Add the option to report WAL usage in EXPLAIN and auto_explain.

This commit adds a new option WAL similar to existing option BUFFERS in the
EXPLAIN command.  This option allows to include information on WAL record
generation added by commit df3b1814 in EXPLAIN output.

This also allows the WAL usage information to be displayed via
the auto_explain module.  A new parameter auto_explain.log_wal controls
whether WAL usage statistics are printed when an execution plan is logged.
This parameter has no effect unless auto_explain.log_analyze is enabled.

Author: Julien Rouhaud
Reviewed-by: Dilip Kumar and Amit Kapila
Discussion: https://postgr.es/m/CAB-hujrP8ZfUkvL5OYETipQwA=e3n7oqHFU=4ZLxWS_Cza3kQQ@mail.gmail.com

contrib/auto_explain/auto_explain.c

@@ -27,6 +27,7 @@ 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_wal = false;
 static bool auto_explain_log_triggers = false;
 static bool auto_explain_log_timing = true;
 static bool auto_explain_log_settings = false;
@@ -148,6 +149,17 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
+	DefineCustomBoolVariable("auto_explain.log_wal",
+							 "Log WAL usage.",
+							 NULL,
+							 &auto_explain_log_wal,
+							 false,
+							 PGC_SUSET,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
+
 	DefineCustomBoolVariable("auto_explain.log_triggers",
 							 "Include trigger statistics in plans.",
 							 "This has no effect unless log_analyze is also set.",
@@ -280,6 +292,8 @@ explain_ExecutorStart(QueryDesc *queryDesc, int eflags)
 				queryDesc->instrument_options |= INSTRUMENT_ROWS;
 			if (auto_explain_log_buffers)
 				queryDesc->instrument_options |= INSTRUMENT_BUFFERS;
+			if (auto_explain_log_wal)
+				queryDesc->instrument_options |= INSTRUMENT_WAL;
 		}
 	}
 
@@ -374,6 +388,7 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
 			es->analyze = (queryDesc->instrument_options && auto_explain_log_analyze);
 			es->verbose = auto_explain_log_verbose;
 			es->buffers = (es->analyze && auto_explain_log_buffers);
+			es->wal = (es->analyze && auto_explain_log_wal);
 			es->timing = (es->analyze && auto_explain_log_timing);
 			es->summary = es->analyze;
 			es->format = auto_explain_log_format;

doc/src/sgml/auto-explain.sgml

@@ -109,6 +109,26 @@ LOAD 'auto_explain';
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term>
+     <varname>auto_explain.log_wal</varname> (<type>boolean</type>)
+     <indexterm>
+      <primary><varname>auto_explain.log_wal</varname> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      <varname>auto_explain.log_wal</varname> controls whether WAL
+      usage statistics are printed when an execution plan is logged; it's
+      equivalent to the <literal>WAL</literal> option of <command>EXPLAIN</command>.
+      This parameter has no effect
+      unless <varname>auto_explain.log_analyze</varname> is enabled.
+      This parameter is off by default.
+      Only superusers can change this setting.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term>
      <varname>auto_explain.log_timing</varname> (<type>boolean</type>)

doc/src/sgml/ref/explain.sgml

@@ -41,6 +41,7 @@ EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replac
     COSTS [ <replaceable class="parameter">boolean</replaceable> ]
     SETTINGS [ <replaceable class="parameter">boolean</replaceable> ]
     BUFFERS [ <replaceable class="parameter">boolean</replaceable> ]
+    WAL [ <replaceable class="parameter">boolean</replaceable> ]
     TIMING [ <replaceable class="parameter">boolean</replaceable> ]
     SUMMARY [ <replaceable class="parameter">boolean</replaceable> ]
     FORMAT { TEXT | XML | JSON | YAML }
@@ -192,6 +193,19 @@ ROLLBACK;
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>WAL</literal></term>
+    <listitem>
+     <para>
+      Include information on WAL record generation. Specifically, include the
+      number of records, number of full page image records and amount of WAL
+      bytes generated.  In text format, only non-zero values are printed.  This
+      parameter may only be used when <literal>ANALYZE</literal> is also
+      enabled.  It defaults to <literal>FALSE</literal>.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>TIMING</literal></term>
     <listitem>

