Add find_duplicate_indexes helper

Relates to #370275

lib/gitlab/database/partitioning_migration_helpers/index_helpers.rb

@@ -77,8 +77,42 @@ def remove_concurrent_partitioned_index_by_name(table_name, index_name)
           end
         end
 
+        # Finds duplicate indexes for a given schema and table. This finds
+        # indexes where the index definition is identical but the names are
+        # different. Returns an array of arrays containing duplicate index name
+        # pairs.
+        #
+        # Example:
+        #
+        #     find_duplicate_indexes('table_name_goes_here')
+        def find_duplicate_indexes(table_name, schema_name: connection.current_schema)
+          find_indexes(table_name, schema_name: schema_name)
+            .group_by { |r| r['index_id'] }
+            .select { |_, v| v.size > 1 }
+            .map { |_, indexes| indexes.map { |index| index['index_name'] } }
+        end
+
         private
 
+        def find_indexes(table_name, schema_name: connection.current_schema)
+          indexes = connection.select_all(<<~SQL, 'SQL', [schema_name, table_name])
+            SELECT n.nspname AS schema_name,
+                   c.relname AS table_name,
+                   i.relname AS index_name,
+                   regexp_replace(pg_get_indexdef(i.oid), 'INDEX .*? USING', '_') AS index_id
+            FROM pg_index x
+              JOIN pg_class c ON c.oid = x.indrelid
+              JOIN pg_class i ON i.oid = x.indexrelid
+              LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
+            WHERE (c.relkind = ANY (ARRAY['r'::"char", 'm'::"char", 'p'::"char"]))
+              AND (i.relkind = ANY (ARRAY['i'::"char", 'I'::"char"]))
+              AND n.nspname = $1
+              AND c.relname = $2;
+          SQL
+
+          indexes.to_a
+        end
+
         def find_partitioned_table(table_name)
           partitioned_table = Gitlab::Database::PostgresPartitionedTable.find_by_name_in_current_schema(table_name)
 

spec/lib/gitlab/database/partitioning_migration_helpers/index_helpers_spec.rb

@@ -26,6 +26,7 @@
       CREATE TABLE #{table_name} (
         id serial NOT NULL,
         created_at timestamptz NOT NULL,
+        updated_at timestamptz NOT NULL,
         PRIMARY KEY (id, created_at)
       ) PARTITION BY RANGE (created_at);
 
@@ -204,4 +205,30 @@ def expect_add_concurrent_index_and_call_original(table, column, index)
       end
     end
   end
+
+  describe '#find_duplicate_indexes' do
+    context 'when duplicate and non-duplicate indexes exist' do
+      let(:nonduplicate_column_name) { 'updated_at' }
+      let(:nonduplicate_index_name) { 'updated_at_idx' }
+      let(:duplicate_column_name) { 'created_at' }
+      let(:duplicate_index_name1) { 'created_at_idx' }
+      let(:duplicate_index_name2) { 'index_on_created_at' }
+
+      before do
+        connection.execute(<<~SQL)
+          CREATE INDEX #{nonduplicate_index_name} ON #{table_name} (#{nonduplicate_column_name});
+          CREATE INDEX #{duplicate_index_name1} ON #{table_name} (#{duplicate_column_name});
+          CREATE INDEX #{duplicate_index_name2} ON #{table_name} (#{duplicate_column_name});
+        SQL
+      end
+
+      subject do
+        migration.find_duplicate_indexes(table_name)
+      end
+
+      it 'finds the duplicate index' do
+        expect(subject).to match_array([match_array([duplicate_index_name1, duplicate_index_name2])])
+      end
+    end
+  end
 end