Add TIMING option to EXPLAIN, to allow eliminating of timing overhead.

Sometimes it may be useful to get actual row counts out of EXPLAIN
(ANALYZE) without paying the cost of timing every node entry/exit.
With this patch, you can say EXPLAIN (ANALYZE, TIMING OFF) to get that.

Tomas Vondra, reviewed by Eric Theise, with minor doc changes by me.

contrib/auto_explain/auto_explain.c

@@ -23,6 +23,7 @@ static int	auto_explain_log_min_duration = -1; /* msec or -1 */
 static bool auto_explain_log_analyze = false;
 static bool auto_explain_log_verbose = false;
 static bool auto_explain_log_buffers = false;
+static bool auto_explain_log_timing  = false;
 static int	auto_explain_log_format = EXPLAIN_FORMAT_TEXT;
 static bool auto_explain_log_nested_statements = false;
 
@@ -133,6 +134,17 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
+	DefineCustomBoolVariable("auto_explain.log_timing",
+							 "Collect timing data, not just row counts.",
+							 NULL,
+							 &auto_explain_log_timing,
+							 true,
+							 PGC_SUSET,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
+
 	EmitWarningsOnPlaceholders("auto_explain");
 
 	/* Install hooks. */
@@ -170,7 +182,12 @@ explain_ExecutorStart(QueryDesc *queryDesc, int eflags)
 		/* Enable per-node instrumentation iff log_analyze is required. */
 		if (auto_explain_log_analyze && (eflags & EXEC_FLAG_EXPLAIN_ONLY) == 0)
 		{
-			queryDesc->instrument_options |= INSTRUMENT_TIMER;
+			if (auto_explain_log_timing)
+				queryDesc->instrument_options |= INSTRUMENT_TIMER;
+			else
+				queryDesc->instrument_options |= INSTRUMENT_ROWS;
+
+
 			if (auto_explain_log_buffers)
 				queryDesc->instrument_options |= INSTRUMENT_BUFFERS;
 		}

doc/src/sgml/ref/explain.sgml

@@ -40,6 +40,7 @@ EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replac
     VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
     COSTS [ <replaceable class="parameter">boolean</replaceable> ]
     BUFFERS [ <replaceable class="parameter">boolean</replaceable> ]
+    TIMING [ <replaceable class="parameter">boolean</replaceable> ]
     FORMAT { TEXT | XML | JSON | YAML }
 </synopsis>
  </refsynopsisdiv>
@@ -171,6 +172,21 @@ ROLLBACK;
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>TIMING</literal></term>
+    <listitem>
+     <para>
+      Include the actual startup time and time spent in the node in the output.
+      The overhead of repeatedly reading the system clock can slow down the
+      query significantly on some systems, so it may be useful to set this
+      parameter to <literal>FALSE</literal> when only actual row counts, and not
+      exact times, are needed.
+      This parameter may only be used when <literal>ANALYZE</literal> is also
+      enabled.  It defaults to <literal>TRUE</literal>.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>FORMAT</literal></term>
     <listitem>

src/backend/commands/explain.c

@@ -116,6 +116,7 @@ ExplainQuery(ExplainStmt *stmt, const char *queryString,
 	TupOutputState *tstate;
 	List	   *rewritten;
 	ListCell   *lc;
+	bool	    timing_set = false;
 
 	/* Initialize ExplainState. */
 	ExplainInitState(&es);
@@ -133,6 +134,11 @@ ExplainQuery(ExplainStmt *stmt, const char *queryString,
 			es.costs = defGetBoolean(opt);
 		else if (strcmp(opt->defname, "buffers") == 0)
 			es.buffers = defGetBoolean(opt);
+		else if (strcmp(opt->defname, "timing") == 0)
+		{
+			timing_set = true;
+			es.timing = defGetBoolean(opt);
+		}
 		else if (strcmp(opt->defname, "format") == 0)
 		{
 			char	   *p = defGetString(opt);
@@ -162,6 +168,15 @@ ExplainQuery(ExplainStmt *stmt, const char *queryString,
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("EXPLAIN option BUFFERS requires ANALYZE")));
+	
+	/* if the timing was not set explicitly, set default value */
+	es.timing = (timing_set) ? es.timing : es.analyze;
+
+	/* check that timing is used with EXPLAIN ANALYZE */
+	if (es.timing && !es.analyze)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("EXPLAIN option TIMING requires ANALYZE")));
 
 	/*
 	 * Parse analysis was done already, but we still have to run the rule
@@ -360,8 +375,11 @@ ExplainOnePlan(PlannedStmt *plannedstmt, ExplainState *es,
 	int			eflags;
 	int			instrument_option = 0;
 
-	if (es->analyze)
+	if (es->analyze && es->timing)
 		instrument_option |= INSTRUMENT_TIMER;
+	else if (es->analyze)
+		instrument_option |= INSTRUMENT_ROWS;
+
 	if (es->buffers)
 		instrument_option |= INSTRUMENT_BUFFERS;
 
