diff --git a/resources/sql/autopatches/20180904.commit.01.unreachable.sql b/resources/sql/autopatches/20180904.commit.01.unreachable.sql
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20180904.commit.01.unreachable.sql
@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_repository.repository_commit
+  ADD isUnreachable BOOL NOT NULL;
diff --git a/resources/sql/autopatches/20180904.commit.02.reachability.sql b/resources/sql/autopatches/20180904.commit.02.reachability.sql
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20180904.commit.02.reachability.sql
@@ -0,0 +1,2 @@
+UPDATE {$NAMESPACE}_repository.repository_commit
+  SET isUnreachable = IF((importStatus & 2048) = 2048, 1, 0);
diff --git a/scripts/repository/rebuild_summaries.php b/scripts/repository/rebuild_summaries.php
deleted file mode 100755
--- a/scripts/repository/rebuild_summaries.php
+++ /dev/null
@@ -1,53 +0,0 @@
-#!/usr/bin/env php
-<?php
-
-$root = dirname(dirname(dirname(__FILE__)));
-require_once $root.'/scripts/__init_script__.php';
-
-$commit = new PhabricatorRepositoryCommit();
-
-$conn_w = id(new PhabricatorRepository())->establishConnection('w');
-$sizes = queryfx_all(
-  $conn_w,
-  'SELECT repositoryID, count(*) N FROM %T GROUP BY repositoryID',
-  $commit->getTableName());
-$sizes = ipull($sizes, 'N', 'repositoryID');
-
-$maxes = queryfx_all(
-  $conn_w,
-  'SELECT repositoryID, max(epoch) maxEpoch FROM %T GROUP BY repositoryID',
-  $commit->getTableName());
-$maxes = ipull($maxes, 'maxEpoch', 'repositoryID');
-
-
-$repository_ids = array_keys($sizes + $maxes);
-
-echo pht('Updating %d repositories', count($repository_ids));
-
-foreach ($repository_ids as $repository_id) {
-  $last_commit = queryfx_one(
-    $conn_w,
-    'SELECT id FROM %T WHERE repositoryID = %d AND epoch = %d LIMIT 1',
-    $commit->getTableName(),
-    $repository_id,
-    idx($maxes, $repository_id, 0));
-  if ($last_commit) {
-    $last_commit = $last_commit['id'];
-  } else {
-    $last_commit = 0;
-  }
-  queryfx(
-    $conn_w,
-    'INSERT INTO %T (repositoryID, lastCommitID, size, epoch)
-      VALUES (%d, %d, %d, %d) ON DUPLICATE KEY UPDATE
-        lastCommitID = VALUES(lastCommitID),
-        size = VALUES(size),
-        epoch = VALUES(epoch)',
-    PhabricatorRepository::TABLE_SUMMARY,
-    $repository_id,
-    $last_commit,
-    idx($sizes, $repository_id, 0),
-    idx($maxes, $repository_id, 0));
-  echo '.';
-}
-echo "\n".pht('Done.')."\n";
diff --git a/src/applications/repository/engine/PhabricatorRepositoryDiscoveryEngine.php b/src/applications/repository/engine/PhabricatorRepositoryDiscoveryEngine.php
--- a/src/applications/repository/engine/PhabricatorRepositoryDiscoveryEngine.php
+++ b/src/applications/repository/engine/PhabricatorRepositoryDiscoveryEngine.php
@@ -912,14 +912,31 @@
   private function rebuildSummaryTable(PhabricatorRepository $repository) {
     $conn_w = $repository->establishConnection('w');
 
