SELECT FOR UPDATE in multi thread environment - mysql

I', writing MySQL queries in a multi threaded environment so this query can be executed on any given number of threads. My db is MySQL 8 using InnoDB engine.
Let says I have a DB table with 10 numbers (1,2,3,4,5,6,7,8,9,10)
I have a SELECT ... FOR UPDATE query with a limit of 2 rows from a table in the database. FOR UPDATE will lock the rows to ensure isolation. If I have 5 threads that start at the same time will thread 1 grab entries 1 and 2, thread 2 see that thread 1 got entries 1 and 2 so it will go grab 3 and 4.. and so on.
Would it behavior this way?

No, the locks should have no influence on the query plan. The queries will try to select whichever rows fit the WHERE and ORDER BY criteria. If they're locked by another thread, it will block.
Also, the locking will depend on whether the WHERE or ORDER BY clauses use indexed columns or not. If you examine non-indexed columns, it will have to scan the entire table to find or order the rows, which will effectively lock the entire table. If you restrict these clauses to indexed columns, it should just be able to set locks on those indexes.
See use of LIMIT, FOR UPDATE in SELECT statement in the MySQL Forum for some more information.

Related

MySQL Locking Tables with millions of rows

I've been running a website, with a large amount of data in the process.
A user's save data like ip , id , date to the server and it is stored in a MySQL database. Each entry is stored as a single row in a table.
Right now there are approximately 24 million rows in the table
Problem 1:
Things are getting slow now, as a full table scan can take too many minutes but I already indexed the table.
Problem 2:
If a user is pulling a select data from table it could potentially block all other users (as the table is locked) access to the site until the query is complete.
Our server
32 Gb Ram
12 core with 24 thread cpu
table use MyISAM engine
EXPLAIN SELECT SUM(impresn), SUM(rae), SUM(reve), `date` FROM `publisher_ads_hits` WHERE date between '2015-05-01' AND '2016-04-02' AND userid='168' GROUP BY date ORDER BY date DESC
Lock to comment from #Max P. If you write to MyIsam Tables ALL SELECTs are blocked. There is only a Table lock. If you use InnoDB there is a ROW Lock that only locks the ROWs they need. Aslo show us the EXPLAIN of your Queries. So it is possible that you must create some new one. MySQL can only handle one Index per Query. So if you use more fields in the Where Condition it can be useful to have a COMPOSITE INDEX over this fields
According to explain, query doesn't use index. Try to add composite index (userid, date).
If you have many update and delete operations, try to change engine to INNODB.
Basic problem is full table scan. Some suggestion are:
Partition the table based on date and dont keep more than 6-12months data in live system
Add an index on user_id

Mysql subquery locking

If I have a query like:
UPDATE table_x SET a = 1 WHERE id = ? AND (
SELECT SUM(a) < 100 FROM table_x
)
And
hundreds of this query could be made at exactly the same time
I need to be certain that a never gets to more than 100
Do I need to lock the table or will table_x be locked automatically as it's a subquery?
Assuming this is innodb table, You will have row level locking . So, even if they are 100 of these happening at a time, only ONE transaction will be able to acquire the lock on those rows and finish processing before the next transaction is to occur. There is no difference between how a transaction is processed for the update and the subquery. To the innodb engine this is all ONE transaction, not two separate transactions.
If you want to see what is going on behind the scenes when you run your query, type 'show engine innodb status' in the command line while the query is running.
Here is a great walkthrough on what all that output means.
If you want to read more about Innodb and row level locking, follow link here.

UPDATE with SELECT, will it lock each row or all SELECTed records

It is unclear to me (by reading MySQL docs) if the following query ran on INNODB tables on MySQL 5.1, would create WRITE LOCK for each of the rows the db updates internally (5000 in total) or LOCK all the rows in the batch. As the database has really heavy load, this is very important.
UPDATE `records`
INNER JOIN (
SELECT id, name FROM related LIMIT 0, 5000
) AS `j` ON `j`.`id` = `records`.`id`
SET `name` = `j`.`name`
I'd expect it to be per row but as I do not know a way to make sure it is so, I decided to ask someone with deeper knowledge. If this is not the case and the db would LOCK all the rows in the set, I'd be thankful if you give me explanation why.
The UPDATE is running in transaction - it's an atomic operation, which means that if one of the rows fails (because of unique constrain for example) it won't update any of the 5000 rows. This is one of the ACID properties of a transactional database.
Because of this the UPDATE hold a lock on all of the rows for the entire transaction. Otherwise another transaction can further update the value of a row, based on it's current value (let's say update records set value = value * '2'). This statement should produce different result depending if the first transaction commits or rollbacks. Because of this it should wait for the first transaction to complete all 5000 updates.
If you want to release the locks, just do the update in (smaller) batches.
P.S. autocommit controls if each statement is issued in own transaction, but does not effect the execution of a single query

