diff --git a/app/sheet.c b/app/sheet.c
index 34796540..996b9d25 100644
--- a/app/sheet.c
+++ b/app/sheet.c
@@ -699,10 +699,6 @@ int ZSV_MAIN_FUNC(ZSV_COMMAND)(int argc, const char *argv[], struct zsv_opts *op
     pthread_mutex_lock(&ub->mutex);
     if (ub->status)
       zsvsheet_priv_set_status(&display_dims, 1, ub->status);
-    if (ub->write_progressed) {
-      handler_state.display_info.update_buffer = true;
-      ub->write_progressed = 0;
-    }
     if (ub->index_ready && ub->dimensions.row_count != ub->index->row_count + 1) {
       ub->dimensions.row_count = ub->index->row_count + 1;
       handler_state.display_info.update_buffer = true;
diff --git a/app/sheet/index.c b/app/sheet/index.c
index 56f5bc7a..98a2eb96 100644
--- a/app/sheet/index.c
+++ b/app/sheet/index.c
@@ -43,6 +43,8 @@ enum zsv_index_status build_memory_index(struct zsvsheet_index_opts *optsp) {
   if (!ixr.ix)
     goto out;
 
+  optsp->uib->index = ixr.ix;
+
   char cancelled = 0;
   while (!cancelled && (zst = zsv_parse_more(ixr.parser)) == zsv_status_ok) {
     pthread_mutex_lock(&optsp->uib->mutex);
@@ -50,16 +52,19 @@ enum zsv_index_status build_memory_index(struct zsvsheet_index_opts *optsp) {
       cancelled = 1;
       zst = zsv_status_cancelled;
     }
+    zsv_index_commit_rows(ixr.ix);
+    optsp->uib->index_ready = 1;
     pthread_mutex_unlock(&optsp->uib->mutex);
   }
 
   zsv_finish(ixr.parser);
+  pthread_mutex_lock(&optsp->uib->mutex);
+  zsv_index_commit_rows(ixr.ix);
+  optsp->uib->index_ready = 1;
+  pthread_mutex_unlock(&optsp->uib->mutex);
 
-  if (zst == zsv_status_no_more_input || zst == zsv_status_cancelled) {
+  if (zst == zsv_status_no_more_input || zst == zsv_status_cancelled)
     ret = zsv_index_status_ok;
-    optsp->uib->index = ixr.ix;
-  } else
-    zsv_index_delete(ixr.ix);
 
 out:
   if (ixr.parser)
diff --git a/app/sheet/read-data.c b/app/sheet/read-data.c
index 90504e12..cb51f496 100644
--- a/app/sheet/read-data.c
+++ b/app/sheet/read-data.c
@@ -204,7 +204,7 @@ static int read_data(struct zsvsheet_ui_buffer **uibufferp,   // a new zsvsheet_
     return 0;
 
   pthread_mutex_lock(&uibuff->mutex);
-  char need_index = !uibuff->index_started && (!uibuff->write_in_progress || uibuff->write_done);
+  char need_index = !uibuff->index_started && !uibuff->write_in_progress;
   pthread_mutex_unlock(&uibuff->mutex);
 
   if (need_index) {
@@ -253,7 +253,6 @@ static void *get_data_index(void *gdi) {
   }
 
   pthread_mutex_lock(mutexp);
-  uib->index_ready = 1;
   uib->status = old_ui_status;
   uib->ixopts = NULL;
   pthread_mutex_unlock(mutexp);
diff --git a/app/sheet/transformation.c b/app/sheet/transformation.c
index 1ebcbe0c..dd50dd83 100644
--- a/app/sheet/transformation.c
+++ b/app/sheet/transformation.c
@@ -15,7 +15,7 @@ struct zsvsheet_transformation {
   FILE *output_stream;
   unsigned char *output_buffer;
   int output_fileno;
-  size_t output_count;
+  char writer_wrote;
   struct zsvsheet_transformation_opts opts;
   void *user_context;
 
@@ -28,7 +28,7 @@ static size_t transformation_write(const void *restrict ptr, size_t size, size_t
   struct zsvsheet_transformation *trn = stream;
 
   const size_t count = fwrite(ptr, size, nitems, trn->output_stream);
-  trn->output_count += count;
+  trn->writer_wrote = count > 0;
 
   return count > 0 ? count : 0;
 }
@@ -134,14 +134,17 @@ static void *zsvsheet_run_buffer_transformation(void *arg) {
   zsv_parser parser = trn->parser;
   pthread_mutex_t *mutex = &uib->mutex;
   enum zsv_status zst;
+  char *default_status = trn->default_status;
 
-  size_t c = trn->output_count;
   char cancelled = 0;
   while (!cancelled && (zst = zsv_parse_more(parser)) == zsv_status_ok) {
     pthread_mutex_lock(mutex);
     cancelled = uib->worker_cancelled;
-    if (trn->output_count != c)
-      uib->write_progressed = 1;
+    if (trn->writer_wrote) {
+      trn->writer_wrote = 0;
+      zsv_index_commit_rows(uib->index);
+      uib->index_ready = 1;
+    }
     pthread_mutex_unlock(mutex);
   }
 