src/backend/commands/explain.c

@@ -113,6 +113,7 @@ static void show_foreignscan_info(ForeignScanState *fsstate, ExplainState *es);
 static void show_eval_params(Bitmapset *bms_params, ExplainState *es);
 static const char *explain_get_index_name(Oid indexId);
 static void show_buffer_usage(ExplainState *es, const BufferUsage *usage);
+static void show_wal_usage(ExplainState *es, const WalUsage *usage);
 static void ExplainIndexScanDetails(Oid indexid, ScanDirection indexorderdir,
 									ExplainState *es);
 static void ExplainScanTarget(Scan *plan, ExplainState *es);
@@ -175,6 +176,8 @@ ExplainQuery(ParseState *pstate, ExplainStmt *stmt,
 			es->costs = defGetBoolean(opt);
 		else if (strcmp(opt->defname, "buffers") == 0)
 			es->buffers = defGetBoolean(opt);
+		else if (strcmp(opt->defname, "wal") == 0)
+			es->wal = defGetBoolean(opt);
 		else if (strcmp(opt->defname, "settings") == 0)
 			es->settings = defGetBoolean(opt);
 		else if (strcmp(opt->defname, "timing") == 0)
@@ -219,6 +222,11 @@ ExplainQuery(ParseState *pstate, ExplainStmt *stmt,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("EXPLAIN option BUFFERS requires ANALYZE")));
 
+	if (es->wal && !es->analyze)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("EXPLAIN option WAL requires ANALYZE")));
+
 	/* if the timing was not set explicitly, set default value */
 	es->timing = (timing_set) ? es->timing : es->analyze;
 
@@ -506,6 +514,8 @@ ExplainOnePlan(PlannedStmt *plannedstmt, IntoClause *into, ExplainState *es,
 
 	if (es->buffers)
 		instrument_option |= INSTRUMENT_BUFFERS;
+	if (es->wal)
+		instrument_option |= INSTRUMENT_WAL;
 
 	/*
 	 * We always collect timing for the entire statement, even when node-level
@@ -1970,12 +1980,14 @@ ExplainNode(PlanState *planstate, List *ancestors,
 		}
 	}
 
-	/* Show buffer usage */
+	/* Show buffer/WAL usage */
 	if (es->buffers && planstate->instrument)
 		show_buffer_usage(es, &planstate->instrument->bufusage);
+	if (es->wal && planstate->instrument)
+		show_wal_usage(es, &planstate->instrument->walusage);
 
-	/* Prepare per-worker buffer usage */
-	if (es->workers_state && es->buffers && es->verbose)
+	/* Prepare per-worker buffer/WAL usage */
+	if (es->workers_state && (es->buffers || es->wal) && es->verbose)
 	{
 		WorkerInstrumentation *w = planstate->worker_instrument;
 
@@ -1988,7 +2000,10 @@ ExplainNode(PlanState *planstate, List *ancestors,
 				continue;
 
 			ExplainOpenWorker(n, es);
+			if (es->buffers)
 				show_buffer_usage(es, &instrument->bufusage);
+			if (es->wal)
+				show_wal_usage(es, &instrument->walusage);
 			ExplainCloseWorker(n, es);
 		}
 	}
@@ -3087,6 +3102,44 @@ show_buffer_usage(ExplainState *es, const BufferUsage *usage)
 	}
 }
 
