Generating transaction id for in-memory databases - database

Question

At the time of this writing, TRANSACTION_ID() does not support in-memory databases. I can generate my own IDs using a sequence table but it's not clear how to communicate existing IDs to triggers. The first trigger should generate a new ID. Subsequent triggers (in the same transaction) should share the existing ID.
I could use thread-local variables to share the existing ID but that seems fragile. Is there a better way to do this?

Answer 1

What about using sequences instead of transaction ids?
CREATE SEQUENCE SEQ;
The first operation in the transaction sets a session variable as follows:
SET #TID = SEQ.NEXTVAL;
The other operations within this transaction use the session variable:
CALL #TID;

Answer 2

I found a (very hacky) workaround:
/**
* Invoked when a transaction completes.
*/
public abstract class TransactionListener extends Value
{
private boolean invoked;
/**
* Invoked when the transaction completes.
*/
protected abstract void onCompleted();
#Override
public String getSQL()
{
return null;
}
#Override
public int getType()
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public long getPrecision()
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public int getDisplaySize()
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public String getString()
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public Object getObject()
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public void set(PreparedStatement prep, int parameterIndex) throws SQLException
{
throw new AssertionError("Unexpected method invocation");
}
#Override
protected int compareSecure(Value v, CompareMode mode)
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public int hashCode()
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public boolean equals(Object other)
{
throw new AssertionError("Unexpected method invocation");
}
#Override
public boolean isLinked()
{
return !invoked;
}
#Override
public void close()
{
invoked = true;
onCompleted();
}
}
// -------------TRIGGER BELOW-----------
public void fire(final Connection connection, ResultSet oldRow, ResultSet newRow)
throws SQLException
{
Statement statement = connection.createStatement();
long transactionId;
ResultSet rs = statement.executeQuery("SELECT #TRANSACTION_ID");
try
{
rs.next();
transactionId = rs.getLong(1);
if (transactionId == 0)
{
// Generate a new transaction id
rs.close();
JdbcConnection jdbcConnection = (JdbcConnection) connection;
final Session session = (Session) jdbcConnection.getSession();
session.unlinkAtCommit(new TransactionListener()
{
#Override
protected void onCompleted()
{
boolean oldAutoCommit = session.getAutoCommit();
session.setAutoCommit(false);
try
{
Statement statement = connection.createStatement();
statement.executeQuery("SELECT SET(#TRANSACTION_ID, NULL)");
statement.close();
}
catch (SQLException e)
{
throw new AssertionError(e);
}
finally
{
session.setAutoCommit(oldAutoCommit);
}
}
});
rs = statement.executeQuery("SELECT SET(#TRANSACTION_ID, "
+ "audit_transaction_sequence.NEXTVAL)");
rs.next();
transactionId = rs.getLong(1);
}
}
finally
{
rs.close();
}
assert (transactionId != 0);
// ...
}
Here's how it works:
We use Session.unlinkAtCommit() to listen for transaction commits (I assume this hooks rollbacks too but I haven't verified this yet)
Since we cannot predict the number and order of trigger invocation we must do the following check in every single trigger:
If #TRANSACTION_ID is null, register a new event listener and increment the sequence.
If #TRANSACTION_ID is not null, grab the current transaction id from it.
The two major problems with this workaround is:
It is extremely fragile. If Session.unlinkAtCommit() changes in the future it will likely break the event listener.
We must repeat a lot of boilerplate code at the top of each trigger just to retrieve the transaction id.
It would be far easier to implement this as a built-in function TRANSACTION_LOCAL_ID(). This funciton would return a database instance-specific transaction id similar to HSQLDB.