about new post in topic)
So when this is met do:
UPDATE forum_subscriptions SET Status=1 where IDTopic=11111 AND IDUser=1
Now I am wondering, I always do a select here to query if a user is subscribed to this topic and he has a status that he visited that topic before so any new posts will not trigger new email notification. When he visits the page again, the update is triggered that resets the visit so any new posts will again send him email.
So select is made on every user if he is subscribed or not to test the subscription. Update is made only when necessary.
Is it better to just use the update? To try to update on every page, if he is not subscribed to the topic it will not update anything.
How fast is update that doesn't produce any valid data? How is it made internally, how does update find if there is any record, does it select and then update? If so it would be better to only update because I would achieve same thing without any slowdowns. If the update is more expensive than select I should try to check first and then update if necessary.
This example is a real life example, but the logic behing this update/select is really what I am interested because I do find this kind of a problem more often.
Thanx
UPDATE: Thanx both guys, but I do not see on your links if UPDATE is locking even without results or not. As you gave different answers I still don't know what to do.
The subscription table really doesn't need to be myisam, I could change it to InnoDB because I don't have a need to fulltext it. Is this a good solution, to only use update and change this small table to inno? Does mixing table types have any drawbacks?
You just do the update, with no previous select:
UPDATE forum_subscriptions SET Status=1 where IDTopic=11111 AND IDUser=1
If the conditions are not met, update will do nothing.
This update is very fast if you have an index on status and IDtopic and IDuser!
An empty update is just as fast as an empty select.
If you do the select first, you will just slow things down for no reason.
If you want to know how many rows where updated do a
SELECT ROW_COUNT() as rows_affected
After doing the update, this will tell you 0 if no rows where updated, or the number of rows updated (or inserted or deleted, if you used those statements).
This function is ultra fast because it just has to fetch one value from memory.
Workarounds for table locking issues
See here: http://dev.mysql.com/doc/refman/5.5/en/table-locking.html
A potential side affect of always calling the UPDATE is the locking that needs to be put to insure that no other connection modifies these rows.
If the table is MyISAM - a lock will be places on the he entire table during the search.
If the table is InnoDB, locks will be places on the indexes/gaps.
From the Docs:
A locking read, an UPDATE, or a DELETE
generally set record locks on every
index record that is scanned in the
processing of the SQL statement. It
does not matter whether there are
WHERE conditions in the statement that
would exclude the row