I have a mysql table that keep gaining new records every 5 seconds.
The questions are
can I run query on this set of data that may takes more than 5 seconds?
if SELECT statement takes more than 5s, will it affect the scheduled INSERT statement?
what happen when INSERT statement invoked while SELECT is still running, will SELECT get the newly inserted records?
I'll go over your questions and some of the comments you added later.
can I run query on this set of data that may takes more than 5 seconds?
Can you? Yes. Should you? It depends. In a MySQL configuration I set up, any query taking longer than 3 seconds was considered slow and logged accordingly. In addition, you need to keep in mind the frequency of the queries you intend to run.
For example, if you try to run a 10 second query every 3 seconds, you can probably see how things won't end well. If you run a 10 second query every few hours or so, then it becomes more tolerable for the system.
That being said, slow queries can often benefit from optimizations, such as not scanning the entire table (i.e. search using primary keys), and using the explain keyword to get the database's query planner to tell you how it intends to work on that internally (e.g. is it using PKs, FKs, indices, or is it scanning all table rows?, etc).
if SELECT statement takes more than 5s, will it affect the scheduled INSERT statement?
"Affect" in what way? If you mean "prevent insert from actually inserting until the select has completed", that depends on the storage engine. For example, MyISAM and InnoDB are different, and that includes locking policies. For example, MyISAM tends to lock entire tables while InnoDB tends to lock specific rows. InnoDB is also ACID-compliant, which means it can provide certain integrity guarantees. You should read the docs on this for more details.
what happen when INSERT statement invoked while SELECT is still running, will SELECT get the newly inserted records?
Part of "what happens" is determined by how the specific storage engine behaves. Regardless of what happens, the database is designed to answer application queries in a way that's consistent.
As an example, if the select statement were to lock an entire table, then the insert statement would have to wait until the select has completed and the lock has been released, meaning that the app would see the results prior to the insert's update.
I understand that locking database can prevent messing up the SELECT statement.
It can also put a potentially unacceptable performance bottleneck, especially if, as you say, the system is inserting lots of rows every 5 seconds, and depending on the frequency with which you're running your queries, and how efficiently they've been built, etc.
what is the good practice to do when I need the data for calculations while those data will be updated within short period?
My recommendation is to simply accept the fact that the calculations are based on a snapshot of the data at the specific point in time the calculation was requested and to let the database do its job of ensuring the consistency and integrity of said data. When the app requests data, it should trust that the database has done its best to provide the most up-to-date piece of consistent information (i.e. not providing a row where some columns have been updated, but others yet haven't).
With new rows coming in at the frequency you mentioned, reasonable users will understand that the results they're seeing are based on data available at the time of request.
All of your questions are related to locking of table.
Your all questions depend on the way database is configured.
Read : http://www.mysqltutorial.org/mysql-table-locking/
Perform Select Statement While insert statement working
If you want to perform a select statement during insert SQL is performing, you should check by open new connection and close connection every time. i.e If I want to insert lots of records, and want to know that last record has inserted by selecting query. I must have to open connection and close connection in for loop or while loop.
# send a request to store data
insert statement working // take a long time
# select statement in while loop.
while true:
cnx.open()
select statement
cnx.close
//break while loop if you get the result
Related
I'm using MySQL 5.6. Let's say we have the following two tables:
Every DataSet has a huge amount of child DataEntry records that the number would be 10000 or 100000 or more. DataSet.md5sum and DataSet.version get updated when its child DataEntry records are inserted or deleted, in one transaction. A DataSet.md5sum is calculated against all of its children DataEntry.content s.
Under this situation, What's the most efficient way to fetch consistent data from those two tables?
If I issue the following two distinct SELECTs, I think I might get inconsistent data due to concurrent INSERT / UPDATEs:
SELECT md5sum, version FROM DataSet WHERE dataset_id = 1000
SELECT dataentry_id, content FROM DataEntry WHERE dataset_id = 1000 -- I think the result of this query will possibly incosistent with the md5sum which fetched by former query
I think I can get consistent data with one query as follows:
SELECT e.dataentry_id, e.content, s.md5sum, s.version
FROM DataSet s
INNER JOIN DataEntry e ON (s.dataset_id = e.dataset_id)
WHERE s.dataset_id = 1000
But it produces redundant dataset which filled with 10000 or 100000 duplicated md5sums, So I guess it's not efficient (EDIT: My concerns are high network bandwidth and memory consumption).
I think using pessimistic read / write lock (SELECT ... LOCK IN SHARE MODE / FOR UPDATE) would be another option but it seems overkill. Are there any other better approaches?
The join will ensure that the data returned is not affected by any updates that would have occurred between the two separate selects, since they are being executed as a single query.
