diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index c3fae16d109..b0a0c753c09 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -874,7 +874,7 @@ class MergeRequest < ActiveRecord::Base
   #
   def all_commit_shas
     if persisted?
-      column_shas = MergeRequestDiffCommit.where(merge_request_diff: merge_request_diffs).pluck('DISTINCT(sha)')
+      column_shas = MergeRequestDiffCommit.where(merge_request_diff: merge_request_diffs).limit(10_000).pluck('sha')
       serialised_shas = merge_request_diffs.where.not(st_commits: nil).flat_map(&:commit_shas)
 
       (column_shas + serialised_shas).uniq
diff --git a/changelogs/unreleased/39054-activerecord-statementinvalid-pg-querycanceled-error-canceling-statement-due-to-statement-timeout.yml b/changelogs/unreleased/39054-activerecord-statementinvalid-pg-querycanceled-error-canceling-statement-due-to-statement-timeout.yml
new file mode 100644
index 00000000000..47bf30ecb5a
--- /dev/null
+++ b/changelogs/unreleased/39054-activerecord-statementinvalid-pg-querycanceled-error-canceling-statement-due-to-statement-timeout.yml
@@ -0,0 +1,5 @@
+---
+title: Stop merge requests with thousands of commits from timing out
+merge_request: 15063
+author:
+type: performance