PERF: Replace posts reply_to_post_number index (#26385)

post_number is a sequence per topic, so it doesn't make sense to have an
index on only reply_to_post_number without also including the topic_id.

db/migrate/20240327000440_replace_post_reply_index.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+class ReplacePostReplyIndex < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def up
+    execute <<~SQL
+      DROP INDEX CONCURRENTLY IF EXISTS "index_posts_on_topic_id_and_reply_to_post_number"
+    SQL
+
+    execute <<~SQL
+      CREATE INDEX CONCURRENTLY IF NOT EXISTS "index_posts_on_topic_id_and_reply_to_post_number"
+      ON "posts" ("topic_id", "reply_to_post_number")
+    SQL
+
+    execute <<~SQL
+      DROP INDEX CONCURRENTLY IF EXISTS "index_posts_on_reply_to_post_number"
+    SQL
+  end
+
+  def down
+    execute <<~SQL
+      DROP INDEX CONCURRENTLY IF EXISTS "index_posts_on_reply_to_post_number"
+    SQL
+
+    execute <<~SQL
+      CREATE INDEX CONCURRENTLY IF NOT EXISTS "index_posts_on_reply_to_post_number"
+      ON "posts" ("reply_to_post_number")
+    SQL
+
+    execute <<~SQL
+      DROP INDEX CONCURRENTLY IF EXISTS "index_posts_on_topic_id_and_reply_to_post_number"
+    SQL
+  end
+end