@@ -956,29 +974,42 @@ ExplainNode(PlanState *planstate, List *ancestors,
 
 		if (es->format == EXPLAIN_FORMAT_TEXT)
 		{
-			appendStringInfo(es->str,
-							 " (actual time=%.3f..%.3f rows=%.0f loops=%.0f)",
-							 startup_sec, total_sec, rows, nloops);
+			if (planstate->instrument->need_timer)
+				appendStringInfo(es->str,
+								 " (actual time=%.3f..%.3f rows=%.0f loops=%.0f)",
+								 startup_sec, total_sec, rows, nloops);
+			else
+				appendStringInfo(es->str,
+								 " (actual rows=%.0f loops=%.0f)",
+								 rows, nloops);
 		}
 		else
 		{
-			ExplainPropertyFloat("Actual Startup Time", startup_sec, 3, es);
-			ExplainPropertyFloat("Actual Total Time", total_sec, 3, es);
+			if (planstate->instrument->need_timer)
+			{
+				ExplainPropertyFloat("Actual Startup Time", startup_sec, 3, es);
+				ExplainPropertyFloat("Actual Total Time", total_sec, 3, es);
+			}
 			ExplainPropertyFloat("Actual Rows", rows, 0, es);
 			ExplainPropertyFloat("Actual Loops", nloops, 0, es);
 		}
 	}
 	else if (es->analyze)
 	{
+
 		if (es->format == EXPLAIN_FORMAT_TEXT)
 			appendStringInfo(es->str, " (never executed)");
-		else
+		else if (planstate->instrument->need_timer)
 		{
 			ExplainPropertyFloat("Actual Startup Time", 0.0, 3, es);
 			ExplainPropertyFloat("Actual Total Time", 0.0, 3, es);
+		}
+		else
+		{
 			ExplainPropertyFloat("Actual Rows", 0.0, 0, es);
 			ExplainPropertyFloat("Actual Loops", 0.0, 0, es);
 		}
+
 	}
 
 	/* in text format, first line ends here */

src/backend/executor/instrument.c

@@ -29,17 +29,19 @@ InstrAlloc(int n, int instrument_options)
 {
 	Instrumentation *instr;
 
-	/* timer is always required for now */
-	Assert(instrument_options & INSTRUMENT_TIMER);
-
 	/* initialize all fields to zeroes, then modify as needed */
 	instr = palloc0(n * sizeof(Instrumentation));
-	if (instrument_options & INSTRUMENT_BUFFERS)
+	if (instrument_options & (INSTRUMENT_BUFFERS | INSTRUMENT_TIMER))
 	{
 		int			i;
+		bool		need_buffers = instrument_options & INSTRUMENT_BUFFERS;
+		bool		need_timer = instrument_options & INSTRUMENT_TIMER;
 
 		for (i = 0; i < n; i++)
-			instr[i].need_bufusage = true;
+		{
+			instr[i].need_bufusage = need_buffers;
+			instr[i].need_timer = need_timer;
+		}
 	}
 
 	return instr;
@@ -49,7 +51,7 @@ InstrAlloc(int n, int instrument_options)
 void
 InstrStartNode(Instrumentation *instr)
 {
-	if (INSTR_TIME_IS_ZERO(instr->starttime))
+	if (instr->need_timer && INSTR_TIME_IS_ZERO(instr->starttime))
 		INSTR_TIME_SET_CURRENT(instr->starttime);
 	else
 		elog(DEBUG2, "InstrStartNode called twice in a row");
@@ -68,16 +70,22 @@ InstrStopNode(Instrumentation *instr, double nTuples)
 	/* count the returned tuples */
 	instr->tuplecount += nTuples;
 
-	if (INSTR_TIME_IS_ZERO(instr->starttime))
+	/* let's update the time only if the timer was requested */
+	if (instr->need_timer)
 	{
-		elog(DEBUG2, "InstrStopNode called without start");
-		return;
-	}
 
-	INSTR_TIME_SET_CURRENT(endtime);
-	INSTR_TIME_ACCUM_DIFF(instr->counter, endtime, instr->starttime);
+		if (INSTR_TIME_IS_ZERO(instr->starttime))
+		{
+			elog(DEBUG2, "InstrStopNode called without start");
+			return;
+		}
 
-	INSTR_TIME_SET_ZERO(instr->starttime);
+		INSTR_TIME_SET_CURRENT(endtime);
+		INSTR_TIME_ACCUM_DIFF(instr->counter, endtime, instr->starttime);
+
+		INSTR_TIME_SET_ZERO(instr->starttime);
+
+	}
 
 	/* Add delta of buffer usage since entry to node's totals */
 	if (instr->need_bufusage)

src/include/commands/explain.h

@@ -31,6 +31,7 @@ typedef struct ExplainState
 	bool		analyze;		/* print actual times */
 	bool		costs;			/* print costs */
 	bool		buffers;		/* print buffer usage */
+	bool		timing;			/* print timing */
 	ExplainFormat format;		/* output format */
 	/* other states */
 	PlannedStmt *pstmt;			/* top of plan */

src/include/executor/instrument.h

@@ -31,14 +31,16 @@ typedef struct BufferUsage
 /* Flag bits included in InstrAlloc's instrument_options bitmask */
 typedef enum InstrumentOption
 {
-	INSTRUMENT_TIMER = 1 << 0,	/* needs timer */
+	INSTRUMENT_TIMER = 1 << 0,	/* needs timer (and row counts) */
 	INSTRUMENT_BUFFERS = 1 << 1,	/* needs buffer usage */
+	INSTRUMENT_ROWS = 1 << 2,	/* needs row count */
 	INSTRUMENT_ALL = 0x7FFFFFFF
 } InstrumentOption;
 
 typedef struct Instrumentation
 {
 	/* Parameters set at node creation: */
+	bool		need_timer;	    /* TRUE if we need timer data */
 	bool		need_bufusage;	/* TRUE if we need buffer usage data */
 	/* Info about current plan cycle: */
 	bool		running;		/* TRUE if we've completed first tuple */