SELECT vs UPDATE performance with index

If I SELECT IDs then UPDATE using those IDs, then the UPDATE query is faster than if I would UPDATE using the conditions in the SELECT.
To illustrate:
SELECT id FROM table WHERE a IS NULL LIMIT 10; -- 0.00 sec
UPDATE table SET field = value WHERE id IN (...); -- 0.01 sec
The above is about 100 times faster than an UPDATE with the same conditions:
UPDATE table SET field = value WHERE a IS NULL LIMIT 10; -- 0.91 sec
Why?
Note: the a column is indexed.
Most likely the second UPDATE statement locks much more rows, while the first one uses unique key and locks only the rows it's going to update.
The two queries are not identical. You only know that the IDs are unique in the table.
UPDATE ... LIMIT 10 will update at most 10 records.
UPDATE ... WHERE id IN (SELECT ... LIMIT 10) may update more than 10 records if there are duplicate ids.
I don't think there can be a one straight-forward answer to your "why?" without doing some sort of analysis and research.
The SELECT queries are normally cached, which means that if you run the same SELECT query multiple times, the execution time of the first query is normally greater than the following queries. Please note that this behavior can only be experienced where the SELECT is heavy and not in scenarios where even the first SELECT is much faster. So, in your example it might be that the SELECT took 0.00s because of the caching. The UPDATE queries are using different WHERE clauses and hence it is likely that their execution times are different.
Though the column a is indexed, but it is not necessary that MySQL must be using the index when doing the SELECT or the UPDATE. Please study the EXPLAIN outputs. Also, see the output of SHOW INDEX and check if the "Comment" column reads "disabled" for any indexes? You may read more here - http://dev.mysql.com/doc/refman/5.0/en/show-index.html and http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html.
Also, if we ignore the SELECT for a while and focus only on the UPDATE queries, it is obvious that they aren't both using the same WHERE condition - the first one runs on id column and the latter on a. Though both columns are indexed but it does not necessarily mean that all the table indexes perform alike. It is possible that some index is more efficient than the other depending on the size of the index or the datatype of the indexed column or if it is a single- or multiple-column index. There sure might be other reasons but I ain't an expert on it.
Also, I think that the second UPDATE is doing more work in the sense that it might be putting more row-level locks compared to the first UPDATE. It is true that both UPDATES are finally updating the same number of rows. But where in the first update, it is 10 rows that are locked, I think in the second UPDATE, all rows with a as NULL (which is more than 10) are locked before doing the UPDATE. Perhaps MySQL first applies the locking and then runs the LIMIT clause to update only limited records.
Hope the above explanation makes sense!
Do you have a composite index or separate indexes?
If it is a composite index of id and a columns,
In 2nd update statement the a column's index would not be used. The reason is that only the left most prefix indexes are used (unless if a is the PRIMARY KEY)
So if you want the a column's index to be used, you need in include id in your WHERE clause as well, with id first then a.
Also it depends on what storage engine you are using since MySQL does indexes at the engine level, not server.
You can try this:
UPDATE table SET field = value WHERE id IN (...) AND a IS NULL LIMIT 10;
By doing this id is in the left most index followed by a
Also from your comments, the lookups are much faster because if you are using InnoDB, updating columns would mean that the InnoDB storage engine would have to move indexes to a different page node, or have to split a page if the page is already full, since InnoDB stores indexes in sequential order. This process is VERY slow and expensive, and gets even slower if your indexes are fragmented, or if your table is very big
The comment by Michael J.V is the best description. This answer assumes a is a column that is not indexed and 'id' is.
The WHERE clause in the first UPDATE command is working off the primary key of the table, id
The WHERE clause in the second UPDATE command is working off a non-indexed column. This makes the finding of the columns to be updated significantly slower.
Never underestimate the power of indexes. A table will perform better if the indexes are used correctly than a table a tenth the size with no indexing.
Regarding "MySQL doesn't support updating the same table you're selecting from"
UPDATE table SET field = value
WHERE id IN (SELECT id FROM table WHERE a IS NULL LIMIT 10);
Just do this:
UPDATE table SET field = value
WHERE id IN (select id from (SELECT id FROM table WHERE a IS NULL LIMIT 10));
The accepted answer seems right but is incomplete, there are major differences.
As much as I understand, and I'm not a SQL expert:
The first query you SELECT N rows and UPDATE them using the primary key.
That's very fast as you have a direct access to all rows based on the fastest possible index.
The second query you UPDATE N rows using LIMIT
That will lock all rows and release again after the update is finished.
The big difference is that you have a RACE CONDITION in case 1) and an atomic UPDATE in case 2)
If you have two or more simultanous calls of the case 1) query you'll have the situation that you select the SAME id's from the table.
Both calls will update the same IDs simultanously, overwriting each other.
This is called "race condition".
The second case is avoiding that issue, mysql will lock all rows during the update.
If a second session is doing the same command it will have a wait time until the rows are unlocked.
So no race condition is possible at the expense of lost time.

MySQL INSERT and SELECT Order of precedence

if an INSERT and a SELECT are done simultaneously on a mysql table which one will go first?
Example: Suppose "users" table row count is 0.
Then this two queries are run at the same time (assume it's at the same mili/micro second):
INSERT into users (id) values (1)
and
SELECT COUNT(*) from users
Will the last query return 0 or 1?
Depends whether your users table is MyISAM or InnoDB.
If it's MyISAM, one statement or the other takes a lock on the table, and there's little you can do to control that, short of locking tables yourself.
If it's InnoDB, it's transaction-based. The multi-versioning architecture allows concurrent access to the table, and the SELECT will see the count of rows as of the instant its transaction started. If there's an INSERT going on simultaneously, the SELECT will see 0 rows. In fact you could even see 0 rows by a SELECT executed some seconds later, if the transaction for the INSERT has not committed yet.
There's no way for the two transactions to start truly simultaneously. Transactions are guaranteed to have some order.
It depends on which statement will be executed first. If first then the second will return 1, if the second one executes first, then it will return 0. Even you are executing them on the computer with multiple physical cores and due to the lock mechanism, they will never ever execute at the exactly same time stamp.