Move estimate into function and fix a test

pkg/migrations/backfill.go

@@ -29,21 +29,9 @@ func Backfill(ctx context.Context, conn db.DB, table *schema.Table, batchSize in
 		return BackfillNotPossibleError{Table: table.Name}
 	}
 
-	var total int64
-
-	// Try and get estimated row count
-	row := conn.QueryRowContext(ctx, `SELECT n_live_tup AS estimate
-	FROM pg_stat_user_tables
-	WHERE relname = $1`, table.Name)
-	if err := row.Scan(&total); err != nil {
-		return fmt.Errorf("scanning row count estimate for %q: %w", table.Name, err)
-	}
-	// If the estimate is zero, fall back to full count
-	if total == 0 {
-		row = conn.QueryRowContext(ctx, fmt.Sprintf(`SELECT count(*) from %s`, table.Name))
-		if err := row.Scan(&total); err != nil {
-			return fmt.Errorf("scanning row count for %q: %w", table.Name, err)
-		}
+	total, err := getRowCount(ctx, conn, table.Name)
+	if err != nil {
+		return fmt.Errorf("get row count for %q: %w", table.Name, err)
 	}
 
 	// Create a batcher for the table.
@@ -72,6 +60,30 @@ func Backfill(ctx context.Context, conn db.DB, table *schema.Table, batchSize in
 	return nil
 }
 
+// getRowCount will attempt to get the row count for the given table. It first attempts to get an
+// estimate and if that is zero, falls back to a full table scan.
+func getRowCount(ctx context.Context, conn db.DB, tableName string) (int64, error) {
+	var total int64
+	// Try and get estimated row count
+	row := conn.QueryRowContext(ctx, `
+	  SELECT n_live_tup AS estimate
+	  FROM pg_stat_user_tables
+	  WHERE relname = $1`, tableName)
+	if err := row.Scan(&total); err != nil {
+		return 0, fmt.Errorf("scanning row count estimate for %q: %w", tableName, err)
+	}
+	if total > 0 {
+		return total, nil
+	}
+
+	// If the estimate is zero, fall back to full count
+	row = conn.QueryRowContext(ctx, fmt.Sprintf(`SELECT count(*) from %s`, tableName))
+	if err := row.Scan(&total); err != nil {
+		return 0, fmt.Errorf("scanning row count for %q: %w", tableName, err)
+	}
+	return total, nil
+}
+
 // checkBackfill will return an error if the backfill operation is not supported.
 func checkBackfill(table *schema.Table) error {
 	cols := getIdentityColumns(table)

pkg/roll/execute_test.go

@@ -655,7 +655,7 @@ func TestCallbacksAreInvokedOnMigrationStart(t *testing.T) {
 
 		// Define a mock callback
 		invoked := false
-		cb := func(n int64) { invoked = true }
+		cb := func(n, total int64) { invoked = true }
 
 		// Start a migration that requires a backfill
 		err = mig.Start(ctx, &migrations.Migration{