backupccl: do not backup or restore stat forecasts

Statistics forecasts are not persisted and are reconstructed on demand
from the persisted statistics. As such, they don't need to be included
in a backup or restored.

This has the additional effect of solving a bug in which we would fail
to write statistics into our metadata SST if it included more than 1
forecast since the in-memory forecasts have a 0 statistic_id.

Fixes #86806

Epic: none

Release note: None

pkg/ccl/backupccl/backup_job.go

@@ -349,39 +349,14 @@ func backup(
 		return roachpb.RowCount{}, err
 	}
 
-	var tableStatistics []*stats.TableStatisticProto
-	for i := range backupManifest.Descriptors {
-		if tbl, _, _, _, _ := descpb.GetDescriptors(&backupManifest.Descriptors[i]); tbl != nil {
-			tableDesc := tabledesc.NewBuilder(tbl).BuildImmutableTable()
-			// Collect all the table stats for this table.
-			tableStatisticsAcc, err := statsCache.GetTableStats(ctx, tableDesc)
-			if err != nil {
-				// Successfully backed up data is more valuable than table stats that can
-				// be recomputed after restore, and so if we fail to collect the stats of a
-				// table we do not want to mark the job as failed.
-				// The lack of stats on restore could lead to suboptimal performance when
-				// reading/writing to this table until the stats have been recomputed.
-				log.Warningf(ctx, "failed to collect stats for table: %s, "+
-					"table ID: %d during a backup: %s", tableDesc.GetName(), tableDesc.GetID(),
-					err.Error())
-				continue
-			}
-			for _, stat := range tableStatisticsAcc {
-				tableStatistics = append(tableStatistics, &stat.TableStatisticProto)
-			}
-		}
-	}
-	statsTable := backuppb.StatsTable{
-		Statistics: tableStatistics,
-	}
-
+	statsTable := getTableStatsForBackup(ctx, statsCache, backupManifest.Descriptors)
 	if err := backupinfo.WriteTableStatistics(ctx, defaultStore, encryption, &kmsEnv, &statsTable); err != nil {
 		return roachpb.RowCount{}, err
 	}
 
 	if backupinfo.WriteMetadataSST.Get(&settings.SV) {
 		if err := backupinfo.WriteBackupMetadataSST(ctx, defaultStore, encryption, &kmsEnv, backupManifest,
-			tableStatistics); err != nil {
+			statsTable.Statistics); err != nil {
 			err = errors.Wrap(err, "writing forward-compat metadata sst")
 			if !build.IsRelease() {
 				return roachpb.RowCount{}, err
@@ -410,6 +385,50 @@ func releaseProtectedTimestamp(
 	return err
 }
 
+// getTableStatsForBackup collects all stats for tables found in descs.
+//
+// We do not fail if we can't retrieve statistiscs. Successfully
+// backed up data is more valuable than table stats that can be
+// recomputed after restore. The lack of stats on restore could lead
+// to suboptimal performance when reading/writing to this table until
+// the stats have been recomputed.
+func getTableStatsForBackup(
+	ctx context.Context, statsCache *stats.TableStatisticsCache, descs []descpb.Descriptor,
+) backuppb.StatsTable {
+	var tableStatistics []*stats.TableStatisticProto
+	for i := range descs {
+		if tbl, _, _, _, _ := descpb.GetDescriptors(&descs[i]); tbl != nil {
+			tableDesc := tabledesc.NewBuilder(tbl).BuildImmutableTable()
+			tableStatisticsAcc, err := statsCache.GetTableStats(ctx, tableDesc)
+			if err != nil {
+				log.Warningf(ctx, "failed to collect stats for table: %s, "+
+					"table ID: %d during a backup: %s", tableDesc.GetName(), tableDesc.GetID(),
+					err.Error())
+				continue
+			}
+
+			for _, stat := range tableStatisticsAcc {
+				if statShouldBeIncludedInBackupRestore(&stat.TableStatisticProto) {
+					tableStatistics = append(tableStatistics, &stat.TableStatisticProto)
+				}
+			}
+		}
+	}
+	return backuppb.StatsTable{
+		Statistics: tableStatistics,
+	}
+}
+
+func statShouldBeIncludedInBackupRestore(stat *stats.TableStatisticProto) bool {
+	// Forecasts and merged stats are computed from the persisted
+	// stats on demand and do not need to be backed up or
+	// restored.
+	if stat.Name == jobspb.ForecastStatsName || stat.Name == jobspb.MergedStatsName {
+		return false
+	}
+	return true
+}
+
 type backupResumer struct {
 	job         *jobs.Job
 	backupStats roachpb.RowCount

pkg/ccl/backupccl/backupinfo/backup_metadata.go

@@ -500,12 +500,12 @@ func writeStatsToMetadata(
 		return stats[i].TableID < stats[j].TableID || (stats[i].TableID == stats[j].TableID && stats[i].StatisticID < stats[j].StatisticID)
 	})
 
