Develop Reference

Counting Number Of Specific Record In Database

I have a application which needs to aware of latest number of some records from a table from database, the solution should be applicable without changing the database code or add triggers or functions to it ,so I need a database vendor independent solution.
My program written in java but database could be (SQLite,MySQL,PostgreSQL or MSSQL),for now I'm doing Like that:
In a separate thread that is set as a daemon my application sends a simple command through JDBC to database to be aware of latest number of the records with condition:
while(true){
SELECT COUNT(*) FROM Mytable WHERE exited='1'
}
and this sort of coding causes DATABASE To lock,slows down the whole system and generates huge DB Logs which finally brings down the whole thing!
how can i do it in a right way to always have latest number of certain records or only counting when the number changed?

A SELECT statement should not -- by itself -- have the behavior that you are describing. For instance, nothing is logged with a SELECT. Now, it is possible that concurrent insert/update/delete statements are going on, and that these cause problems because the SELECT locks the table.
Two general things you can do:
Be sure that the comparison is of the same type. So, if exited is a number, do not use single quotes (mixing of types can confuse some databases).
Create an index on (exited). In basically all databases, this is a single command: create index idx_mytable_exited on mytable(exited).
If locking and concurrent transactions are an issue, then you will need to do more database specific things, to avoid that problem.

As others have said, make sure that exited is indexed.
Also, you can set the transaction isolation on your query to do a "dirty read"; this indicates to the database server that you do not need to wait for other processes' transactions to commit, and instead you wish to read the current value of exited on rows that are being updated by those other processes.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED is the standard syntax for using "dirty read".

Netbeans Entity Manager not updating Derby Database

I am currently using the automatically created class and Entity manager which is created when a table is bound to a database from NetBeans to get and set values to a derby database.
However when I want to update/edit the field using:
LessonTb Obj = new LessonTb();
Obj.setAdditionalResources(Paths);
Obj.setDescription(LessonDescription);
Obj.setLessonName(LessonName);
Obj.setLessonPath(LessonName + ".txt");
Obj.setRecommendedTest(RecommendedTest);
EUCLIDES_DBPUEntityManager.getTransaction().begin();
EUCLIDES_DBPUEntityManager.getTransaction().commit();
lessonTbList.clear();
lessonTbList.addAll(lessonTbQuery.getResultList());
The current Entry does not update in the database despite knowing that the code worked in other projects. I use the same get and set methods from the same LessonTb class which works to add a new entry and delete and entry.
What could possibly be wrong and how do I solve my problem? No exceptions are thrown.

Here's several possibilities. Perhaps you can do more research to rule at least some of them out:
You're using an in-memory database, and you didn't realize that all the database contents are lost when your application terminates.
You're not in auto-commit mode, and your application failed to issue a commit statement after making your update
You're not actually issuing the update statement that you think you're issuing. For some reason, your program flow is not reaching that code.
Your update statement has encountered an error, but it's not the sort of error that results in an exception. Instead, there's an error code returned, but no exception is thrown.
There are multiple copies of the database, or multiple copies of the schema within the database, and you're updating one copy of the database but querying a different one.
One powerful tool for helping you diagnose things more deeply is to learn how to use -Dderby.language.logStatementText=true and read in derby.log what actual SQL statements you're issuing, and what the results of those statements are. Here's a couple links to help you get started doing that: https://db.apache.org/derby/docs/10.4/tuning/rtunproper43517.html and http://apache-database.10148.n7.nabble.com/How-to-log-queries-in-Apache-Derby-td136818.html

Need a program that continously listens to the Oracle DB and if any DML's are made it calls another program?

I have an Oracle DB with a lot of tables ... How can I write something which listens to the Oracle DB and if any changes are made to the DB, it calls another program which does some processing
It would be asnync
I just need to trigger a java program if there is any kind of DML happening .. I dont want details about which table or what rows or new/old values ...
If there is a DML on any table in a DB call a java program thats it

http://docs.oracle.com/cd/B14117_01/win.101/b10118/o4o00118.htm
OracleDB has some events. Just try to use them; For example, when the db is updated - And to call another java program:
Runtime.getRuntime().exec(/Here the argument/);

I fear you're setting yourself up for failure here. I suggest a related, but slightly different course of action.
Rather than trigging your processing on every data change, consider instead having your processing run every X minutes/hours/whatever using something like Cron or Quartz.
If you're worried about having it run when no changes have been made, you can add triggers to your tables to update a "last updated" table and you can abort if no changes have been made since the last run.
This avoids the biggest concern you would have with an "on-update-processor", namely what do you do if there's an update, which triggers a process, and while that process is running another update happens. Do you trigger another process? What if they conflict? I think you'll find it better to allow there to be a slight delay between the update and the synchronization process.

