graph/db: use batch loading for ChanUpdatesInHorizon

The following performance gains were measured using the new benchmark
test.

```
name                                      old time/op  new time/op  delta
ChanUpdatesInHorizon-native-sqlite-10     18.5s ± 3%    2.0s ± 5%  -89.11%  (p=0.000 n=9+9)
ChanUpdatesInHorizon-native-postgres-10   59.0s ± 3%    0.8s ±10%  -98.65%  (p=0.000 n=10+9)
```

graph/db/sql_store.go

@@ -911,6 +911,7 @@ func (s *SQLStore) ChanUpdatesInHorizon(startTime,
 		edges        []ChannelEdge
 		hits         int
 	)
+
 	err := s.db.ExecTx(ctx, sqldb.ReadTxOpt(), func(db SQLQueries) error {
 		rows, err := db.GetChannelsByPolicyLastUpdateRange(
 			ctx, sqlc.GetChannelsByPolicyLastUpdateRangeParams{
@@ -923,72 +924,61 @@ func (s *SQLStore) ChanUpdatesInHorizon(startTime,
 			return err
 		}
 
+		if len(rows) == 0 {
+			return nil
+		}
+
+		// We'll pre-allocate the slices and maps here with a best
+		// effort size in order to avoid unnecessary allocations later
+		// on.
+		uncachedRows := make(
+			[]sqlc.GetChannelsByPolicyLastUpdateRangeRow, 0,
+			len(rows),
+		)
+		edgesToCache = make(map[uint64]ChannelEdge, len(rows))
+		edgesSeen = make(map[uint64]struct{}, len(rows))
+		edges = make([]ChannelEdge, 0, len(rows))
+
+		// Separate cached from non-cached channels since we will only
+		// batch load the data for the ones we haven't cached yet.
 		for _, row := range rows {
-			// If we've already retrieved the info and policies for
-			// this edge, then we can skip it as we don't need to do
-			// so again.
 			chanIDInt := byteOrder.Uint64(row.GraphChannel.Scid)
+
+			// Skip duplicates.
 			if _, ok := edgesSeen[chanIDInt]; ok {
 				continue
 			}
+			edgesSeen[chanIDInt] = struct{}{}
 
+			// Check cache first.
 			if channel, ok := s.chanCache.get(chanIDInt); ok {
 				hits++
-				edgesSeen[chanIDInt] = struct{}{}
 				edges = append(edges, channel)
-
 				continue
 			}
 
-			node1, node2, err := buildNodes(
-				ctx, db, row.GraphNode, row.GraphNode_2,
-			)
-			if err != nil {
-				return err
-			}
-
-			channel, err := getAndBuildEdgeInfo(
-				ctx, db, s.cfg.ChainHash, row.GraphChannel,
-				node1.PubKeyBytes, node2.PubKeyBytes,
-			)
-			if err != nil {
-				return fmt.Errorf("unable to build channel "+
-					"info: %w", err)
-			}
-
-			dbPol1, dbPol2, err := extractChannelPolicies(row)
-			if err != nil {
-				return fmt.Errorf("unable to extract channel "+
-					"policies: %w", err)
-			}
-
-			p1, p2, err := getAndBuildChanPolicies(
-				ctx, db, dbPol1, dbPol2, channel.ChannelID,
-				node1.PubKeyBytes, node2.PubKeyBytes,
-			)
-			if err != nil {
-				return fmt.Errorf("unable to build channel "+
-					"policies: %w", err)
-			}
-
-			edgesSeen[chanIDInt] = struct{}{}
-			chanEdge := ChannelEdge{
-				Info:    channel,
-				Policy1: p1,
-				Policy2: p2,
-				Node1:   node1,
-				Node2:   node2,
-			}
-			edges = append(edges, chanEdge)
-			edgesToCache[chanIDInt] = chanEdge
+			// Mark this row as one we need to batch load data for.
+			uncachedRows = append(uncachedRows, row)
 		}
 
+		// If there are no uncached rows, then we can return early.
+		if len(uncachedRows) == 0 {
+			return nil
+		}
+
+		// Batch load data for all uncached channels.
+		newEdges, err := batchBuildChannelEdges(
+			ctx, s.cfg, db, uncachedRows,
+		)
+		if err != nil {
+			return fmt.Errorf("unable to batch build channel "+
+				"edges: %w", err)
+		}
+
+		edges = append(edges, newEdges...)
+
 		return nil
-	}, func() {
-		edgesSeen = make(map[uint64]struct{})
-		edgesToCache = make(map[uint64]ChannelEdge)
-		edges = nil
-	})
+	}, sqldb.NoOpReset)
 	if err != nil {
 		return nil, fmt.Errorf("unable to fetch channels: %w", err)
 	}
@@ -5298,3 +5288,121 @@ func buildDirectedChannel(chain chainhash.Hash, nodeID int64,
 
