I am using SQLite in JAVA. I created a table with column id AUTOINCREMENT but I want to reset id after deletion so that no gaps exist between ids. How can I reset it ?
Although I highly recommend agains, like laalto
What problem would that solve? Generally, identifiers should stay stable. Having gaps between identifiers is not a problem.
It's possible by itinerating through all records with an auxiliary variable starting at 1 and incrementing. After reseting all id's you need to change the AUTOINCREMENT seed by running this SQL command DBCC CHECKIDENT('TABLE_NAME', RESEED, LATEST_RECORD)
EDIT
My bad here's the code for SQLite
UPDATE SQLITE_SEQUENCE SET SEQ=NEWSQUENCE WHERE NAME='table_name';
Related
I have a problem with MYSQL Workbench. Let's say I have this table, the user 'x' has index 8. Let's say I delete this user, so now I have only 7 users. If I create a new user with Java Console, the new user will be created with ID 9 because it's in auto increment. Is there a way to tell Workbench to start from where it is the last number? in that specific case, start creating the new user from 8 and not 9?
I tried to check everywhere on the web, but found only some query to recreate the table, but its not what I want because I have some foreign keys linked to that table, so I can't just delete the table and recreate another one.
I'm not familiar with MYSQL Workbench, but I think you might be able to alter your table and to change the value of the AUTO_INCREMENT.
Here's a link that talk about it in more details and even has an example:https://www.w3schools.com/mysql/mysql_autoincrement.asp
I hope this help you.
There is not a way to do that. Using Auto Increment or Identity makes a value no longer usable once it is used.
So a user with an ID of 1 will always have an ID of 1. A possible workaround is to not delete users, but instead put in a column of TinyInt(1) labeled 'Deleted'
New Users will have Deleted set to 0. When a user is deleted, their record gets set to 1. It will keep your IDs in order and then you just need to account for querying users who are not deleted.
Is it possible to restart the ID column of an HSQLDB after rows were inserted? Can I even set it to restart at a value lower than existing IDs in the table?
The Situation
I have a simple Java program which connects to a HSQLDB like so:
DriverManager.getConnection("jdbc:hsqldb:file:" + hsqldbPath, "", "");
This gives me an HsqlException when executing the following script (this is an excerpt, the complete script for HSQLDB 2.2.4 can be found here):
SET SCHEMA PUBLIC
CREATE MEMORY TABLE PUBLIC.MAP(
ID BIGINT GENERATED BY DEFAULT AS IDENTITY(START WITH 0) NOT NULL PRIMARY KEY,
FOO VARCHAR(16) NOT NULL)
ALTER TABLE PUBLIC.MAP ALTER COLUMN ID RESTART WITH 1
// [...]
SET SCHEMA PUBLIC
INSERT INTO MAP VALUES(1,'Foo')
INSERT INTO MAP VALUES(2,'Bar')
ALTER TABLE PUBLIC.MAP ALTER COLUMN ID RESTART WITH 42
The message is:
HsqlException: error in script file: ALTER TABLE PUBLIC.MAP ALTER COLUMN ID RESTART WITH 42
The exception goes away when I move the RESTART-command before the INSERTs. The documentation gives no hint as to why that would be necessary.
I will eventually have to make this work on version 2.2.4 but have the same problem with the current version 2.3.2.
Background
What I am trying to do here is to recreate a situation which apparently occurred in production: An unlucky interaction with the database (I don't know what exactly happened) seems to have caused newly inserted rows to collide with existing ones because they were issued the same IDs. I want to create a test replicating the scenario in order to write a proper fix.
The .script file of the database follows a predefined order for the statements. This shouldn't be altered if it is edited and only certain manual changes are allowed (see the guide for details).
You can execute the ALTER TABLE statement via JDBC at the start of your test instead of inserting it in the script.
If IDENTITY values for the PRIMARY KEY collide, you will get an exception when you insert the values.
The actual fix for a problem like this is to RESTART WITH the max value in the primary key column plus one.
I think SEQUENCES are much more flexiblee than IDENTITY. The IDENTITY generator disabled JDBC batching, by the way.
But if you use SEQUENCE identifiers, you must pay attention to the hilo optimizers as well, because identifier are generated by Hibernate using a sequence value as a base calculation starting point.
With a SEQUENCE the restart goes like this:
ALTER SEQUENCE my_seqeunce RESTART WITH 105;
Just a quick question about locking tables in a postgres database using JDBC. I have a table for which I want to add a new record to, however, To do this for the primary key, I use an increasing integer value.
I want to be able to retrieve the max value of this column in Java and store it as a variable to be used as a new primary key when adding a new row.