-    $data = queryfx_one(
+    // See PHI842. This awkward construction allows us to hit keys on both
+    // parts of the query.
+
+    $repository_table = id(new PhabricatorRepositoryCommit())->getTableName();
+    $repository_id = $repository->getID();
+
+    $data = queryfx_all(
       $conn_w,
-      'SELECT COUNT(*) N, MAX(id) id, MAX(epoch) epoch
-        FROM %T WHERE repositoryID = %d AND (importStatus & %d) != %d',
-      id(new PhabricatorRepositoryCommit())->getTableName(),
-      $repository->getID(),
-      PhabricatorRepositoryCommit::IMPORTED_UNREACHABLE,
-      PhabricatorRepositoryCommit::IMPORTED_UNREACHABLE);
+      '(SELECT %s Label, COUNT(*) N, MAX(id) Max
+        FROM %T WHERE repositoryID = %d AND isUnreachable = 0)
+       UNION ALL
+       (SELECT %s Label, 0 N, MAX(epoch)
+         FROM %T WHERE repositoryID = %d AND isUnreachable = 0)',
+      'size',
+      $repository_table,
+      $repository_id,
+      'epoch',
+      $repository_table,
+      $repository_id);
+
+    $data = ipull($data, null, 'Label');
+
+    $repository_size = (int)$data['size']['N'];
+    $repository_max = (int)$data['size']['Max'];
+    $repository_epoch = (int)$data['epoch']['Max'];
 
     queryfx(
       $conn_w,
@@ -930,10 +947,10 @@
           lastCommitID = VALUES(lastCommitID),
           epoch = VALUES(epoch)',
       PhabricatorRepository::TABLE_SUMMARY,
-      $repository->getID(),
-      $data['N'],
-      $data['id'],
-      $data['epoch']);
+      $repository_id,
+      $repository_size,
+      $repository_max,
+      $repository_epoch);
   }
 
 }
diff --git a/src/applications/repository/storage/PhabricatorRepositoryCommit.php b/src/applications/repository/storage/PhabricatorRepositoryCommit.php
--- a/src/applications/repository/storage/PhabricatorRepositoryCommit.php
+++ b/src/applications/repository/storage/PhabricatorRepositoryCommit.php
@@ -30,6 +30,7 @@
   protected $auditStatus = PhabricatorAuditCommitStatusConstants::NONE;
   protected $summary = '';
   protected $importStatus = 0;
+  protected $isUnreachable = 0;
 
   const IMPORTED_MESSAGE = 1;
   const IMPORTED_CHANGE = 2;
@@ -87,26 +88,40 @@
     $table_name = $this->getTableName();
     $id = $this->getID();
 
+    if ($set) {
+      $new_flags = ($this->getImportStatus() | $flag);
+    } else {
+      $new_flags = ($this->getImportStatus() & ~$flag);
+    }
+
+    $is_unreachable = (bool)($new_flags & self::IMPORTED_UNREACHABLE);
+
     if ($set) {
       queryfx(
         $conn_w,
-        'UPDATE %T SET importStatus = (importStatus | %d) WHERE id = %d',
+        'UPDATE %T SET
+            importStatus = (importStatus | %d),
+            isUnreachable = %d
+          WHERE id = %d',
         $table_name,
         $flag,
+        (int)$is_unreachable,
         $id);
-
-      $this->setImportStatus($this->getImportStatus() | $flag);
     } else {
       queryfx(
         $conn_w,
-        'UPDATE %T SET importStatus = (importStatus & ~%d) WHERE id = %d',
+        'UPDATE %T SET
+            importStatus = (importStatus & ~%d),
+            isUnreachable = %d
+          WHERE id = %d',
         $table_name,
         $flag,
+        (int)$is_unreachable,
         $id);
-
-      $this->setImportStatus($this->getImportStatus() & ~$flag);
     }
 
+    $this->setImportStatus($new_flags);
+
     return $this;
   }
 
@@ -123,6 +138,7 @@
         'auditStatus' => 'uint32',
         'summary' => 'text255',
         'importStatus' => 'uint32',
+        'isUnreachable' => 'bool',
       ),
       self::CONFIG_KEY_SCHEMA => array(
         'key_phid' => null,
@@ -149,6 +165,12 @@
         'key_author' => array(
           'columns' => array('authorPHID', 'epoch'),
         ),
+        'key_epoch_summary' => array(
+          'columns' => array('repositoryID', 'isUnreachable', 'epoch'),
+        ),
+        'key_count_summary' => array(
+          'columns' => array('repositoryID', 'isUnreachable', 'id'),
+        ),
       ),
       self::CONFIG_NO_MUTATE => array(
         'importStatus',