I use transactions in MySQL to store orders. Each order has OrderID (BIGINT), which looks like this: XXXXXX0001, with last four digits incrementing (1620200001, 1620200002, 1620200003, ...).
The transaction works as follows:
start transaction
get new OrderID (increment by 1)
do some stuff
commit/rollback
Saving the transaction can take up to several seconds and if multiple orders are created in very short timespan, duplicate OrderID's can be inserted into database. Before first order is commited, second is assigned the same OrderID, which, at the moment is next in line.
What is best way to prevent this? Having UNIQUE OrderID does not solve it (there would be rollback in second order). I could get rid of transaction and save OrderID quicker, but this leads to other potential problems and does not entirely solve this (just reduces chances of problem happening).
Any help would be appreciated.
Read about AUTO_INCREMENT. Search for it in the manual on CREATE TABLE. It's a long page, but AUTO_INCREMENT is documented about 1/4 of the way down the page.
Briefly, you just declare the primary key with a column option:
CREATE TABLE mytable (
id BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
...other columns...
);
The initial value is 1, or you can make it start at a higher value:
ALTER TABLE mytable AUTO_INCREMENT=1620200001;
A table with an auto-increment column ensures that each concurrent transaction gets a unique, increasing value. There is no race condition, because the INSERT acquires a brief table-lock during which it increments the value. Unlike transaction-based locks, the auto-increment table lock is released immediately. So concurrent sessions don't have to wait for your transaction to finish.
Auto-increment is guaranteed to be unique. That is, the same value will not be allocated to multiple sessions. However, it's not guaranteed to allocate consecutive values. Also, it may allocate a value to one session, but that session decides to rollback its transaction. The value it had allocated is NOT returned to any kind of queue of values, because there has probably other sessions that have allocated the next few values in the meantime. So it's possible to "lose" values and then your table has "gaps" or non-consecutive values.
Do not worry about gaps. These could also happen even if the values were consecutive, because you might delete a row later.
What is the correct implementation of handling concurrent SQL inserts while ensuring a non-primary key column (i.e. wid) unique and auto-incremental?
Note: There was a design constraint that led this field (i.e. wid) in not becoming the primary key. A separate auto-increment PK is included in the column called id. Changing the wid to PK is not feasible as the database design is now heavily coupled to multiple software projects. So I need to devise a workaround.
I'm thinking of 3 possible ways:
1.) Reserving the wid by executing a transaction and proceeding with other SQL statements which should have been included in the same transaction. The reservation is executed in a separate transaction to make the locking as short as possible. However, if the subsequent transaction failed (e.g. due to database connection issues), then the invalidated row containing the reserved wid will remain in the table.
START TRANSACTION;
SET #wid_d = 0;
SELECT COALESCE(MAX(`wid`), 0) INTO #wid_d FROM table
WHERE `wid >= 0 AND `wid <= 1000 FOR UPDATE;
INSERT INTO table (`wid`) VALUES (IF (#wid_d = 0, 1, #wid_d+1) )
COMMIT;
2.) Including the reservation of wid in the entire transaction to eliminate the problem of insertion of invalidated row, should the transaction fail but this approach prolongs the locking of tables.
3.) Rely on the failure of transaction calls for re-executing the transaction until it performs successfully; should the transaction fail due to the race condition of inserting the same MAX(wid) + 1. This approach will prevent insertion of invalidated rows but the problem is the number of times the transaction should be re-run. There is also no distinction of knowing the problem that is caused by the conflicting wid or other problems, hence, this is a problematic approach.
Well, depending on how brave you are you have other options, like create separate sequence table that will be used to maintain WID values:
CREATE TABLE wid_seq (wid INT PRIMARY KEY AUTO_INCREMENT);
and in your transaction:
START TRANSACTION;
INSERT INTO wid_seq VALUES (null);
INSERT yourtable (WID) values (LAST_INSERT_ID());
COMMIT;
We have various tables to represent various types of data. Each table has a corresponding revisions table to track history of this data. Each revision (entry in a revisions table) has a unique number. This number is stored in a change metadata table. Each of these tables references a parent_id. Before we make any changes to the tables we lock the parent row with SELECT … FOR UPDATE.
After making an update/insert we also increment the change number and write that number to the change metadata table. To do so we do a SELECT MAX on the change metadata number and then increment it.
The issue we’re seeing is that somehow a transaction is getting an old change number from the select max statement. To illustrate:
Transaction 1:
START TRANSACTION
lock with FOR UPDATE
do stuff...
Get Latest Change Number (9)
Insert Revision with Number 10
COMMIT
Transaction 2:
START TRANSACTION
lock with FOR UPDATE
do stuff...
