Remove redundant index

This index was substituted with
(project_id, status, created_at) index

changelogs/unreleased/31528-index-cleanup.yml

+---
+title: Remove redundant index
+merge_request: 47072
+author:
+type: performance

db/migrate/20201106135608_remove_redundant_pipelines_index.rb

+# frozen_string_literal: true
+
+class RemoveRedundantPipelinesIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name :ci_pipelines, :index_ci_pipelines_on_project_id_and_created_at
+  end
+
+  def down
+    add_concurrent_index :ci_pipelines, [:project_id, :created_at]
+  end
+end

db/schema_migrations/20201106135608

+52c8fb75035a08e212db52d032638a0c8f9d91306bfb8015fb3fc403a1cff1ec
\ No newline at end of file

db/structure.sql

@@ -20874,8 +20874,6 @@ CREATE INDEX index_ci_pipelines_on_merge_request_id ON ci_pipelines USING btree
 
 CREATE INDEX index_ci_pipelines_on_pipeline_schedule_id ON ci_pipelines USING btree (pipeline_schedule_id);
 
-CREATE INDEX index_ci_pipelines_on_project_id_and_created_at ON ci_pipelines USING btree (project_id, created_at);
-
 CREATE INDEX index_ci_pipelines_on_project_id_and_id_desc ON ci_pipelines USING btree (project_id, id DESC);
 
 CREATE UNIQUE INDEX index_ci_pipelines_on_project_id_and_iid ON ci_pipelines USING btree (project_id, iid) WHERE (iid IS NOT NULL);