Extract logic filling pg_stat_get_io()'s tuplestore into its own routine

This commit adds pg_stat_io_build_tuples(), a helper routine for
pg_stat_get_io(), that fills its result tuplestore based on the contents
of PgStat_BktypeIO.  This will be used in a follow-up commit that uses
the same structures as pg_stat_io for reporting, including the same
object types and contexts, but for a different statistics kind.

Author: Bertrand Drouvot, Michael Paquier
Discussion: https://postgr.es/m/ZtXR+CtkEVVE/LHF@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/utils/adt/pgstatfuncs.c

@@ -1365,38 +1365,20 @@ pg_stat_us_to_ms(PgStat_Counter val_ms)
 	return val_ms * (double) 0.001;
 }
 
-Datum
+/*
-pg_stat_get_io(PG_FUNCTION_ARGS)
+ * pg_stat_io_build_tuples
+ *
+ * Helper routine for pg_stat_get_io() filling a result tuplestore with one
+ * tuple for each object and each context supported by the caller, based on the
+ * contents of bktype_stats.
+ */
+static void
+pg_stat_io_build_tuples(ReturnSetInfo *rsinfo,
+						PgStat_BktypeIO *bktype_stats,
+						BackendType bktype,
+						TimestampTz stat_reset_timestamp)
 {
-	ReturnSetInfo *rsinfo;
-	PgStat_IO  *backends_io_stats;
-	Datum		reset_time;
-
-	InitMaterializedSRF(fcinfo, 0);
-	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
-
-	backends_io_stats = pgstat_fetch_stat_io();
-
-	reset_time = TimestampTzGetDatum(backends_io_stats->stat_reset_timestamp);
-
-	for (int bktype = 0; bktype < BACKEND_NUM_TYPES; bktype++)
-	{
 	Datum		bktype_desc = CStringGetTextDatum(GetBackendTypeDesc(bktype));
-		PgStat_BktypeIO *bktype_stats = &backends_io_stats->stats[bktype];
-
-		/*
-		 * In Assert builds, we can afford an extra loop through all of the
-		 * counters checking that only expected stats are non-zero, since it
-		 * keeps the non-Assert code cleaner.
-		 */
-		Assert(pgstat_bktype_io_stats_valid(bktype_stats, bktype));
-
-		/*
-		 * For those BackendTypes without IO Operation stats, skip
-		 * representing them in the view altogether.
-		 */
-		if (!pgstat_tracks_io_bktype(bktype))
-			continue;
 
 	for (int io_obj = 0; io_obj < IOOBJECT_NUM_TYPES; io_obj++)
 	{
@@ -1410,9 +1392,9 @@ pg_stat_get_io(PG_FUNCTION_ARGS)
 			bool		nulls[IO_NUM_COLUMNS] = {0};
 
 			/*
-				 * Some combinations of BackendType, IOObject, and IOContext
+			 * Some combinations of BackendType, IOObject, and IOContext are
-				 * are not valid for any type of IOOp. In such cases, omit the
+			 * not valid for any type of IOOp. In such cases, omit the entire
-				 * entire row from the view.
+			 * row from the view.
 			 */
 			if (!pgstat_tracks_io_object(bktype, io_obj, io_context))
 				continue;
@@ -1420,13 +1402,16 @@ pg_stat_get_io(PG_FUNCTION_ARGS)
 			values[IO_COL_BACKEND_TYPE] = bktype_desc;
 			values[IO_COL_CONTEXT] = CStringGetTextDatum(context_name);
 			values[IO_COL_OBJECT] = CStringGetTextDatum(obj_name);
-				values[IO_COL_RESET_TIME] = reset_time;
+			if (stat_reset_timestamp != 0)
+				values[IO_COL_RESET_TIME] = TimestampTzGetDatum(stat_reset_timestamp);
+			else
+				nulls[IO_COL_RESET_TIME] = true;
 
 			/*
-				 * Hard-code this to the value of BLCKSZ for now. Future
+			 * Hard-code this to the value of BLCKSZ for now. Future values
-				 * values could include XLOG_BLCKSZ, once WAL IO is tracked,
+			 * could include XLOG_BLCKSZ, once WAL IO is tracked, and constant
-				 * and constant multipliers, once non-block-oriented IO (e.g.
+			 * multipliers, once non-block-oriented IO (e.g. temporary file
-				 * temporary file IO) is tracked.
+			 * IO) is tracked.
 			 */
 			values[IO_COL_CONVERSION] = Int64GetDatum(BLCKSZ);
 
@@ -1436,9 +1421,9 @@ pg_stat_get_io(PG_FUNCTION_ARGS)
 				int			time_idx = pgstat_get_io_time_index(io_op);
 
 				/*
-					 * Some combinations of BackendType and IOOp, of IOContext
+				 * Some combinations of BackendType and IOOp, of IOContext and
-					 * and IOOp, and of IOObject and IOOp are not tracked. Set
+				 * IOOp, and of IOObject and IOOp are not tracked. Set these
-					 * these cells in the view NULL.
+				 * cells in the view NULL.
 				 */
 				if (pgstat_tracks_io_op(bktype, io_obj, io_context, io_op))
 				{
@@ -1469,6 +1454,41 @@ pg_stat_get_io(PG_FUNCTION_ARGS)
 								 values, nulls);
 		}
 	}
+}
+
+Datum
+pg_stat_get_io(PG_FUNCTION_ARGS)
+{
+	ReturnSetInfo *rsinfo;
+	PgStat_IO  *backends_io_stats;
+
+	InitMaterializedSRF(fcinfo, 0);
+	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+
+	backends_io_stats = pgstat_fetch_stat_io();
+
+	for (int bktype = 0; bktype < BACKEND_NUM_TYPES; bktype++)
+	{
+		PgStat_BktypeIO *bktype_stats = &backends_io_stats->stats[bktype];
+
+		/*
+		 * In Assert builds, we can afford an extra loop through all of the
+		 * counters (in pg_stat_io_build_tuples()), checking that only
+		 * expected stats are non-zero, since it keeps the non-Assert code
+		 * cleaner.
+		 */
+		Assert(pgstat_bktype_io_stats_valid(bktype_stats, bktype));
+
+		/*
+		 * For those BackendTypes without IO Operation stats, skip
+		 * representing them in the view altogether.
+		 */
+		if (!pgstat_tracks_io_bktype(bktype))
+			continue;
+
+		/* save tuples with data from this PgStat_BktypeIO */
+		pg_stat_io_build_tuples(rsinfo, bktype_stats, bktype,
+								backends_io_stats->stat_reset_timestamp);
 	}
 
 	return (Datum) 0;