Get Latest Change Number (7)
Insert Revision with Number 8
COMMIT
This causes the revision insert for transaction 2 to fail as the change number is a unique key. I’m leaning towards it being an issue of repeatable reads but I’m not sure how the old data can persist across transactions in such a way. For each transactions there's a START TRANSACTION statement and then immediately the parent id is locked with FOR UPDATE. We have a high traffic site with multiple concurrent transactions. It's possible there are many waiting on the lock at any one time. I'd be happy to clarify any point and would appreciate any insight anyone could offer.
SELECT MAX on the change metadata number
That needs FOR UPDATE, too.
Another approach:
Have a "sequence number generator" table.
CREATE TABLE Sequence (
pk TINYINT NOT NULL,
seq INT UNSIGNED NOT NULL AUTO_INCREMENT,
PRIMARY KEY(pk), -- For ON DUPLICATE KEY UPDATE
INDEX(seq) -- Sufficient for AUTO_INCREMENT
);
The only action (once initialized) should be
INSERT INTO Sequence (pk, seq) VALUE (1, 0)
ON DUPLICATE KEY UPDATE seq := LAST_INSERT_ID(seq+1);
That will update the one row atomically. Then (in the same connection), do this to get the new seq:
SELECT LAST_INSERT_ID();
That statement is tied to the connection, so there is no chance of someone else getting your number.
I have a table with bills. Every bill has an id that comes from the DB after I insert a new record. The field is an INTEGER with AUTO_INCREMENT set.
If I insert a new record as part of a transaction and I have to rollback this transaction, the ID is taken and gone. So the next record becomes the ID one higher, even though this ID is not in use.
It would be better for the bills to have a linear numbering, so the accounting can figure out if something is wrong.
For concurrency reasons, the auto increment value cannot "rollback" with your transaction. If another process had inserted records while your transaction was in process, you'd risk a collision with their IDs later.
As an example, let's say your transaction in process "A" grabs IDs 1,2 and 3. Another process "B" runs and gets IDs 4 and 5. If the identity rolled back with your transaction and the next process "C" needed 5 IDs, it would get 1,2,3,4,5 but 4 and 5 were already taken by process "B".
I'm using MySQL's AUTO_INCREMENT field and InnoDB to support transactions. I noticed when I rollback the transaction, the AUTO_INCREMENT field is not rollbacked? I found out that it was designed this way but are there any workarounds to this?
It can't work that way. Consider:
program one, you open a transaction and insert into a table FOO which has an autoinc primary key (arbitrarily, we say it gets 557 for its key value).
Program two starts, it opens a transaction and inserts into table FOO getting 558.
Program two inserts into table BAR which has a column which is a foreign key to FOO. So now the 558 is located in both FOO and BAR.
Program two now commits.
Program three starts and generates a report from table FOO. The 558 record is printed.
After that, program one rolls back.
How does the database reclaim the 557 value? Does it go into FOO and decrement all the other primary keys greater than 557? How does it fix BAR? How does it erase the 558 printed on the report program three output?
Oracle's sequence numbers are also independent of transactions for the same reason.
If you can solve this problem in constant time, I'm sure you can make a lot of money in the database field.
Now, if you have a requirement that your auto increment field never have gaps (for auditing purposes, say). Then you cannot rollback your transactions. Instead you need to have a status flag on your records. On first insert, the record's status is "Incomplete" then you start the transaction, do your work and update the status to "compete" (or whatever you need). Then when you commit, the record is live. If the transaction rollsback, the incomplete record is still there for auditing. This will cause you many other headaches but is one way to deal with audit trails.
Let me point out something very important:
You should never depend on the numeric features of autogenerated keys.
That is, other than comparing them for equality (=) or unequality (<>), you should not do anything else. No relational operators (<, >), no sorting by indexes, etc. If you need to sort by "date added", have a "date added" column.
Treat them as apples and oranges: Does it make sense to ask if an apple is the same as an orange? Yes. Does it make sense to ask if an apple is larger than an orange? No. (Actually, it does, but you get my point.)
If you stick to this rule, gaps in the continuity of autogenerated indexes will not cause problems.
I had a client needed the ID to rollback on a table of invoices, where the order must be consecutive
My solution in MySQL was to remove the AUTO-INCREMENT and pull the latest Id from the table, add one (+1) and then insert it manually.
If the table is named "TableA" and the Auto-increment column is "Id"
INSERT INTO TableA (Id, Col2, Col3, Col4, ...)
VALUES (
(SELECT Id FROM TableA t ORDER BY t.Id DESC LIMIT 1)+1,
Col2_Val, Col3_Val, Col4_Val, ...)
Why do you care if it is rolled back? AUTO_INCREMENT key fields are not supposed to have any meaning so you really shouldn't care what value is used.
