PostgreSQL provides facilities to support dynamic tracing of the database server. This allows an external utility to be called at specific points in the code and thereby trace execution. Currently, this facility is primarily intended for use by database developers, as it requires substantial familiarity with the code.
A number of trace points, often called probes, are already inserted into the source code. By default these probes are disabled, and the user needs to explicitly tell the configure script to make the probes available in PostgreSQL.
Currently, only the DTrace utility is supported, which is only available
on Solaris Express and Solaris 10+. It is expected that DTrace will
be available in the future on FreeBSD and Mac OS X.
Supporting other dynamic tracing utilities is theoretically possible by
changing the definitions for the PG_TRACE
macros in
src/include/pg_trace.h
.
By default, trace points are disabled, so you will need to
explicitly tell the configure script to make the probes available
in PostgreSQL. To include DTrace support
specify --enable-dtrace
to configure. See Section 14.5, “Installation Procedure” for further information.
A few standard trace points are provided in the source code (of course, more can be added as needed for a particular problem). These are shown in Table 25.3, “Built-in Trace Points”.
Table 25.3. Built-in Trace Points
Name | Parameters | Overview |
---|---|---|
transaction__start | (int transactionId) | The start of a new transaction. |
transaction__commit | (int transactionId) | The successful completion of a transaction. |
transaction__abort | (int transactionId) | The unsuccessful completion of a transaction. |
lwlock__acquire | (int lockid, int mode) | An LWLock has been acquired. |
lwlock__release | (int lockid, int mode) | An LWLock has been released. |
lwlock__startwait | (int lockid, int mode) | An LWLock was not immediately available and a backend has begun to wait for the lock to become available. |
lwlock__endwait | (int lockid, int mode) | A backend has been released from its wait for an LWLock. |
lwlock__condacquire | (int lockid, int mode) | An LWLock was successfully acquired when the caller specified no waiting. |
lwlock__condacquire__fail | (int lockid, int mode) | An LWLock was not successfully acquired when the caller specified no waiting. |
lock__startwait | (int locktag_field2, int lockmode) | A request for a heavyweight lock (lmgr lock) has begun to wait because the lock is not available. |
lock__endwait | (int locktag_field2, int lockmode) | A request for a heavyweight lock (lmgr lock) has finished waiting (i.e., has acquired the lock). |
The example below shows a DTrace script for analyzing transaction
counts on the system, as an alternative to snapshotting
pg_stat_database
before and after a performance test.
#!/usr/sbin/dtrace -qs postgresql$1:::transaction-start { @start["Start"] = count(); self->ts = timestamp; } postgresql$1:::transaction-abort { @abort["Abort"] = count(); } postgresql$1:::transaction-commit /self->ts/ { @commit["Commit"] = count(); @time["Total time (ns)"] = sum(timestamp - self->ts); self->ts=0; }
Note how the double underline in trace point names needs to be replaced by a hyphen when using D script. When executed, the example D script gives output such as:
# ./txn_count.d `pgrep -n postgres` ^C Start 71 Commit 70 Total time (ns) 2312105013
You should remember that trace programs need to be carefully written and debugged prior to their use, otherwise the trace information collected may be meaningless. In most cases where problems are found it is the instrumentation that is at fault, not the underlying system. When discussing information found using dynamic tracing, be sure to enclose the script used to allow that too to be checked and discussed.
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_TRACE
n
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