Let's say I have a table defined by:
CREATE TABLE People (
id SERIAL,
name TEXT
);
If I first find the maximum id in the table and then run the following query:
SELECT (id, name)
FROM People
WHERE id <= [maximum id found before];
I'll get a list of people. If I run the same query again with the same maximum id:
Am I guaranteed to get the same results?
Or is it possible that the first query returned a list with gaps which were filled in before the second query was executed, causing the second query to have more rows?
Assume that no changes are made to the database except sequential insert operations from any number of concurrent connections.
EDIT:
I'll try to clarify the specific case I'm concerned about. Let's say MySQL gets five transactions at around the same time. Transactions A and B both insert a person into People. Transaction C finds the maximum id. Transactions D and E both perform the query written above.
Is it possible for this sequence of events:
A is assigned an id
B is assigned the next id
B is committed
C is committed and returns the id of B
D is committed and returns a list that does not include the row inserted by A
A is committed
E is committed and returns a list that does include the row inserted by A
EDIT:
I'm thinking this scenario is impossible due to atomicity, but I'm hoping for confirmation from someone who understands ACID a little better than I do.
I think you are guaranteed to get the same list, gaps should never be filed in unless you're manually inserting them somehow. (although, if there are gaps i assume rows can be deleted, so it may not be exactly the same list because more may have been deleted.)
from How to reset AUTO_INCREMENT in MySQL?
You can reset the counter with:
ALTER TABLE tablename AUTO_INCREMENT = 1 For InnoDB you cannot set the
auto_increment value lower or equal to the highest current index.
(quote from ViralPatel):
Note that you cannot reset the counter to a value less than or equal
to any that have already been used. For MyISAM, if the value is less
than or equal to the maximum value currently in the AUTO_INCREMENT
column, the value is reset to the current maximum plus one. For
InnoDB, if the value is less than the current maximum value in the
column, no error occurs and the current sequence value is not changed.
See How to Reset an MySQL AutoIncrement using a MAX value from another
table? on how to dynamically get an acceptable value.
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'm having problem implementing auto-increment logic in my app. Says I inserts a 'group', and in mysql it has value 10 for its Id, next one would be 11, 12 and so forth.
But once the record (assume it's Id 12) got deleted, the next new item is 12 again. So it may have conflict.
Could possibly makes the auto increment don't repeat the same Int? I want every Id be unique, once it's delete means it never come back.
InnoDB really has this "Feature" or Bug, that the recent auto_increment is NOT stored in the table space. As soon as you restart the MySQL Server, the "auto_increment" value is taken from the highest recent value of the table, thus conflicting with possibly deleted values.
The solution to this is really ugly. You could create a table with the highest unused values per table, in the form
tablename maxvalue
tableA 375
tableB 12
and you could write a Post-Startup-Script, if you manage the MySQL-Server. So after every delete of a row of such a table you would check per AFTER DELETE, if that row was the max-value. That is a bit easier with newer versions of MySQL, since table informations are stored in INFORMATION_SCHEMA, and not only calculated with every select (which means reading INFORMATION_SCHEMA does not fire heavy and blocking queries so often).
You only have to update maxvalue if the deleted row was that max value.
It is a bit easier to update the maxalue on every insert on a row, if that does not slow down the system.
In some cases you have just one table with critical references, and that table has an index, so you can retrieve maxvalue from that table.
All in all that is a big problem with InnoDB, and writing a lot of Triggers just for this single unsaved number auto_increment is really not nice.
I think you not set id is primary key and auto increment
When using MySQL START TRANSACTION and the decision is made by MySQL to roll back -
In the case that a table had an AUTO_INCREMENT column - does the column get... decremented during the roll back?
Or should it?
I am having some issues where the transaction data is being properly rolled back - but it looks like the table was auto incremented and not decremented in the rollback.
# BOTH TABLES START OUT EMPTY // TABLE1 ID is **auto_increment**
START TRANSACTION;
INSERT INTO `TABLE1` (`ID` ,`NAME`) VALUES (NULL , 'Ted'); # MySQL TABLE1 **ID** is Auto incremented to 1
INSERT INTO `TABLE2` (`ID` ,`WRONGVALUE`) VALUES (NULL , 'some value'); # error. This TRANSACTION will not succeed
COMMIT; # Because of the error - this TRANSACTION is now rolled back and Ted is NOT added
Because MySQL will auto_increment the ID on the first table - regardless of if the transaction succeeds or fails - is the standard practice for this to decrement the table yourself?
No, auto-increment mechanisms must work outside the scope of transactions, because another user may be inserting to the same table before you finish your transaction. The other user's transaction must be able to allocate the next value, before knowing whether your transaction is using the value you just allocated.
Re your comment: If I may say it more clearly, any change in the scope of a transaction may be rolled back. The auto-increment counter is not rolled back, so it doesn't obey atomicity of transactions. Nor does it obey isolation because another transaction gets the next value even though your transaction hasn't committed yet.
The way auto-increment works means that sometimes, if you insert some rows and then roll back your transaction, the values you allocated with auto-increment are lost forever!
But this is okay. Primary key values must be unique, but they don't need to be consecutive. In other words, they are not row numbers, and you shouldn't use them like that. So you should never need to decrement values created by auto-increment.
Disclosure: I am relatively new to SQL so some of this may be wrong. But this is how I understand it.
Auto increment must function outside of transactions otherwise you can compromise data. Let us say you have 2 users both trying to connect to your dB. Both are creating accounts. When Account 1 is being created it increments up to 1. Then Account 2 is created and it increments to 2. So far it would look like the following:
(1, Account1)
(2, Account2)
Increment: 2.
Now let us say that Account 2 commits, but Account 1 fails. Now your table looks as follows:
(2, Account2)
Increment: 1.
Because you decremented your Auto Increment, you will fail at having a unique value when someone tries to register a new account, because the pair will be:
(2,Account2)
Attempting to Insert (2, NewAccount)
Which will fail because 2, the primary key, will not be unique.
If you need to fix your table (for development purposes) you can always run the following code:
ALTER TABLE `DB_NAME`.`TABLE_NAME`
AUTO_INCREMENT = 1 (or some other number);
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";