Remove import columns from projects table

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21497, we
migrated all project import data into a separate table,
`project_import_data`.  In addition, we also added:

```
ignore_column :import_status, :import_jid, :import_error
```

In https://gitlab.com/gitlab-com/gl-infra/production/issues/908, we
observed some of these `import_error` columns consumed megabytes of
error backtraces and caused slow loading of projects whenever a `SELECT
* from projects` query loaded the row into memory.

Since we have long migrated away from these columns, we can now drop
these columns entirely.

app/models/project.rb

@@ -55,8 +55,6 @@ class Project < ApplicationRecord
   VALID_MIRROR_PORTS = [22, 80, 443].freeze
   VALID_MIRROR_PROTOCOLS = %w(http https ssh git).freeze
 
-  ignore_column :import_status, :import_jid, :import_error
-
   cache_markdown_field :description, pipeline: :description
 
   delegate :feature_available?, :builds_enabled?, :wiki_enabled?,

changelogs/unreleased/sh-remove-import-columns-from-projects.yml

@@ -0,0 +1,5 @@
+---
+title: Remove import columns from projects table
+merge_request: 29863
+author:
+type: performance

db/post_migrate/20190619175843_remove_import_columns_from_projects.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class RemoveImportColumnsFromProjects < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  def change
+    remove_column :projects, :import_status, :string
+    remove_column :projects, :import_jid, :string
+    remove_column :projects, :import_error, :text
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20190613030606) do
+ActiveRecord::Schema.define(version: 20190619175843) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -2595,11 +2595,9 @@ ActiveRecord::Schema.define(version: 20190613030606) do
     t.integer "visibility_level", default: 0, null: false
     t.boolean "archived", default: false, null: false
     t.string "avatar"
-    t.string "import_status"
     t.integer "star_count", default: 0, null: false
     t.string "import_type"
     t.string "import_source"
-    t.text "import_error"
     t.boolean "shared_runners_enabled", default: true, null: false
     t.string "runners_token"
     t.string "build_coverage_regex"
@@ -2621,7 +2619,6 @@ ActiveRecord::Schema.define(version: 20190613030606) do
     t.boolean "only_allow_merge_if_all_discussions_are_resolved"
     t.boolean "printing_merge_request_link_enabled", default: true, null: false
     t.integer "auto_cancel_pending_pipelines", default: 1, null: false
-    t.string "import_jid"
     t.integer "cached_markdown_version"
     t.text "delete_error"
     t.datetime "last_repository_updated_at"

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -461,9 +461,9 @@ describe Gitlab::Database::MigrationHelpers do
       end
 
       it 'updates all the rows in a table' do
-        model.update_column_in_batches(:projects, :import_error, 'foo')
+        model.update_column_in_batches(:projects, :description_html, 'foo')
 
-        expect(Project.where(import_error: 'foo').count).to eq(5)
+        expect(Project.where(description_html: 'foo').count).to eq(5)
       end
 
       it 'updates boolean values correctly' do