Add index for merge_requests.merge_commit_sha

This is a composite index on (target_project_id, merge_commit_sha, id)
that allows queries such as the following to use a full backwards index
scan:

    SELECT "merge_requests".*
    FROM "merge_requests"
    WHERE "merge_requests"."deleted_at" IS NULL
    AND "merge_requests"."target_project_id" = 13083
    AND "merge_requests"."merge_commit_sha" = 'e80a893f'
    ORDER BY "merge_requests"."id" DESC
    LIMIT 1;

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38507

changelogs/unreleased/add-composite-index-on-merge-requests-merge-commit-sha.yml

+---
+title: Add index for merge_requests.merge_commit_sha
+merge_request:
+author:
+type: other

db/migrate/20170928100231_add_composite_index_on_merge_requests_merge_commit_sha.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddCompositeIndexOnMergeRequestsMergeCommitSha < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # The default index name is too long for PostgreSQL and would thus be
+  # truncated.
+  INDEX_NAME = 'index_merge_requests_on_tp_id_and_merge_commit_sha_and_id'
+
+  COLUMNS = [:target_project_id, :merge_commit_sha, :id]
+
+  disable_ddl_transaction!
+
+  def up
+    return if index_is_present?
+
+    add_concurrent_index(:merge_requests, COLUMNS, name: INDEX_NAME)
+  end
+
+  def down
+    return unless index_is_present?
+
+    remove_concurrent_index(:merge_requests, COLUMNS, name: INDEX_NAME)
+  end
+
+  def index_is_present?
+    index_exists?(:merge_requests, COLUMNS, name: INDEX_NAME)
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170921115009) do
+ActiveRecord::Schema.define(version: 20170928100231) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -895,6 +895,7 @@ ActiveRecord::Schema.define(version: 20170921115009) do
   add_index "merge_requests", ["source_project_id", "source_branch"], name: "index_merge_requests_on_source_project_id_and_source_branch", using: :btree
   add_index "merge_requests", ["target_branch"], name: "index_merge_requests_on_target_branch", using: :btree
   add_index "merge_requests", ["target_project_id", "iid"], name: "index_merge_requests_on_target_project_id_and_iid", unique: true, using: :btree
+  add_index "merge_requests", ["target_project_id", "merge_commit_sha", "id"], name: "index_merge_requests_on_tp_id_and_merge_commit_sha_and_id", using: :btree
   add_index "merge_requests", ["title"], name: "index_merge_requests_on_title", using: :btree
   add_index "merge_requests", ["title"], name: "index_merge_requests_on_title_trigram", using: :gin, opclasses: {"title"=>"gin_trgm_ops"}