diff --git a/geojson.c b/geojson.c
index 2320482e..0f2b866d 100644
--- a/geojson.c
+++ b/geojson.c
@@ -65,9 +65,21 @@ int indexcmp(const void *v1, const void *v2) {
 		return -1;
 	} else if (i1->index > i2->index) {
 		return 1;
-	} else {
-		return 0;
 	}
+
+	if (i1->fpos < i2->fpos) {
+		return -1;
+	} else if (i1->fpos > i2->fpos) {
+		return 1;
+	}
+
+	if (i1->maxzoom < i2->maxzoom) {
+		return -1;
+	} else if (i1->maxzoom > i2->maxzoom) {
+		return 1;
+	}
+
+	return 0;
 }
 
 size_t fwrite_check(const void *ptr, size_t size, size_t nitems, FILE *stream, char *fname, json_pull *source) {
@@ -180,6 +192,10 @@ void check(struct index *ix, long long n, char *metabase, unsigned *file_bbox, s
 	for (z = maxzoom; z >= minzoom; z--) {
 		struct index *i, *j = NULL;
 		for (i = ix; i < ix + n && i != NULL; i = j) {
+			if (i > ix && indexcmp(i - 1, i) > 0) {
+				fprintf(stderr, "index out of order\n");
+				exit(EXIT_FAILURE);
+			}
 			unsigned wx, wy;
 			decode(i->index, &wx, &wy);
 
@@ -221,6 +237,55 @@ void check(struct index *ix, long long n, char *metabase, unsigned *file_bbox, s
 	fprintf(stderr, "\n");
 }
 
+struct merge {
+	long long start;
+	long long end;
+
+	struct merge *next;
+};
+
+static void insert(struct merge *m, struct merge **head, unsigned char *map, int bytes) {
+	while (*head != NULL && indexcmp(map + m->start, map + (*head)->start) > 0) {
+		head = &((*head)->next);
+	}
+
+	m->next = *head;
+	*head = m;
+}
+
+static void merge(struct merge *merges, int nmerges, unsigned char *map, FILE *f, int bytes, long long nrec) {
+	int i;
+	struct merge *head = NULL;
+	long long along = 0;
+	long long reported = -1;
+
+	for (i = 0; i < nmerges; i++) {
+		if (merges[i].start < merges[i].end) {
+			insert(&(merges[i]), &head, map, bytes);
+		}
+	}
+
+	while (head != NULL) {
+		fwrite(map + head->start, bytes, 1, f);
+		head->start += bytes;
+
+		struct merge *m = head;
+		head = m->next;
+		m->next = NULL;
+
+		if (m->start < m->end) {
+			insert(m, &head, map, bytes);
+		}
+
+		along++;
+		long long report = 100 * along / nrec;
+		if (report != reported) {
+			fprintf(stderr, "Merging: %lld%%\r", report);
+			reported = report;
+		}
+	}
+}
+
 void read_json(FILE *f, char *fname, char *layername, int maxzoom, int minzoom, sqlite3 *outdb, struct pool *exclude, int exclude_all, double droprate, int buffer) {
 	char metaname[] = "/tmp/meta.XXXXXXXX";
 	char indexname[] = "/tmp/index.XXXXXXXX";
@@ -421,11 +486,6 @@ next_feature:
 
 	struct stat indexst;
 	fstat(indexfd, &indexst);
-	struct index *index = mmap(NULL, indexst.st_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, indexfd, 0);
-	if (index == MAP_FAILED) {
-		perror("mmap index");
-		exit(EXIT_FAILURE);
-	}
 
 	struct stat metast;
 	fstat(metafd, &metast);
@@ -468,7 +528,100 @@ next_feature:
 		printf("using layer name %s\n", trunc);
 	}
 
-	qsort(index, indexst.st_size / sizeof(struct index), sizeof(struct index), indexcmp);
+	{
+		int bytes = sizeof(struct index);
+
+		fprintf(stderr,
+		 	"Sorting %lld indices for %lld features\n",
+			(long long) indexst.st_size / bytes,
+			seq);
+
+		int page = sysconf(_SC_PAGESIZE);
+		long long unit = (50 * 1024 * 1024 / bytes) * bytes;
+		while (unit % page != 0) {
+			unit += bytes;
+		}
+
+		int nmerges = (indexst.st_size + unit - 1) / unit;
+		struct merge merges[nmerges];
+
+		long long start;
+		for (start = 0; start < indexst.st_size; start += unit) {
+			long long end = start + unit;
+			if (end > indexst.st_size) {
+				end = indexst.st_size;
+			}
+
+			if (nmerges != 1) {
+				fprintf(stderr, "Sorting part %lld of %d\r", start / unit + 1, nmerges);
+			}
+
+			merges[start / unit].start = start;
+			merges[start / unit].end = end;
+			merges[start / unit].next = NULL;
+
+			void *map = mmap(NULL, end - start, PROT_READ | PROT_WRITE, MAP_PRIVATE, indexfd, start);
+			if (map == MAP_FAILED) {
+				perror("mmap");
+				exit(EXIT_FAILURE);
+			}
+
+			qsort(map, (end - start) / bytes, bytes, indexcmp);
+
+			// Sorting and then copying avoids the need to
+			// write out intermediate stages of the sort.
+
+			void *map2 = mmap(NULL, end - start, PROT_READ | PROT_WRITE, MAP_SHARED, indexfd, start);
+			if (map2 == MAP_FAILED) {
+				perror("mmap (write)");
+				exit(EXIT_FAILURE);
+			}
+
+			memcpy(map2, map, end - start);
+
+			munmap(map, end - start);
+			munmap(map2, end - start);
+		}
+
+		if (nmerges != 1) {
+			fprintf(stderr, "\n");
+		}
+
+		void *map = mmap(NULL, indexst.st_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, indexfd, 0);
+		if (map == MAP_FAILED) {
+			perror("mmap");
+			exit(EXIT_FAILURE);
+		}
+
+		FILE *f = fopen(indexname, "w");
+		if (f == NULL) {
+			perror(indexname);
+			exit(EXIT_FAILURE);
+		}
+
+		merge(merges, nmerges, map, f, bytes, indexst.st_size / bytes);
+
+		munmap(map, indexst.st_size);
+		fclose(f);
+		close(indexfd);
+	}
+
+	indexfd = open(indexname, O_RDONLY);
+	if (indexfd < 0) {
+		perror(indexname);
+		exit(EXIT_FAILURE);
+	}
+	if (unlink(indexname) != 0) {
+		perror(indexname);
+		exit(EXIT_FAILURE);
+	}
+
+	struct index *index = mmap(NULL, indexst.st_size, PROT_READ, MAP_PRIVATE, indexfd, 0);
+	if (index == MAP_FAILED) {
+		perror("mmap index");
+		exit(EXIT_FAILURE);
+	}
+
 	check(index, indexst.st_size / sizeof(struct index), meta, file_bbox, &file_keys, &midx, &midy, layername, maxzoom, minzoom, outdb, droprate, buffer);
 
 	munmap(index, indexst.st_size);