diff --git a/tools/backfill-commentcontent-id.sql b/tools/backfill-commentcontent-id.sql
new file mode 100644
index 0000000..2b2e3c2
--- /dev/null
+++ b/tools/backfill-commentcontent-id.sql
@@ -0,0 +1,101 @@
+-- Copyright 2016 The Chromium Authors. All Rights Reserved.
+--
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file or at
+-- https://developers.google.com/open-source/licenses/bsd
+
+
+DROP PROCEDURE IF EXISTS BackfillCommentContentID;
+
+delimiter //
+
+CREATE PROCEDURE BackfillCommentContentID(
+    IN in_start INT, IN in_stop INT, IN in_step INT)
+BEGIN
+  comment_loop: LOOP
+    IF in_start >= in_stop THEN
+      LEAVE comment_loop;
+    END IF;
+
+    SELECT in_start AS StartingAt;
+    SELECT count(*)
+    FROM CommentContent
+    WHERE comment_id >= in_start
+    AND comment_id < in_start + in_step;
+
+    DROP TEMPORARY TABLE IF EXISTS temp_comment;
+    CREATE TEMPORARY TABLE temp_comment (
+      id INT NOT NULL,
+      issue_id INT NOT NULL,
+      created INT NOT NULL,
+      project_id SMALLINT UNSIGNED NOT NULL,
+      commenter_id INT UNSIGNED NOT NULL,
+      commentcontent_id INT UNSIGNED NOT NULL,
+      deleted_by INT UNSIGNED,
+      is_spam BOOLEAN DEFAULT FALSE,
+      is_description BOOLEAN DEFAULT FALSE,
+
+      PRIMARY KEY(id)
+    ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+
+    INSERT INTO temp_comment
+    SELECT Comment.id, Comment.issue_id, Comment.created, Comment.project_id,
+           Comment.commenter_id, CommentContent.id,
+           Comment.deleted_by, Comment.is_spam, Comment.is_description
+    FROM Comment
+    LEFT JOIN CommentContent on CommentContent.comment_id = Comment.id
+    WHERE CommentContent.comment_id >= in_start
+    AND CommentContent.comment_id < in_start + in_step;
+
+
+    REPLACE INTO Comment (id, issue_id, created, project_id, commenter_id,
+        commentcontent_id, deleted_by, is_spam, is_description)
+    SELECT id, issue_id, created, project_id, commenter_id,
+        commentcontent_id, deleted_by, is_spam, is_description
+    FROM temp_comment;
+
+    SET in_start = in_start + in_step;
+
+  END LOOP;
+
+END;
+
+
+//
+
+
+delimiter ;
+
+
+-- Temporarily disable these foreign key references so that we can do
+-- REPLACE commands.
+-- ALTER TABLE SpamReport DROP FOREIGN KEY spamreport_ibfk_2;
+-- ALTER TABLE SpamVerdict DROP FOREIGN KEY spamverdict_ibfk_2;
+
+-- If run locally do all at once:
+-- CALL BackfillCommentContentID(           0,  1 * 1000000, 10000);
+
+-- If run on staging or production, do it in steps and check that
+-- users are not hitting errors or timeouts as you go:
+-- CALL BackfillCommentContentID(           0, 13 * 1000000, 10000);
+-- CALL BackfillCommentContentID(13 * 1000000, 16 * 1000000, 10000);
+-- CALL BackfillCommentContentID(16 * 1000000, 17 * 1000000, 10000);
+-- CALL BackfillCommentContentID(17 * 1000000, 18 * 1000000, 10000);
+-- CALL BackfillCommentContentID(18 * 1000000, 19 * 1000000, 10000);
+-- CALL BackfillCommentContentID(19 * 1000000, 20 * 1000000, 10000);
+-- CALL BackfillCommentContentID(20 * 1000000, 21 * 1000000, 10000);
+-- CALL BackfillCommentContentID(21 * 1000000, 22 * 1000000, 10000);
+-- CALL BackfillCommentContentID(22 * 1000000, 23 * 1000000, 10000);
+-- CALL BackfillCommentContentID(23 * 1000000, 24 * 1000000, 10000);
+-- CALL BackfillCommentContentID(24 * 1000000, 25 * 1000000, 10000);
+-- CALL BackfillCommentContentID(25 * 1000000, 26 * 1000000, 10000);
+-- CALL BackfillCommentContentID(26 * 1000000, 27 * 1000000, 10000);
+-- CALL BackfillCommentContentID(27 * 1000000, 28 * 1000000, 10000);
+-- CALL BackfillCommentContentID(28 * 1000000, 29 * 1000000, 10000);
+-- CALL BackfillCommentContentID(29 * 1000000, 30 * 1000000, 10000);
+-- CALL BackfillCommentContentID(30 * 1000000, 40 * 1000000, 10000);
+
+-- Add back foreign key constraints.
+-- ALTER TABLE SpamReport ADD FOREIGN KEY (comment_id) REFERENCES Comment(id);
+-- ALTER TABLE SpamVerdict ADD FOREIGN KEY (comment_id) REFERENCES Comment(id);