@@ -151,20 +154,22 @@ static void *zsvsheet_run_buffer_transformation(void *arg) {
   if (trn->on_done)
     trn->on_done(trn);
 
+  if (trn->user_context)
+    free(trn->user_context);
+
+  zsvsheet_transformation_delete(trn);
+
   pthread_mutex_lock(mutex);
   char *buff_status_old = uib->status;
-  uib->write_progressed = 1;
   uib->write_done = 1;
+  zsv_index_commit_rows(uib->index);
   uib->index_ready = 1;
-  if (buff_status_old == trn->default_status)
+  if (buff_status_old == default_status)
     uib->status = NULL;
   pthread_mutex_unlock(mutex);
 
-  if (buff_status_old == trn->default_status)
+  if (buff_status_old == default_status)
     free(buff_status_old);
-  if (trn->user_context)
-    free(trn->user_context);
-  zsvsheet_transformation_delete(trn);
 
   return NULL;
 }
@@ -214,9 +219,10 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   // TODO: If the transformation is a reduction that doesn't output for some time this will caus a pause
   zsv_parser parser = zsvsheet_transformation_parser(trn);
   while ((zst = zsv_parse_more(parser)) == zsv_status_ok) {
-    if (trn->output_count > 0)
+    if (trn->writer_wrote)
       break;
   }
+  trn->writer_wrote = 0;
 
   switch (zst) {
   case zsv_status_no_more_input:
@@ -242,14 +248,18 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
 
   struct zsvsheet_ui_buffer *nbuff = zsvsheet_buffer_current(ctx);
   trn->ui_buffer = nbuff;
-  nbuff->write_progressed = 1;
+  zsv_index_commit_rows(index);
   nbuff->index_started = 1;
   nbuff->index = index;
 
   if (zst != zsv_status_ok) {
     nbuff->write_done = 1;
     nbuff->index_ready = 1;
-    goto out;
+    if (trn->on_done)
+      opts.on_done(trn);
+    zsvsheet_transformation_delete(trn);
+    zsv_index_commit_rows(index);
+    return stat;
   }
 
   asprintf(&trn->default_status, "(working) Press ESC to cancel ");
@@ -259,9 +269,8 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   return stat;
 
 error:
-  free(index);
+  zsv_index_delete(index);
 
-out:
   if (trn && trn->on_done)
     opts.on_done(trn);
   if (trn)
diff --git a/app/sheet/ui_buffer.c b/app/sheet/ui_buffer.c
index 7cc2faff..ebf30d1f 100644
--- a/app/sheet/ui_buffer.c
+++ b/app/sheet/ui_buffer.c
@@ -44,11 +44,10 @@ struct zsvsheet_ui_buffer {
   unsigned char has_row_num : 1;
   unsigned char mutex_inited : 1;
   unsigned char write_in_progress : 1;
-  unsigned char write_progressed : 1;
   unsigned char write_done : 1;
   unsigned char worker_active : 1;
   unsigned char worker_cancelled : 1;
-  unsigned char _ : 6;
+  unsigned char _ : 7;
 };
 
 void zsvsheet_ui_buffer_create_worker(struct zsvsheet_ui_buffer *ub, void *(*start_func)(void *), void *arg) {
diff --git a/app/test/Makefile b/app/test/Makefile
index 5fc7a505..fd039592 100644
--- a/app/test/Makefile
+++ b/app/test/Makefile
@@ -623,7 +623,7 @@ test-sheet-3: ${BUILD_DIR}/bin/zsv_sheet${EXE}
 	sleep 2 && \
 	tmux capture-pane -t $@ -p ${REDIRECT1} ${TMP_DIR}/$@.out && \
 	tmux send-keys -t $@ "q" && \
-	${CMP} ${TMP_DIR}/$@.out expected/$@.out && ${TEST_PASS} || ${TEST_FAIL})
+	${CMP} ${TMP_DIR}/$@.out expected/$@.out && ${TEST_PASS} || (echo 'Incorrect output:' && cat ${TMP_DIR}/$@.out && ${TEST_FAIL}))
 
 test-sheet-4: ${BUILD_DIR}/bin/zsv_sheet${EXE}
 	@${TEST_INIT}
diff --git a/app/utils/index.c b/app/utils/index.c
index 96720969..ef656b6d 100644
--- a/app/utils/index.c
+++ b/app/utils/index.c
@@ -7,30 +7,34 @@
 #include <zsv/utils/index.h>
 
 struct zsv_index *zsv_index_new(void) {
-  struct zsv_index *ix = malloc(sizeof(*ix));
+  struct zsv_index *ix = calloc(1, sizeof(*ix));
 
   if (!ix)
     return ix;
 
