module sis batch index migration

Further experience has shown this to be a better fit for the job and
admin page queries.

This migration has only run on dev boxes so far, so this is a
replacement migration rather than a second migration.

Change-Id: Ibdde713efa3940d6e05af71a4ee792ef505c4ae9
Reviewed-on: https://gerrit.instructure.com/11727
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>

db/migrate/20120608154850_add_sis_batches_index.rb

@@ -1,11 +0,0 @@
-class AddSisBatchesIndex < ActiveRecord::Migration
-  tag :predeploy
-
-  def self.up
-    add_index :sis_batches, [:account_id, :workflow_state, :created_at], :name => "index_sis_batches_for_accounts"
-  end
-
-  def self.down
-    remove_index :sis_batches, :name => "index_sis_batches_for_accounts"
-  end
-end

db/migrate/20120620171523_add_sis_batches_index.rb

@@ -0,0 +1,28 @@
+class AddSisBatchesIndex < ActiveRecord::Migration
+  tag :predeploy
+
+  self.transactional = false
+
+  def self.up
+    # this index may or may not have been created on dev boxes
+    remove_index :sis_batches, :name => "index_sis_batches_for_accounts" rescue nil
+
+    case connection.adapter_name
+    when 'PostgreSQL'
+      # select * from sis_batches where account_id = ? and workflow_state = 'created' order by created_at
+      # select count(*) from sis_batches where account_id = ? and workflow_state = 'created'
+      # this index is highly optimized for the sis batch job processor workflow
+      connection.execute "CREATE INDEX CONCURRENTLY index_sis_batches_pending_for_accounts ON sis_batches (account_id, created_at) WHERE workflow_state = 'created'"
+      # select * from sis_batches where account_id = ? order by created_at desc limit 1
+      connection.execute "CREATE INDEX CONCURRENTLY index_sis_batches_account_id_created_at ON sis_batches (account_id, created_at)"
+    else
+      add_index :sis_batches, [:workflow_state, :account_id, :created_at], :name => "index_sis_batches_pending_for_accounts"
+      add_index :sis_batches, [:account_id, :created_at], :name => "index_sis_batches_account_id_created_at"
+    end
+  end
+
+  def self.down
+    remove_index :sis_batches, :name => "index_sis_batches_pending_for_accounts"
+    remove_index :sis_batches, :name => "index_sis_batches_account_id_created_at"
+  end
+end