From a004f9ec8c66ea9dcce0e76e28b0a343437c0b16 Mon Sep 17 00:00:00 2001
From: Yorick Peterse <yorickpeterse@gmail.com>
Date: Fri, 14 Jul 2017 13:19:00 +0200
Subject: Added cop to blacklist the use of hash indexes

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.
---
 rubocop/cop/migration/hash_index.rb | 51 +++++++++++++++++++++++++++++++++++++
 rubocop/rubocop.rb                  |  1 +
 2 files changed, 52 insertions(+)
 create mode 100644 rubocop/cop/migration/hash_index.rb

(limited to 'rubocop')

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'
-- 
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