 	return directedChannel, nil
 }
+
+// batchBuildChannelEdges builds a slice of ChannelEdge instances from the
+// provided rows. It uses batch loading for channels, policies, and nodes.
+func batchBuildChannelEdges[T sqlc.ChannelAndNodes](ctx context.Context,
+	cfg *SQLStoreConfig, db SQLQueries, rows []T) ([]ChannelEdge, error) {
+
+	var (
+		channelIDs = make([]int64, len(rows))
+		policyIDs  = make([]int64, 0, len(rows)*2)
+		nodeIDs    = make([]int64, 0, len(rows)*2)
+
+		// nodeIDSet is used to ensure we only collect unique node IDs.
+		nodeIDSet = make(map[int64]bool)
+
+		// edges will hold the final channel edges built from the rows.
+		edges = make([]ChannelEdge, 0, len(rows))
+	)
+
+	// Collect all IDs needed for batch loading.
+	for i, row := range rows {
+		channelIDs[i] = row.Channel().ID
+
+		// Collect policy IDs
+		dbPol1, dbPol2, err := extractChannelPolicies(row)
+		if err != nil {
+			return nil, fmt.Errorf("unable to extract channel "+
+				"policies: %w", err)
+		}
+		if dbPol1 != nil {
+			policyIDs = append(policyIDs, dbPol1.ID)
+		}
+		if dbPol2 != nil {
+			policyIDs = append(policyIDs, dbPol2.ID)
+		}
+
+		var (
+			node1ID = row.Node1().ID
+			node2ID = row.Node2().ID
+		)
+
+		// Collect unique node IDs.
+		if !nodeIDSet[node1ID] {
+			nodeIDs = append(nodeIDs, node1ID)
+			nodeIDSet[node1ID] = true
+		}
+
+		if !nodeIDSet[node2ID] {
+			nodeIDs = append(nodeIDs, node2ID)
+			nodeIDSet[node2ID] = true
+		}
+	}
+
+	// Batch the data for all the channels and policies.
+	channelBatchData, err := batchLoadChannelData(
+		ctx, cfg.QueryCfg, db, channelIDs, policyIDs,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("unable to batch load channel and "+
+			"policy data: %w", err)
+	}
+
+	// Batch the data for all the nodes.
+	nodeBatchData, err := batchLoadNodeData(ctx, cfg.QueryCfg, db, nodeIDs)
+	if err != nil {
+		return nil, fmt.Errorf("unable to batch load node data: %w",
+			err)
+	}
+
+	// Build all channel edges using batch data.
+	for _, row := range rows {
+		// Build nodes using batch data.
+		node1, err := buildNodeWithBatchData(row.Node1(), nodeBatchData)
+		if err != nil {
+			return nil, fmt.Errorf("unable to build node1: %w", err)
+		}
+
+		node2, err := buildNodeWithBatchData(row.Node2(), nodeBatchData)
+		if err != nil {
+			return nil, fmt.Errorf("unable to build node2: %w", err)
+		}
+
+		// Build channel info using batch data.
+		channel, err := buildEdgeInfoWithBatchData(
+			cfg.ChainHash, row.Channel(), node1.PubKeyBytes,
+			node2.PubKeyBytes, channelBatchData,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("unable to build channel "+
+				"info: %w", err)
+		}
+
+		// Extract and build policies using batch data.
+		dbPol1, dbPol2, err := extractChannelPolicies(row)
+		if err != nil {
+			return nil, fmt.Errorf("unable to extract channel "+
+				"policies: %w", err)
+		}
+
+		p1, p2, err := buildChanPoliciesWithBatchData(
+			dbPol1, dbPol2, channel.ChannelID,
+			node1.PubKeyBytes, node2.PubKeyBytes, channelBatchData,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("unable to build channel "+
+				"policies: %w", err)
+		}
+
+		edges = append(edges, ChannelEdge{
+			Info:    channel,
+			Policy1: p1,
+			Policy2: p2,
+			Node1:   node1,
+			Node2:   node2,
+		})
+	}
+
+	return edges, nil
+}

sqldb/sqlc/db_custom.go

@@ -37,3 +37,37 @@ func makeQueryParams(numTotalArgs, numListArgs int) string {
 
 	return b.String()
 }
+
+// ChannelAndNodes is an interface that provides access to a channel and its
+// two nodes.
+type ChannelAndNodes interface {
+	// Channel returns the GraphChannel associated with this interface.
+	Channel() GraphChannel
+
+	// Node1 returns the first GraphNode associated with this channel.
+	Node1() GraphNode
+
+	// Node2 returns the second GraphNode associated with this channel.
+	Node2() GraphNode
+}
+
+// Channel returns the GraphChannel associated with this interface.
+//
+// NOTE: This method is part of the ChannelAndNodes interface.
+func (r GetChannelsByPolicyLastUpdateRangeRow) Channel() GraphChannel {
+	return r.GraphChannel
+}
+
+// Node1 returns the first GraphNode associated with this channel.
+//
+// NOTE: This method is part of the ChannelAndNodes interface.
+func (r GetChannelsByPolicyLastUpdateRangeRow) Node1() GraphNode {
+	return r.GraphNode
+}
+
+// Node2 returns the second GraphNode associated with this channel.
+//
+// NOTE: This method is part of the ChannelAndNodes interface.
+func (r GetChannelsByPolicyLastUpdateRangeRow) Node2() GraphNode {
+	return r.GraphNode_2
+}