Do not build explain output unless we need to

Instead of building the explain text on every execution start we only do
so if we are actually intending to use the result. This is especially a
gain for nested queries when we only log top level ones.

Author: jeltz

src/pg_stat_monitor.c

@@ -702,50 +702,49 @@ static void
 pgsm_ExecutorEnd(QueryDesc *queryDesc)
 {
 	int64		queryId = queryDesc->plannedstmt->queryId;
-	PlanInfo	plan_info;
-	PlanInfo   *plan_ptr = NULL;
-
-	/* Extract the plan information in case of SELECT statement */
-	if (queryDesc->operation == CMD_SELECT && pgsm_enable_query_plan)
-	{
-		int			plan_len;
-		MemoryContext oldctx;
-
-		/*
-		 * Run explain in a per query context so that there's no memory leak
-		 * when executor ends.
-		 */
-		oldctx = MemoryContextSwitchTo(queryDesc->estate->es_query_cxt);
-
-		plan_len = strlcpy(plan_info.plan_text, pgsm_explain(queryDesc), PLAN_TEXT_LEN);
-
-		MemoryContextSwitchTo(oldctx);
-
-		plan_info.plan_len = plan_len < PLAN_TEXT_LEN ? plan_len : PLAN_TEXT_LEN - 1;
-		plan_info.planid = pgsm_hash_string(plan_info.plan_text, plan_info.plan_len);
-		plan_ptr = &plan_info;
-	}
 
 	if (queryId != INT64CONST(0) && pgsm_query_instr(queryDesc) && pgsm_enabled(nesting_level))
 	{
+		PlanInfo	plan_info;
+		PlanInfo   *plan_ptr = NULL;
 		pgsmQueryStats *stats;
 		struct rusage rusage_end;
 		SysInfo		sys_info;
-		int64		planid = plan_ptr ? plan_ptr->planid : 0;
+
+		/* Extract the plan information in case of SELECT statement */
+		if (queryDesc->operation == CMD_SELECT && pgsm_enable_query_plan)
+		{
+			int			plan_len;
+			MemoryContext oldctx;
+			char	   *plan_text;
+
+			/*
+			 * Run explain in a per query context so that there's no memory
+			 * leak when executor ends.
+			 */
+			oldctx = MemoryContextSwitchTo(queryDesc->estate->es_query_cxt);
+			plan_text = pgsm_explain(queryDesc);
+			MemoryContextSwitchTo(oldctx);
+
+			plan_len = strlcpy(plan_info.plan_text, plan_text, PLAN_TEXT_LEN);
+			plan_info.plan_len = plan_len < PLAN_TEXT_LEN ? plan_len : PLAN_TEXT_LEN - 1;
+			plan_info.planid = pgsm_hash_string(plan_info.plan_text, plan_info.plan_len);
+			plan_ptr = &plan_info;
+		}
 
 		stats = pgsm_find_query_stats(queryId);
 		if (stats == NULL)
 		{
 			int			query_len = strlen(queryDesc->sourceText);
 
-			stats = pgsm_add_query_stats(queryId, planid,
+			stats = pgsm_add_query_stats(queryId, plan_ptr ? plan_ptr->planid : 0,
 										 get_pgsm_query_id_hash(queryDesc->sourceText, query_len),
 										 queryDesc->sourceText, query_len,
 										 queryDesc->operation);
 		}
 
-		if (stats->key.planid == 0 && planid != 0)
-			stats->key.planid = planid;
+		if (stats->key.planid == 0 && plan_ptr != NULL)
+			stats->key.planid = plan_ptr->planid;
 
 #if PG_VERSION_NUM < 190000