When you say that md5sum and version are updated, do you mean the child table has a trigger on it for inserts and updates?
When you join the tables, you will get a "duplicate md5sum and version" because you are pulling the matching record for each item in the DataEntry table. It is perfectly fine and isn't going to be an efficiency issue. The alternative would be to use the two individual selects, but depending upon the frequency of inserts/updates, without a transaction, you run the very slight risk of getting data that may be slightly off.
I would just go with the join. You can run explain plans on your query from within mysql and look at how the query is executed and see any differences between the two approaches based upon your data and if you have any indexes, etc...
Perhaps it would be more beneficial to run these groups of records into a staging table of sorts. Before processing, you could call a pre-processor function that takes a "snapshot" of the data about to be processed, putting a copy into a staging table. Then you could select just the version and md5sum alone, and then all of the records, as two different selects. Since these are copied into a separate staging table, you wont have to worry about immediate updates corrupting your session of processing. You could set up timed jobs to do this or have it as an on-demand call. Again though, this would be something you would need to research the best approach given the hardware/network setup you are working with. And any job scheduling software you have available to you.
Use this pattern:
START TRANSACTION;
SELECT ... FOR UPDATE; -- this locks the row
...
UPDATE ...
COMMIT;
(and check for errors after every statement, including COMMIT.)
"100000" is not "huge", but "BIGINT" is. Recomment INT UNSIGNED instead.
For an MD5, make sure you are not using utf8: CHAR(32) CHARACTER SET ascii. This goes for any other hex strings.
Or, use BINARY(16) for half the space. Then use UNHEX(md5...) when inserting, and HEX(...) when fetching.
You are concerned about bandwidth, etc. Please describe your client (PHP? Java? ...). Please explain how much (100K rows?) needs to be fetched to re-do the MD5.
Note that there is a MD5 function in MySQL. If each of your items had an MD5, you could take the MD5 of the concatenation of those -- and do it entirely in the server; no bandwidth needed. (Be sure to increase group_concat_max_len)
I have a MySQL server with many innodb tables.
I have a background script that does A LOT a delete/insert with one request : it deletes many millions of rows from table 2, then insert many millions of rows to table 2 using data from table 1 :
INSERT INTO table 2 (date)
SELECT date from table 1 GROUP BY date
(The request is actually more complex but it is to shown what kind of request I am doing).
At the same time, I am going to run a second background script, that does about a million INSERT or UPDATE requests, but separately (I mean, I execute the first update query, then I execute an insert query, etc...) in table 3.
My issue is that when a script is running, it is fast, like let's say it takes 30minutes each, so 1h total. But when the two scripts are running at the same time, it is VERY slow, like it will take 5h, instead of 1h.
So first, I would like to know what can cause this ? Is it because of IO performance ? (like mysql is writing in two different tables so it is slow to switch between the two ?)
And how could I fix this ? If I could say that the big INSERT query is paused while my second background script is running, it would be great, for example... But I can't find a way to do something like this.
I am not an expert at MySQL administration.. If you need more information, please let me know !
Thank you !!
30 minutes for million INSERT is not fast. Do you have an index on date column? (or whatever column you are using to pivot on)
Regarding your original question.It's difficult to say much without knowing the details of both your scripts and the table structures, but one possible reason why the scripts are running reasonably quickly separately is because you are doing similar kinds of SELECT queries, which might be getting cached by MySQL and then reused for subsequent queries. But if you are running two queries in parallel, then the SELECT's for the corresponding query might not stay in the cache (because there are two concurrent processes which send new queries all the time).
You might want to explicitly disable cache for some queries which you are sure you only run once (using SQL_NO_CACHE modifier) and see if it changes anything. But I'd look into indexing and into your table structure first, because 30 minutes seems to be extremely slow :) E.g. you might also want to introduce partitioning by date for your tables, if you know that you always choose entries in a given period (say by month). The exact tricks depend on your data.
UPDATE: Another issue might be that both your queries work with the same table (table 1), and the default transaction isolation level in MySQL is REPEATABLE READS afair. So it might be that one query is waiting until the other is done with the table to satisfy the transaction isolation level. You might want to lower the transaction isolation level if you are sure that your table 1 is not changed when scripts are working on it.
You can use an event scheduler so you can set mysql to launch this queries at different hours of the day, in another stackoverflow related question you have an exmaple of how to do it: MySQL Event Scheduler on a specific time everyday
Another thing to have in mind is to use the explain plan to see what could be the reason the query is that slow.
We recently switched our tables to use InnoDB (from MyISAM) specifically so we could take advantage of the ability to make updates to our database while still allowing SELECT queries to occur (i.e. by not locking the entire table for each INSERT)
We have a cycle that runs weekly and INSERTS approximately 100 million rows using "INSERT INTO ... ON DUPLICATE KEY UPDATE ..."