That's a pretty tall order for a question.
For starters, you are going to need a way to detect that a DML operation (INSERT, UPDATE, DELETE) has occurred.
For auditing on individual tables, we can use TRIGGERS.
CREATE TRIGGER [BEFORE|AFTER] [INSERT][UPDATE][DELETE] ... [FOR EACH ROW]
The normative action is to record information about the information (whatever is needed later) in an "audit log" table. You have to make a decision whether the operations you need to perform are synchronous (should happen before the DML completes) or whether those operations can be asynchronous (can happen anytime after the DML completes).
Normally, synchronous processing is done in PL/SQL, but Oracle does provide a mechanism for calling external procedures (EXTPROC), though you wouldn't want to do that from a TRIGGER.
Oracle also provides "fine grained auditing". The information gathered by FGA is not at a "row by row" level, like we can get with a FOR EACH ROW trigger.
So, it really depends on what you are trying to achieve.
The next step is figuring out how you are going to get this information to your external process. Is that process going to periodically poll table (or set of tables), are you going to use AQ Advanced Queueing, or some other queue mechanism.

H2 Database - Creating Indexes

I'm using the H2 database - running in embedded mode - and when my app starts up I load the H2 database with data from a mySQL database. I'm using linked tables to point to the mySQL tables.
My issue is that I'm trying to speed up the time that H2 takes to create the indexes on the tables, particularly for larger tables (5Million+).
Does anyone know if it is safe to run the CREATE INDEX commands in a separate thread while I load the next table's data into H2?
For example:
Thread 1: Loads table 1 -> tells Thread 2 to start creating indexes and then Thread 1 loads table 2, etc.
I can't use the MVCC mode when loading the tables because later on I need to use the MULTI_THREADED mode when I do my selects. When I try using the MULTI_THREADED mode I got locking errors even though I was loading data into discrete tables.
Many thanks!

What might work (but I'm not sure if it's faster) is to create the tables and indexes first, and then load the tables in parallel. This should avoid locking problems in the system table.

I would also like to add the method rst.findColumn("columnName") to find the indexes AFTER getting the result set of the table. rst is a ResultSet object. This is what I have used.

Another way to dramatically improve H2 loading and especially indexing performance is to set the initial memory close to what the expected memory requirement is. As one example, this one change allowed an app with about a 1.5GB requirement to startup in 47 seconds instead of failing after 15 - 20 minutes. Prior to this, we were seeing GC Overhead limit exceeded and JVMTI errors.
Add the following to your VM arguments (as an example):
-Xms2g
-Xmx4g

Oracle SQL from Java using Spring returns nothing, and doesnt throw exception

I have a Java code that uses Spring to connect and execute sql on an Oracle DB. I have a query that takes long time to execute (20 minutes or sometimes more). I have a Executor Service and it has a Thread that will execute the query and process the results. If i put a timeout to the DB and Spring, the system will time out correctly but will return nothing else before that. If i run the query from SQL plus, it will return values. The time out is set up 3 times what it takes to execute on SQL Developer.
Any ideas!?

Assuming that your Spring query is using bind variables, are you using bind variables when you execute the query in SQL*Plus/ SQL Developer? Or are you using literals?
What version of Oracle are you using?
Have you checked to see whether the query plans for the two environments are different?

20 minutes for a query in Oracle? I'll bet you don't have appropriate indexes on the columns in your WHERE clause.
The dead giveaway is to do an EXPLAIN PLAN on the query. If you see a TABLE SCAN, take appropriate measures.
If you can run the same query in SQL*Plus and see it return in a reasonable time, then I'm incorrect and the problem is due to something else that you did in Java code.
I don't see why you need a separate thread for a query. I'd run the code straight, without a thread, and see how it behaves. If you aren't indexed properly, add some; if the query brings back too much data, add WHERE clauses to restrict it. You've taken extraordinary measures without really understanding what the root cause is.