-  memset(ix, 0, sizeof(*ix));
-
-  const size_t init_cap = 256;
-  ix->array = malloc(sizeof(*ix->array) + init_cap * sizeof(ix->array->u64s[0]));
-  ix->array->capacity = init_cap;
-  ix->array->len = 0;
+  const size_t init_cap = 512;
+  ix->first = calloc(1, sizeof(*ix->first) + init_cap * sizeof(ix->first->u64s[0]));
+  ix->first->capacity = init_cap;
 
   return ix;
 }
 
 void zsv_index_delete(struct zsv_index *ix) {
   if (ix) {
-    free(ix->array);
+    struct zsv_index_array *arr = ix->first;
+
+    while (arr) {
+      struct zsv_index_array *a = arr;
+      arr = arr->next;
+      free(a);
+    }
+
     free(ix);
   }
 }
 
 enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, uint64_t line_end) {
-  struct zsv_index_array *arr = ix->array;
+  struct zsv_index_array *arr = ix->first;
   size_t len = arr->len, cap = arr->capacity;
 
   if (!ix->header_line_end) {
@@ -38,19 +42,27 @@ enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, uint64_t line_end)
     return zsv_index_status_ok;
   }
 
-  ix->row_count++;
+  ix->row_count_local++;
 
-  if ((ix->row_count & (ZSV_INDEX_ROW_N - 1)) != 0)
+  if ((ix->row_count_local & (ZSV_INDEX_ROW_N - 1)) != 0)
     return zsv_index_status_ok;
 
-  if (len >= cap) {
-    cap *= 2;
-    arr = realloc(arr, sizeof(*arr) + cap * sizeof(arr->u64s[0]));
-    if (!arr)
-      return zsv_index_status_memory;
-
-    arr->capacity = cap;
-    ix->array = arr;
+  while (len >= cap) {
+    assert(len == cap);
+
+    if (!arr->next) {
+      len = 0;
+      cap *= 2;
+      arr->next = calloc(1, sizeof(*arr) + cap * sizeof(arr->u64s[0]));
+      arr = arr->next;
+      if (!arr)
+        return zsv_index_status_memory;
+      arr->capacity = cap;
+    } else {
+      arr = arr->next;
+      len = arr->len;
+      cap = arr->capacity;
+    }
   }
 
   arr->u64s[len] = line_end;
@@ -59,22 +71,38 @@ enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, uint64_t line_end)
   return zsv_index_status_ok;
 }
 
+void zsv_index_commit_rows(struct zsv_index *ix) {
+  ix->row_count = ix->row_count_local;
+}
+
 enum zsv_index_status zsv_index_row_end_offset(const struct zsv_index *ix, uint64_t row, uint64_t *offset_out,
                                                uint64_t *remaining_rows_out) {
+  assert(ix->row_count <= ix->row_count_local);
+
   if (row > ix->row_count)
     return zsv_index_status_error;
 
   if (row < ZSV_INDEX_ROW_N) {
     *offset_out = ix->header_line_end;
     *remaining_rows_out = row;
-  } else {
-    const size_t i = (row >> ZSV_INDEX_ROW_SHIFT) - 1;
 
-    assert(i < ix->array->len);
-    *offset_out = (long)ix->array->u64s[i];
-    *remaining_rows_out = row & (ZSV_INDEX_ROW_N - 1);
+    return zsv_index_status_ok;
+  }
+
+  const size_t i = (row >> ZSV_INDEX_ROW_SHIFT) - 1;
+  struct zsv_index_array *arr = ix->first;
+  size_t lens = 0;
+
+  while (i >= lens + arr->len) {
+    assert(arr->next);
+
+    lens += arr->len;
+    arr = arr->next;
   }
 
+  *offset_out = (long)arr->u64s[i - lens];
+  *remaining_rows_out = row & (ZSV_INDEX_ROW_N - 1);
+
   return zsv_index_status_ok;
 }
 
diff --git a/include/zsv/utils/index.h b/include/zsv/utils/index.h
index f5805c94..9735fc40 100644
--- a/include/zsv/utils/index.h
+++ b/include/zsv/utils/index.h
@@ -23,20 +23,25 @@ enum zsv_index_status {
 struct zsv_index_array {
   size_t capacity;
   size_t len;
+  struct zsv_index_array *next;
   uint64_t u64s[];
 };
 
 struct zsv_index {
   uint64_t header_line_end;
+  // Reading and writing should be protected with a lock
   uint64_t row_count;
+  // Should only be updated by the thread building the index
+  uint64_t row_count_local;
 
   // array containing the offsets of every ZSV_INDEX_ROW_N line end
-  struct zsv_index_array *array;
+  struct zsv_index_array *first;
 };
 
 struct zsv_index *zsv_index_new(void);
 void zsv_index_delete(struct zsv_index *ix);
 enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, uint64_t line_end);
+void zsv_index_commit_rows(struct zsv_index *ix);
 enum zsv_index_status zsv_index_row_end_offset(const struct zsv_index *ix, uint64_t row, uint64_t *offset_out,
                                                uint64_t *remaining_rows_out);
 enum zsv_index_status zsv_index_seek_row(const struct zsv_index *ix, struct zsv_opts *opts, uint64_t row);