--- a/pkg/controllers/diff.go	Mon Mar 11 15:53:31 2019 +0100
+++ b/pkg/controllers/diff.go	Mon Mar 11 15:58:26 2019 +0100
@@ -28,6 +28,20 @@
 	"gemma.intevation.de/gemma/pkg/octree"
 )
 
+const (
+	diffIDSQL = `
+SELECT sd.id FROM
+  caching.sounding_differences sd JOIN 
+  waterway.sounding_results srm ON sd.minuend = srm.id JOIN
+  waterway.sounding_results srs ON sd.subtrahend = srs.id JOIN
+  waterway.bottlenecks      bns ON srm.bottleneck_id = bns.id
+WHERE srm.bottleneck_id = srs.bottleneck_id AND
+  bns.objnam = $1 AND
+  srs.date_info = $2::date AND
+  srm.date_info = $3::date AND
+`
+)
+
 // Only allow three diffence calculation at once.
 // TODO: Make this configurable?
 var diffCalculationSemaphore = semaphore.NewWeighted(int64(3))
@@ -40,16 +54,38 @@
 
 	start := time.Now()
 
+	dci := input.(models.DiffCalculationInput)
+
 	ctx := req.Context()
 
+	var id int64
+	err = conn.QueryRowContext(
+		ctx,
+		diffIDSQL,
+		dci.Bottleneck,
+		dci.Minuend.Time,
+		dci.Subtrahend.Time,
+	).Scan(&id)
+
+	switch {
+	case err == sql.ErrNoRows:
+		// We need to calculate it.
+	case err != nil:
+		return
+	default:
+		// We already have this diff
+		jr = JSONResult{
+			Result: map[string]int64{"id": id},
+		}
+		return
+	}
+
 	// DoS counter measure.
 	if err = diffCalculationSemaphore.Acquire(ctx, 1); err != nil {
 		return
 	}
 	defer diffCalculationSemaphore.Release(1)
 
-	dci := input.(models.DiffCalculationInput)
-
 	minuendTree, err := octree.FromCache(
 		ctx, conn,
 		dci.Bottleneck, dci.Minuend.Time)
@@ -176,6 +212,13 @@
 	heights := octree.SampleDiffHeights(tree.Min.Z, tree.Max.Z, 0.1)
 	log.Printf("info: num heights: %d\n", len(heights))
 
+	// XXX: Maybe we should start this transaction earlier!?
+	var tx *sql.Tx
+	if tx, err = conn.BeginTx(ctx, nil); err != nil {
+		return
+	}
+	defer tx.Rollback()
+
 	// TODO: Implement me!
 
 	return