+/*
+ * Show WAL usage details.
+ */
+static void
+show_wal_usage(ExplainState *es, const WalUsage *usage)
+{
+	if (es->format == EXPLAIN_FORMAT_TEXT)
+	{
+		/* Show only positive counter values. */
+		if ((usage->wal_records > 0) || (usage->wal_num_fpw > 0) ||
+			(usage->wal_bytes > 0))
+		{
+			ExplainIndentText(es);
+			appendStringInfoString(es->str, "WAL:");
+
+			if (usage->wal_records > 0)
+				appendStringInfo(es->str, "  records=%ld",
+								 usage->wal_records);
+			if (usage->wal_num_fpw > 0)
+				appendStringInfo(es->str, "  full page writes=%ld",
+								 usage->wal_num_fpw);
+			if (usage->wal_bytes > 0)
+				appendStringInfo(es->str, "  bytes=" UINT64_FORMAT,
+								 usage->wal_bytes);
+			appendStringInfoChar(es->str, '\n');
+		}
+	}
+	else
+	{
+		ExplainPropertyInteger("WAL records", NULL,
+							   usage->wal_records, es);
+		ExplainPropertyInteger("WAL full page writes", NULL,
+							   usage->wal_num_fpw, es);
+		ExplainPropertyUInteger("WAL bytes", NULL,
+								usage->wal_bytes, es);
+	}
+}
+
 /*
  * Add some additional details about an IndexScan or IndexOnlyScan
  */
@@ -3871,6 +3924,19 @@ ExplainPropertyInteger(const char *qlabel, const char *unit, int64 value,
 	ExplainProperty(qlabel, unit, buf, true, es);
 }
 
+/*
+ * Explain an unsigned integer-valued property.
+ */
+void
+ExplainPropertyUInteger(const char *qlabel, const char *unit, uint64 value,
+						ExplainState *es)
+{
+	char		buf[32];
+
+	snprintf(buf, sizeof(buf), UINT64_FORMAT, value);
+	ExplainProperty(qlabel, unit, buf, true, es);
+}
+
 /*
  * Explain a float-valued property, using the specified number of
  * fractional digits.

src/bin/psql/tab-complete.c

@@ -3045,8 +3045,8 @@ psql_completion(const char *text, int start, int end)
 		 */
 		if (ends_with(prev_wd, '(') || ends_with(prev_wd, ','))
 			COMPLETE_WITH("ANALYZE", "VERBOSE", "COSTS", "SETTINGS",
-						  "BUFFERS", "TIMING", "SUMMARY", "FORMAT");
-		else if (TailMatches("ANALYZE|VERBOSE|COSTS|SETTINGS|BUFFERS|TIMING|SUMMARY"))
+						  "BUFFERS", "WAL", "TIMING", "SUMMARY", "FORMAT");
+		else if (TailMatches("ANALYZE|VERBOSE|COSTS|SETTINGS|BUFFERS|WAL|TIMING|SUMMARY"))
 			COMPLETE_WITH("ON", "OFF");
 		else if (TailMatches("FORMAT"))
 			COMPLETE_WITH("TEXT", "XML", "JSON", "YAML");

src/include/commands/explain.h

@@ -42,6 +42,7 @@ typedef struct ExplainState
 	bool		analyze;		/* print actual times */
 	bool		costs;			/* print estimated costs */
 	bool		buffers;		/* print buffer usage */
+	bool		wal;			/* print WAL usage */
 	bool		timing;			/* print detailed node timing */
 	bool		summary;		/* print total planning and execution timing */
 	bool		settings;		/* print modified settings */
@@ -111,6 +112,8 @@ extern void ExplainPropertyText(const char *qlabel, const char *value,
 								ExplainState *es);
 extern void ExplainPropertyInteger(const char *qlabel, const char *unit,
 								   int64 value, ExplainState *es);
+extern void ExplainPropertyUInteger(const char *qlabel, const char *unit,
+									uint64 value, ExplainState *es);
 extern void ExplainPropertyFloat(const char *qlabel, const char *unit,
 								 double value, int ndigits, ExplainState *es);
 extern void ExplainPropertyBool(const char *qlabel, bool value,