This gives me a small problem, as this is going to be modelled as a multi-user system, what happens when 2 locations request the same max value? This will of course create a problem when trying to add the same primary key.
I realise that I should be using an EXCLUSIVE lock on the table to prevent reading or writing while getting the key and adding a new row. However, I can't seem to find any way to deal with table locking in JDBC, just standard transactions.
psuedo code as such:
primaryKey = "SELECT MAX(id) FROM table1;";
primary key++;
//id retrieved again from 2nd source
"INSERT INTO table1 (primaryKey, value 1, value 2);"
You're absolutely right, if two locations request at around the same time, you'll run into a race condition.
The way to handle this is to create a sequence in postgres and select the nextval as the primary key.
I don't know exactly what direction you're heading and how your handle your data, but you could also set the column as a serial and not even include the column in your insert query. The column will automatically auto increment.
I am using a field with a prefix + auto increment id. For each instance i am taking the max+1 of ID and adding that to prefix. Can anyone suggest me a way to get this as unique please?
You can try this:
Insert into table1 (id, user_id)
SELECT MAX(id)+1, CONCAT('a',CAST(MAX(id)+1 AS char))
FROM table1;
See this SQLFiddle
The problem with using max(id)+1 is that there may be multiple threads making the same call, and so the result would not be unique. There are several ways to solve this problem. The first is to use a sequence, where the database server will increment the number every time a new id is requested. You can use a table, with a number in it, but you have to lock the table when you update the number. Or you can allow the database to create the key for you when the table is inserted and retrieve the key after the insert. All are valid.
I prefer to use Hibernate and make it determine how to implement the ID for the database I am currently using.
I have a webservice in java that receives a list of information to be inserted or updated in a database. I don't know which one is to insert or update.
Which one is the best approach to abtain better performance results:
Iterate over the list(a object list, with the table pk on it), try to insert the entry on Database. If the insert failed, run a update
Try to load the entry from database. if the results retrieved update, if not insert the entry.
another option? tell me about it :)
In first calls, i believe that most of the entries will be new bd entries, but there will be a saturation point that most of the entries will be to update.
I'm talking about a DB table that could reach over 100 million entries in a mature form.
What will be your approach? Performance is my most important goal.
If your database supports MERGE, I would have thought that was most efficient (and treats all the data as a single set).
See:
http://www.oracle.com/technology/products/oracle9i/daily/Aug24.html
https://web.archive.org/web/1/http://blogs.techrepublic%2ecom%2ecom/datacenter/?p=194
If performance is your goal then first get rid of the word iterate from your vocabulary! learn to do things in sets.
If you need to update or insert, always do the update first. Otherwise it is easy to find yourself updating the record you just inserted by accident. If you are doing this it helps to have an identifier you can look at to see if the record exists. If the identifier exists, then do the update otherwise do the insert.
The important thing is to understand the balance or ratio between the number of inserts versus the number of updates on the list you receive. IMHO you should implement an abstract strategy that says "persists this on database". Then create concrete strategies that (for example):
checks for primary key, if zero records are found does the insert, else updates
Does the update and, if fails, does the insert.
others
And then pull the strategy to use (the class fully qualified name for example) from a configuration file. This way you can switch from one strategy to another easily. If it is feasible, could be depending on your domain, you can put an heuristic that selects the best strategy based on the input entities on the set.
MySQL supports this:
INSERT INTO foo
SET bar='baz', howmanybars=1
ON DUPLICATE KEY UPDATE howmanybars=howmanybars+1
Option 2 is not going to be the most efficient. The database will already be making this check for you when you do the actual insert or update in order to enforce the primary key. By making this check yourself you are incurring the overhead of a table lookup twice as well as an extra round trip from your Java code. Choose which case is the most likely and code optimistically.
Expanding on option 1, you can use a stored procedure to handle the insert/update. This example with PostgreSQL syntax assumes the insert is the normal case.
CREATE FUNCTION insert_or_update(_id INTEGER, _col1 INTEGER) RETURNS void
AS $$
BEGIN
INSERT INTO
my_table (id, col1)
SELECT
_id, _col1;
EXCEPTION WHEN unique_violation THEN
UPDATE
my_table
SET
col1 = _col1
WHERE
id = _id;
END;
END;
$$
LANGUAGE plpgsql;
You could also make the update the normal case and then check the number of rows affected by the update statement to determine if the row is actually new and you need to do an insert.
As alluded to in some other answers, the most efficient way to handle this operation is in one batch:
Take all of the rows passed to the web service and bulk insert them into a temporary table
Update rows in the mater table from the temp table
Insert new rows in the master table from the temp table
Dispose of the temp table
The type of temporary table to use and most efficient way to manage it will depend on the database you are using.