Do not reindex exclusion constraint indexes

Postgres exclusion constraints are based on indexes. Recreating those
will interfere with constraint checking.

For now, we only have one of this type.

db/migrate/20200922093004_add_postgres_index_view.rb

@@ -14,6 +14,7 @@ class AddPostgresIndexView < ActiveRecord::Migration[6.0]
         pg_index.indisunique as unique,
         pg_index.indisvalid as valid_index,
         pg_class.relispartition as partitioned,
+        pg_index.indisexclusion as exclusion,
         pg_indexes.indexdef as definition,
         pg_relation_size(pg_class.oid) as ondisk_size_bytes
       FROM pg_index

db/structure.sql

@@ -14419,6 +14419,7 @@ CREATE VIEW postgres_indexes AS
     pg_index.indisunique AS "unique",
     pg_index.indisvalid AS valid_index,
     pg_class.relispartition AS partitioned,
+    pg_index.indisexclusion AS exclusion,
     pg_indexes.indexdef AS definition,
     pg_relation_size((pg_class.oid)::regclass) AS ondisk_size_bytes
    FROM (((pg_index

lib/gitlab/database/postgres_index.rb

@@ -12,8 +12,10 @@ module Gitlab
         find(identifier)
       end
 
-      scope :non_unique, -> { where(unique: false) }
-      scope :non_partitioned, -> { where(partitioned: false) }
+      # A 'regular' index is a non-unique index,
+      # that does not serve an exclusion constraint and
+      # is defined on a table that is not partitioned.
+      scope :regular, -> { where(unique: false, partitioned: false, exclusion: false)}
 
       scope :random_few, ->(how_many) do
         limit(how_many).order(Arel.sql('RANDOM()'))

lib/gitlab/database/reindexing/concurrent_reindex.rb

@@ -23,6 +23,7 @@ module Gitlab
         def perform
           raise ReindexError, 'UNIQUE indexes are currently not supported' if index.unique?
           raise ReindexError, 'partitioned indexes are currently not supported' if index.partitioned?
+          raise ReindexError, 'indexes serving an exclusion constraint are currently not supported' if index.exclusion?
 
           logger.info "Starting reindex of #{index}"
 

lib/tasks/gitlab/db.rake

@@ -177,7 +177,7 @@ namespace :gitlab do
       indexes = if args[:index_name]
                   Gitlab::Database::PostgresIndex.by_identifier(args[:index_name])
                 else
-                  Gitlab::Database::PostgresIndex.non_unique.non_partitioned.random_few(2)
+                  Gitlab::Database::PostgresIndex.regular.random_few(2)
                 end
 
       Gitlab::Database::Reindexing.perform(indexes)

spec/lib/gitlab/database/postgres_index_spec.rb

@@ -30,15 +30,17 @@ RSpec.describe Gitlab::Database::PostgresIndex do
     end
   end
 
-  describe '#non_unique' do
+  describe '#regular' do
     it 'only non-unique indexes' do
-      expect(described_class.non_unique).to all(have_attributes(unique: false))
-    end
+      expect(described_class.regular).to all(have_attributes(unique: false))
     end
 
-  describe '#non_partitioned' do
     it 'only non partitioned indexes ' do
-      expect(described_class.non_partitioned).to all(have_attributes(partitioned: false))
+      expect(described_class.regular).to all(have_attributes(partitioned: false))
+    end
+
+    it 'only indexes that dont serve an exclusion constraint' do
+      expect(described_class.regular).to all(have_attributes(exclusion: false))
     end
   end
 

spec/lib/gitlab/database/reindexing/concurrent_reindex_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Gitlab::Database::Reindexing::ConcurrentReindex, '#perform' do
   let(:table_name) { '_test_reindex_table' }
   let(:column_name) { '_test_column' }
   let(:index_name) { '_test_reindex_index' }
-  let(:index) { instance_double(Gitlab::Database::PostgresIndex, indexrelid: 42, name: index_name, schema: 'public', partitioned?: false, unique?: false, definition: 'CREATE INDEX _test_reindex_index ON public._test_reindex_table USING btree (_test_column)') }
+  let(:index) { instance_double(Gitlab::Database::PostgresIndex, indexrelid: 42, name: index_name, schema: 'public', partitioned?: false, unique?: false, exclusion?: false, definition: 'CREATE INDEX _test_reindex_index ON public._test_reindex_table USING btree (_test_column)') }
   let(:logger) { double('logger', debug: nil, info: nil, error: nil ) }
   let(:connection) { ActiveRecord::Base.connection }
 
@@ -46,6 +46,18 @@ RSpec.describe Gitlab::Database::Reindexing::ConcurrentReindex, '#perform' do
     end
   end
 
+  context 'when the index serves an exclusion constraint' do
+    before do
+      allow(index).to receive(:exclusion?).and_return(true)
+    end
+
+    it 'raises an error' do
+      expect do
+        subject.perform
+      end.to raise_error(described_class::ReindexError, /indexes serving an exclusion constraint are currently not supported/)
+    end
+  end
+
   context 'replacing the original index with a rebuilt copy' do
     let(:replacement_name) { 'tmp_reindex_42' }
     let(:replaced_name) { 'old_reindex_42' }

spec/tasks/gitlab/db_rake_spec.rb

@@ -170,7 +170,7 @@ RSpec.describe 'gitlab:db namespace rake task' do
 
     context 'when no index_name is given' do
       it 'rebuilds a random number of large indexes' do
-        expect(Gitlab::Database::PostgresIndex).to receive_message_chain('non_unique.non_partitioned.random_few').and_return(indexes)
+        expect(Gitlab::Database::PostgresIndex).to receive_message_chain('regular.random_few').and_return(indexes)
         expect(Gitlab::Database::Reindexing).to receive(:perform).with(indexes)
 
         run_rake_task('gitlab:db:reindex')