count(*) before fetch in postgres map

server/db/postgres/initialblocks.go

@@ -16,6 +16,31 @@ const (
 	SETTING_LAST_Y_BLOCK = "last_y_block"
 )
 
+func (this *PostgresAccessor) countBlocks(x1, y1, z1, x2, y2, z2 int) (int, error) {
+	rows, err := this.db.Query(getBlockCountByInitialTileQuery,
+		x1, y1, z1, x2, y2, z2,
+	)
+
+	if err != nil {
+		return 0, err
+	}
+
+	defer rows.Close()
+
+	for rows.Next() {
+		var count int
+
+		err = rows.Scan(&count)
+		if err != nil {
+			return 0, err
+		}
+
+		return count, nil
+	}
+
+	return 0, nil
+}
+
 // x -> 0 ... 256
 
 //zoom/mapblock-width
@@ -131,6 +156,17 @@ func (this *PostgresAccessor) FindNextInitialBlocks(s settings.Settings, layers
 		}
 	}
 
+	count, err := this.countBlocks(minX, minY, minZ, maxX, maxY, maxZ)
+
+	if err != nil {
+		return nil, err
+	}
+
+	blocks := make([]*db.Block, 0)
+	var lastmtime int64
+
+	if count > 0 {
+
 		rows, err := this.db.Query(getBlocksByInitialTileQuery,
 			minX, minY, minZ, maxX, maxY, maxZ,
 		)
@@ -140,8 +176,6 @@ func (this *PostgresAccessor) FindNextInitialBlocks(s settings.Settings, layers
 		}
 
 		defer rows.Close()
-	blocks := make([]*db.Block, 0)
-	var lastmtime int64
 
 		for rows.Next() {
 			var posx, posy, posz int
@@ -160,6 +194,7 @@ func (this *PostgresAccessor) FindNextInitialBlocks(s settings.Settings, layers
 			mb := convertRows(posx, posy, posz, data, mtime)
 			blocks = append(blocks, mb)
 		}
+	}
 
 	s.SetInt(SETTING_LAST_LAYER, lastlayer)
 	s.SetInt(SETTING_LAST_X_BLOCK, lastxblock)

server/db/postgres/sql.go

@@ -11,6 +11,17 @@ and b.posy <= $5
 and b.posz <= $6
 `
 
+const getBlockCountByInitialTileQuery = `
+select count(*)
+from blocks b
+where b.posx >= $1
+and b.posy >= $2
+and b.posz >= $3
+and b.posx <= $4
+and b.posy <= $5
+and b.posz <= $6
+`
+
 const getBlocksByMtimeQuery = `
 select posx,posy,posz,data,mtime
 from blocks b