release-24.1: sql: add gauge for object count

docs/generated/metrics/metrics.html

@@ -1570,6 +1570,7 @@
 <tr><td>APPLICATION</td><td>sql.savepoint.started.count</td><td>Number of SQL SAVEPOINT statements started</td><td>SQL Statements</td><td>COUNTER</td><td>COUNT</td><td>AVG</td><td>NON_NEGATIVE_DERIVATIVE</td></tr>
 <tr><td>APPLICATION</td><td>sql.savepoint.started.count.internal</td><td>Number of SQL SAVEPOINT statements started (internal queries)</td><td>SQL Internal Statements</td><td>COUNTER</td><td>COUNT</td><td>AVG</td><td>NON_NEGATIVE_DERIVATIVE</td></tr>
 <tr><td>APPLICATION</td><td>sql.schema.invalid_objects</td><td>Gauge of detected invalid objects within the system.descriptor table (measured by querying crdb_internal.invalid_objects)</td><td>Objects</td><td>GAUGE</td><td>COUNT</td><td>AVG</td><td>NONE</td></tr>
+<tr><td>APPLICATION</td><td>sql.schema_changer.object_count</td><td>Counter of the number of objects in the cluster</td><td>Objects</td><td>GAUGE</td><td>COUNT</td><td>AVG</td><td>NONE</td></tr>
 <tr><td>APPLICATION</td><td>sql.schema_changer.permanent_errors</td><td>Counter of the number of permanent errors experienced by the schema changer</td><td>Errors</td><td>COUNTER</td><td>COUNT</td><td>AVG</td><td>NON_NEGATIVE_DERIVATIVE</td></tr>
 <tr><td>APPLICATION</td><td>sql.schema_changer.retry_errors</td><td>Counter of the number of retriable errors experienced by the schema changer</td><td>Errors</td><td>COUNTER</td><td>COUNT</td><td>AVG</td><td>NON_NEGATIVE_DERIVATIVE</td></tr>
 <tr><td>APPLICATION</td><td>sql.schema_changer.running</td><td>Gauge of currently running schema changes</td><td>Schema changes</td><td>GAUGE</td><td>COUNT</td><td>AVG</td><td>NONE</td></tr>

pkg/sql/catalog/descs/collection.go

@@ -218,6 +218,23 @@ func (tc *Collection) HasUncommittedDescriptors() bool {
 	return tc.uncommitted.uncommitted.Len() > 0
 }
 
+// HasUncommittedNewOrDroppedDescriptors returns true if the collection contains
+// any uncommitted descriptors that are newly created or dropped.
+func (tc *Collection) HasUncommittedNewOrDroppedDescriptors() bool {
+	isNewDescriptor := false
+	err := tc.uncommitted.iterateUncommittedByID(func(desc catalog.Descriptor) error {
+		if desc.GetVersion() == 1 || desc.Dropped() {
+			isNewDescriptor = true
+			return iterutil.StopIteration()
+		}
+		return nil
+	})
+	if err != nil {
+		return false
+	}
+	return isNewDescriptor
+}
+
 // HasUncommittedTypes returns true if the Collection contains uncommitted
 // types.
 func (tc *Collection) HasUncommittedTypes() (has bool) {

pkg/sql/conn_executor.go

@@ -4028,7 +4028,12 @@ func (ex *connExecutor) txnStateTransitionsApplyWrapper(
 		}(); err != nil {
 			return ai, err
 		}
-
+		if ex.extraTxnState.descCollection.HasUncommittedNewOrDroppedDescriptors() {
+			execCfg := ex.planner.ExecCfg()
+			if err := UpdateDescriptorCount(ex.Ctx(), execCfg, execCfg.SchemaChangerMetrics); err != nil {
+				log.Warningf(ex.Ctx(), "failed to scan descriptor table: %v", err)
+			}
+		}
 		fallthrough
 	case txnRollback:
 		ex.resetExtraTxnState(ex.Ctx(), advInfo.txnEvent, payloadErr)

pkg/sql/isql/isql_db.go

@@ -87,7 +87,7 @@ type Executor interface {
 	) (int, error)
 
 	// QueryRow executes the supplied SQL statement and returns a single row, or
-	// nil if no row is found, or an error if more that one row is returned.
+	// nil if no row is found, or an error if more than one row is returned.
 	//
 	// QueryRow is deprecated. Use QueryRowEx() instead.
 	QueryRow(

pkg/sql/schema_changer.go

@@ -3341,3 +3341,20 @@ func (p *planner) CanCreateCrossDBSequenceRef() error {
 	}
 	return nil
 }
+
+// UpdateDescriptorCount updates our sql.schema_changer.object_count gauge with
+// a fresh count of objects in the system.descriptor table.
+func UpdateDescriptorCount(
+	ctx context.Context, execCfg *ExecutorConfig, metric *SchemaChangerMetrics,
+) error {
+	return DescsTxn(ctx, execCfg, func(ctx context.Context, txn isql.Txn, col *descs.Collection) error {
+		row, err := txn.QueryRow(ctx, "sql-schema-changer-object-count", txn.KV(),
+			`SELECT count(*) FROM system.descriptor`)
+		if err != nil {
+			return err
+		}
+		count := *row[0].(*tree.DInt)
+		metric.ObjectCount.Update(int64(count))
+		return nil
+	})
+}

pkg/sql/schema_changer_metrics.go

@@ -38,6 +38,12 @@ var (
 		Measurement: "Errors",
 		Unit:        metric.Unit_COUNT,
 	}
+	metaObjects = metric.Metadata{
+		Name:        "sql.schema_changer.object_count",
+		Help:        "Counter of the number of objects in the cluster",
+		Measurement: "Objects",
+		Unit:        metric.Unit_COUNT,
+	}
 )
 
 // SchemaChangerMetrics are metrics corresponding to the schema changer.
@@ -48,6 +54,7 @@ type SchemaChangerMetrics struct {
 	PermanentErrors      *metric.Counter
 	ConstraintErrors     telemetry.Counter
 	UncategorizedErrors  telemetry.Counter
+	ObjectCount          *metric.Gauge
 }
 
 // MetricStruct makes SchemaChangerMetrics a metric.Struct.
@@ -64,5 +71,6 @@ func NewSchemaChangerMetrics() *SchemaChangerMetrics {
 		PermanentErrors:      metric.NewCounter(metaPermanentErrors),
 		ConstraintErrors:     sqltelemetry.SchemaChangeErrorCounter("constraint_violation"),
 		UncategorizedErrors:  sqltelemetry.SchemaChangeErrorCounter("uncategorized"),
+		ObjectCount:          metric.NewGauge(metaObjects),
 	}
 }