-	for _, i := range stats {
-		b, err := protoutil.Marshal(i)
+	for _, s := range stats {
+		b, err := protoutil.Marshal(s)
 		if err != nil {
 			return err
 		}
-		if err := sst.PutUnversioned(encodeStatSSTKey(i.TableID, i.StatisticID), b); err != nil {
+		if err := sst.PutUnversioned(encodeStatSSTKey(s.TableID, s.StatisticID), b); err != nil {
 			return err
 		}
 	}

pkg/ccl/backupccl/restore_job.go

@@ -524,13 +524,16 @@ func (r *restoreResumer) ForceRealSpan() bool {
 	return true
 }
 
-// remapRelevantStatistics changes the table ID references in the stats
-// from those they had in the backed up database to what they should be
-// in the restored database.
-// It also selects only the statistics which belong to one of the tables
-// being restored. If the descriptorRewrites can re-write the table ID, then that
-// table is being restored.
-func remapRelevantStatistics(
+// remapAndFilterRelevantStatistics changes the table ID references in
+// the stats from those they had in the backed up database to what
+// they should be in the restored database.
+//
+// It also selects only the statistics which belong to one of the
+// tables being restored. If the descriptorRewrites can re-write the
+// table ID, then that table is being restored.
+//
+// Any statistics forecasts are ignored.
+func remapAndFilterRelevantStatistics(
 	ctx context.Context,
 	tableStatistics []*stats.TableStatisticProto,
 	descriptorRewrites jobspb.DescRewriteMap,
@@ -540,11 +543,13 @@ func remapRelevantStatistics(
 
 	tableHasStatsInBackup := make(map[descpb.ID]struct{})
 	for _, stat := range tableStatistics {
-		tableHasStatsInBackup[stat.TableID] = struct{}{}
-		if tableRewrite, ok := descriptorRewrites[stat.TableID]; ok {
-			// Statistics imported only when table re-write is present.
-			stat.TableID = tableRewrite.ID
-			relevantTableStatistics = append(relevantTableStatistics, stat)
+		if statShouldBeIncludedInBackupRestore(stat) {
+			tableHasStatsInBackup[stat.TableID] = struct{}{}
+			if tableRewrite, ok := descriptorRewrites[stat.TableID]; ok {
+				// Statistics imported only when table re-write is present.
+				stat.TableID = tableRewrite.ID
+				relevantTableStatistics = append(relevantTableStatistics, stat)
+			}
 		}
 	}
 
@@ -1555,7 +1560,7 @@ func (r *restoreResumer) doResume(ctx context.Context, execCtx interface{}) erro
 	backupStats, err := backupinfo.GetStatisticsFromBackup(ctx, defaultStore, details.Encryption,
 		&kmsEnv, latestBackupManifest)
 	if err == nil {
-		remappedStats = remapRelevantStatistics(ctx, backupStats, details.DescriptorRewrites,
+		remappedStats = remapAndFilterRelevantStatistics(ctx, backupStats, details.DescriptorRewrites,
 			details.TableDescs)
 	} else {
 		// We don't want to fail the restore if we are unable to resolve statistics

pkg/ccl/backupccl/testdata/backup-restore/metadata

@@ -0,0 +1,42 @@
+new-cluster name=s
+----
+
+# Make sure that the presence of forecasted stats doesn't break
+# metadata SST writing. This is a regression test for #86806.
+#
+# We create 3 statistics manually which is the minimum needed to
+# create a forecast. Automatic collection is disabled to avoid
+# flakiness.
+exec-sql
+SET CLUSTER SETTING kv.bulkio.write_metadata_sst.enabled = true;
+----
+
+exec-sql
+SET CLUSTER SETTING sql.stats.automatic_collection.enabled = false;
+----
+
+exec-sql
+CREATE DATABASE db1;
+USE db1;
+CREATE TABLE tab (a INT PRIMARY KEY, b INT);
+----
+
+exec-sql
+CREATE STATISTICS __auto__ ON a FROM tab;
+CREATE STATISTICS __auto__ ON b FROM tab;
+CREATE STATISTICS __auto__ ON a FROM tab;
+CREATE STATISTICS __auto__ ON b FROM tab;
+CREATE STATISTICS __auto__ ON a FROM tab;
+CREATE STATISTICS __auto__ ON b FROM tab;
+----
+
+# The previous bug occurs when there are two or more forecasts since
+# forecasts do not hava a statistics ID.
+query-sql
+SELECT count(1) FROM [ SHOW STATISTICS FOR TABLE tab WITH FORECAST ] WHERE statistics_name = '__forecast__'
+----
+2
+
+exec-sql
+BACKUP DATABASE db1 INTO 'nodelocal://0/test/'
+----