If you have information you're trying to preserve, perhaps another non-key column is needed.
I do not know of any way to do that. According to the MySQL Documentation, this is expected behavior and will happen with all innodb_autoinc_lock_mode lock modes. The specific text is:
In all lock modes (0, 1, and 2), if a
transaction that generated
auto-increment values rolls back,
those auto-increment values are
“lost.” Once a value is generated for
an auto-increment column, it cannot be
rolled back, whether or not the
“INSERT-like” statement is completed,
and whether or not the containing
transaction is rolled back. Such lost
values are not reused. Thus, there may
be gaps in the values stored in an
AUTO_INCREMENT column of a table.
If you set auto_increment to 1 after a rollback or deletion, on the next insert, MySQL will see that 1 is already used and will instead get the MAX() value and add 1 to it.
This will ensure that if the row with the last value is deleted (or the insert is rolled back), it will be reused.
To set the auto_increment to 1, do something like this:
ALTER TABLE tbl auto_increment = 1
This is not as efficient as simply continuing on with the next number because MAX() can be expensive, but if you delete/rollback infrequently and are obsessed with reusing the highest value, then this is a realistic approach.
Be aware that this does not prevent gaps from records deleted in the middle or if another insert should occur prior to you setting auto_increment back to 1.
INSERT INTO prueba(id)
VALUES (
(SELECT IFNULL( MAX( id ) , 0 )+1 FROM prueba target))
If the table doesn't contain values or zero rows
add target for error mysql type update FROM on SELECT
If you need to have the ids assigned in numerical order with no gaps, then you can't use an autoincrement column. You'll need to define a standard integer column and use a stored procedure that calculates the next number in the insert sequence and inserts the record within a transaction. If the insert fails, then the next time the procedure is called it will recalculate the next id.
Having said that, it is a bad idea to rely on ids being in some particular order with no gaps. If you need to preserve ordering, you should probably timestamp the row on insert (and potentially on update).
Concrete answer to this specific dilemma (which I also had) is the following:
1) Create a table that holds different counters for different documents (invoices, receipts, RMA's, etc..); Insert a record for each of your documents and add the initial counter to 0.
2) Before creating a new document, do the following (for invoices, for example):
UPDATE document_counters SET counter = LAST_INSERT_ID(counter + 1) where type = 'invoice'
3) Get the last value that you just updated to, like so:
SELECT LAST_INSERT_ID()
or just use your PHP (or whatever) mysql_insert_id() function to get the same thing
4) Insert your new record along with the primary ID that you just got back from the DB. This will override the current auto increment index, and make sure you have no ID gaps between you records.
This whole thing needs to be wrapped inside a transaction, of course. The beauty of this method is that, when you rollback a transaction, your UPDATE statement from Step 2 will be rolled back, and the counter will not change anymore. Other concurrent transactions will block until the first transaction is either committed or rolled back so they will not have access to either the old counter OR a new one, until all other transactions are finished first.
SOLUTION:
Let's use 'tbl_test' as an example table, and suppose the field 'Id' has AUTO_INCREMENT attribute
CREATE TABLE tbl_test (
Id int NOT NULL AUTO_INCREMENT ,
Name varchar(255) NULL ,
PRIMARY KEY (`Id`)
)
;
Let's suppose that table has houndred or thousand rows already inserted and you don't want to use AUTO_INCREMENT anymore; because when you rollback a transaction the field 'Id' is always adding +1 to AUTO_INCREMENT value.
So to avoid that you might make this:
Let's remove AUTO_INCREMENT value from column 'Id' (this won't delete your inserted rows):
ALTER TABLE tbl_test MODIFY COLUMN Id int(11) NOT NULL FIRST;
Finally, we create a BEFORE INSERT Trigger to generate an 'Id' value automatically. But using this way won't affect your Id value even if you rollback any transaction.
CREATE TRIGGER trg_tbl_test_1
BEFORE INSERT ON tbl_test
FOR EACH ROW
BEGIN
SET NEW.Id= COALESCE((SELECT MAX(Id) FROM tbl_test),0) + 1;
END;
That's it! You're done!
You're welcome.
$masterConn = mysql_connect("localhost", "root", '');
mysql_select_db("sample", $masterConn);
for($i=1; $i<=10; $i++) {
mysql_query("START TRANSACTION",$masterConn);
$qry_insert = "INSERT INTO `customer` (id, `a`, `b`) VALUES (NULL, '$i', 'a')";
mysql_query($qry_insert,$masterConn);
if($i%2==1) mysql_query("COMMIT",$masterConn);
else mysql_query("ROLLBACK",$masterConn);
mysql_query("ALTER TABLE customer auto_increment = 1",$masterConn);
}
echo "Done";