Added cop to blacklist the use of hash indexesrubocop-hash-index

These indexes are not recorded in the WAL (at least until PostgreSQL
10) and this isn't worth the minor performance improvement over btree
indexes.

2 files changed, 52 insertions, 0 deletions

diff --git a/rubocop/cop/migration/hash_index.rb b/rubocop/cop/migration/hash_index.rb
new file mode 100644
index 00000000000..2cc59691d84
--- /dev/null
+++ b/rubocop/cop/migration/hash_index.rb
@@ -0,0 +1,51 @@
+require 'set'
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that prevents the use of hash indexes in database migrations
+      class HashIndex < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = 'hash indexes should be avoided at all costs since they are not ' \
+          'recorded in the PostgreSQL WAL, you should use a btree index instead'.freeze
+
+        NAMES = Set.new([:add_index, :index, :add_concurrent_index]).freeze
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          name = node.children[1]
+
+          return unless NAMES.include?(name)
+
+          opts = node.children.last
+
+          return unless opts && opts.type == :hash
+
+          opts.each_node(:pair) do |pair|
+            next unless hash_key_type(pair) == :sym &&
+                hash_key_name(pair) == :using
+
+            if hash_key_value(pair).to_s == 'hash'
+              add_offense(pair, :expression)
+            end
+          end
+        end
+
+        def hash_key_type(pair)
+          pair.children[0].type
+        end
+
+        def hash_key_name(pair)
+          pair.children[0].children[0]
+        end
+
+        def hash_key_value(pair)
+          pair.children[1].children[0]
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index f76144275c9..3fbd5b0163c 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -13,6 +13,7 @@ require_relative 'cop/migration/add_concurrent_index'
 require_relative 'cop/migration/add_index'
 require_relative 'cop/migration/add_timestamps'
 require_relative 'cop/migration/datetime'
+require_relative 'cop/migration/hash_index'
 require_relative 'cop/migration/remove_concurrent_index'
 require_relative 'cop/migration/remove_index'
 require_relative 'cop/migration/reversible_add_column_with_default'