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12.4.4 Defining Trace Points

New trace points can be defined within the code wherever the developer desires, though this will require a recompilation.

A trace point can be inserted by using one of the trace macros. These are chosen according to how many variables will be made available for inspection at that trace point. Tracing the occurrence of an event can be achieved with a single line, using just the trace point name, e.g.

PG_TRACE (my__new__trace__point);

More complex trace points can be provided with one or more variables for inspection by the dynamic tracing utility by using the PG_TRACEn macro that corresponds to the number of parameters after the trace point name:

PG_TRACE3 (my__complex__event, varX, varY, varZ);

The definition of the transaction__start trace point is shown below:

static void
StartTransaction(void)
{
    ...

    /*
     * generate a new transaction id
     */
    s->transactionId = GetNewTransactionId(false);

    XactLockTableInsert(s->transactionId);

    PG_TRACE1(transaction__start, s->transactionId);

    ...
}

Note how the transaction ID is made available to the dynamic tracing utility.

The dynamic tracing utility may require you to further define these trace points. For example, DTrace requires you to add new probes to the file ‘src/backend/utils/probes.d’ as shown here:

provider postgresql {
      ...
      probe transaction__start(int);
      ...
 };

You should take care that the data types specified for the probe arguments match the datatypes of the variables used in the PG_TRACE macro. This is not checked at compile time. You can check that your newly added trace point is available by recompiling, then running the new binary, and as root, executing a DTrace command such as:

dtrace -l -n transaction-start