We are fairly pleased with the current update performance of around 2000 insert/updates per second.
However, while this process is running, we have observed that regular queries take very long.
For example, this took about 5 minutes to execute:
SELECT itemid FROM items WHERE itemid = 950768
(When the INSERTs are not happening, the above query takes several milliseconds.)
Is there any way to force SELECT queries to take a higher priority? Otherwise, are there any parameters that I could change in the MySQL configuration that would improve the performance?
We would ideally perform these updates when traffic is low, but anything more than a couple seconds per SELECT query would seem to defeat the purpose of being able to simultaneously update and read from the database. I am looking for any suggestions.
We are using Amazon's RDS as our MySQL server.
Thanks!
I imagine you have already solved this nearly a year later :) but I thought I would chime in. According to MySQL's documentation on internal locking (as opposed to explicit, user-initiated locking):
Table updates are given higher priority than table retrievals. Therefore, when a lock is released, the lock is made available to the requests in the write lock queue and then to the requests in the read lock queue. This ensures that updates to a table are not “starved” even if there is heavy SELECT activity for the table. However, if you have many updates for a table, SELECT statements wait until there are no more updates.
So it sounds like your SELECT is getting queued up until your inserts/updates finish (or at least there's a pause.) Information on altering that priority can be found on MySQL's Table Locking Issues page.
I'm currently building a system that does running computations, and every 5 seconds inserts or updates information based on those computations to a few rows in MySQL. I'm working on running this system on a few different servers at once right now with a few agents that are each doing similar processing and then writing on the same set of rows. I already randomize the order in which each agent writes its set of rows, but there's still a lot of deadlock happening. What's the best/fastest way to get through those deadlocks? Should I just rerun the query each time one happens, or do row locks, or something else entirely?
I suggest you try something that won't require more than one client to update your 'few rows.'
For example, you could have each agent that produces results do an INSERT to a staging table with the MEMORY access method.
Then, every five seconds you can run a MySQL event (a stored procedure within the server) that loops through all the rows in that table, posting their results to your 'few rows' and then deleting them. If it's important for the rows in your staging table to be processed in order, then you can use an AUTO_INCREMENT id field. But it might not be important for them to be in order.
If you want to get fancier and more scalable than that, you'll need a queue management system like Apache ActiveMQ.
I've got a theoretical question and can't find a good solution for this on the net:
For a tblA with 100,000 recs.
I want to have multiple processes/apps running, each of which accesses tblA.
I don't want the apps to access the same recs. ie, I want appA to access the 1st 50 rows, with appB accessing the next 50, and appC accessing the next 50 after that..
So basically I want the apps to do a kind of fetch on the next "N" recs in the table. I'm looking for a way to access/process the row data as fast as possible, essentially running the apps in a simultaneous manner. but I don't want the apps to process the same rows.
So, just how should this kind of process be set up?
Is it simply doing a kind of:
select from tblA limit 50
and doing some kind of row locking for each row (which requires innodb)
Pointers/psuedo code would be useful.
Here is some posts from the DBA StackExchange on this
https://dba.stackexchange.com/q/10017/877
https://dba.stackexchange.com/a/4470/877
It discusses SELECT ... LOCK IN SHARE MODE and potential headcahes that comes with it.
Percona wrote a nice article on this along with SELECT ... FOR UPDATE
Your application should handle what data it wants to access. Create a pointer in that. If you're using stored procedures, use another table to store the pointers. Each process would "reserve" a set of rows before beginning processing. Every process should check for the max of that and also see if it is greater than the length of the table.
If you are specifically looking for processing first set, second set, etc. The you can use LIMIT # (i.e. 0,50 51,100 101,150) with an ORDER BY. Locking is not necessary since the processes won't even try to access each others record sets. But I can't imagine a scenario where that would be a good implementation.
An alternative is to just to use update with a limit, then select the records that were updated. You can use the process ID, random number or something else that is almost guaranteed to be unique across processes. Add a "status" field to your table indicating if the record is available for processing (i.e. value is NULL). Then each process would update the status field to "own" the record for processing.
UPDATE tblA SET status=1234567890 WHERE status IS NULL LIMIT 50;
SELECT * FROM tblA WHERE status=1234567890;
This would work for MyISAM or Innodb. With Innodb you would be able to have multiple updates running at once, improving performance.
The problem with these solutions is lag time. If process A executes at 12:00:00 and proccess B also executes at precisely the same time, and in an application, there are several blocks of distinct code leading up to the locks/DMLs, the process time for each would vary. So process A may complete first, or it may be process B. If process A is setting the lock, and process B modifies the record first, you're in trouble. This is the trouble with forking.