From b3920e05e73fd383dd56fac5d44ab659987f7e55 Mon Sep 17 00:00:00 2001
From: Sam <sam.saffron@gmail.com>
Date: Thu, 23 Nov 2023 18:15:40 +1100
Subject: [PATCH] PERF: avoid table scan while performing a very large update
 (#24525)

We were seeing lots of deadlocks deploying this migration. This improves
the situation in 2 ways.

1. ddl transaction is avoided, so we hold locks for far shorter times
2. we operate in chunks of a maximum of 100_000 posts (though it is heavily filtered down)

* improve code so it is clearer
---
 ...igger_post_rebake_category_style_quotes.rb | 27 ++++++++++++++++---
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/db/post_migrate/20231120190818_trigger_post_rebake_category_style_quotes.rb b/db/post_migrate/20231120190818_trigger_post_rebake_category_style_quotes.rb
index 7adb3ed7e9f..ea3837f529e 100644
--- a/db/post_migrate/20231120190818_trigger_post_rebake_category_style_quotes.rb
+++ b/db/post_migrate/20231120190818_trigger_post_rebake_category_style_quotes.rb
@@ -1,12 +1,31 @@
 # frozen_string_literal: true
 
 class TriggerPostRebakeCategoryStyleQuotes < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
   def up
-    DB.exec(<<~SQL)
-      UPDATE posts
-      SET baked_version = NULL
-      WHERE cooked LIKE '%blockquote%'
+    max_id = DB.query_single(<<~SQL).first.to_i
+      SELECT MAX(id)
+      FROM posts
     SQL
+
+    chunk_size = 100_000
+    while max_id > 0
+      ids = DB.query_single(<<~SQL, start: max_id - chunk_size, finish: max_id)
+        SELECT id
+        FROM posts
+        WHERE cooked LIKE '%blockquote%'
+        AND id >= :start AND id <= :finish
+      SQL
+
+      DB.exec(<<~SQL, ids: ids) if ids && ids.length > 0
+          UPDATE posts
+          SET baked_version = NULL
+          WHERE id IN (:ids)
+        SQL
+
+      max_id -= chunk_size
+    end
   end
 
   def down