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.
Related
This is not working for the row level locking
I just want to know if I can select the row like
select * from table where folder like %344443%**
then update the row with
update table set folder = '{"bin":"44456","venv":4366}' where id = 'i-instanceid'
You can't.
The problem is that the UPDATE must scan the entire table to find the row(s) you need to change. In doing so, it locks the entire table.
Don't bury things that you want to search on inside JSON strings. Have them as indexed columns on their own. This should let you lock a single row, and run the Update much faster.
Or look into indexing parts JSON columns. Such is still evolving. What version of MySQL are you using?
Furthermore, why select the id first, then do the update? Can't you simply do the update? If you are actually doing something else in the "transaction", say so. You may need the SELECT. At that point, it would need to be SELECT ... FOR UPDATE.
Example tables (not actual database):
In this example, I would have the SecurityCode(Unique), and Time. My current solution involves attempting to add a new Person using the security code, then querying the ID, then adding to the Times table. This is 3 separate statements and could likely be a lot faster. Any advice on how to optimise this?
Thanks.
Edit: I previously forgot to mention that this is normally done in a batch of 30-40 records.
I am also considering using SecurityCode as the foreign key in Times.
I think there are many ways of achieve this, the easiest:
Try using "IF", you only need it for the first step of your statement, the last two are independent to the result of this evaluation.
Plus, save your security code in a variable, then you will save one table scan (you already have it)
**please note its just pseudo-code**
IF (exists select * from person where securityCode = #securityCode) then
Step 1
End
Step 2
Step 3
Can you try it?
The fastest way seemed to be to batch ignore insert all security codes, then batch insert all Times with a subquery to select the correct ID from Person.
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.
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 know that this might seem like a strange question, but let me try and explain it. I have a database table called 'plan' and in it the first column is called 'username' and the columns after it are called 'question1', 'question2' and so on. I now need to add a hundred or so more columns named like this, but it would be nice to have a sql statement that would automatically do that for me.
I know this wasn't set up in the best way, but if you have a solution, please let me know :)
There isn't any SQL command or feature that would do this automatically; sure you can generate the alter table statements and add the columns programmatically; however, your design would be terribly flawed.
Instead of adding columns, you should create a table containing the question, the user_id (or username, whatever is the PK) to hold the records. If you need to identify a question by number (or ID), simply add another column called question_id.
Write the query in sql to excel. Seperate the incrementing number. Drag down until excel row 100. Hard to explain but i guess you ll figure it out. You'll have 100 incrementing add column sql statements. copy paste run it on a query tool.