1 files changed, 191 insertions, 0 deletions
diff --git a/lib/unnested_in_filters/rewriter.rb b/lib/unnested_in_filters/rewriter.rb
new file mode 100644
index 00000000000..cba002a5632
--- /dev/null
+++ b/lib/unnested_in_filters/rewriter.rb
@@ -0,0 +1,191 @@
+# frozen_string_literal: true
+
+# rubocop:disable CodeReuse/ActiveRecord (This module is generating ActiveRecord relations therefore using AR methods is necessary)
+module UnnestedInFilters
+  class Rewriter
+    include Gitlab::Utils::StrongMemoize
+
+    class ValueTable
+      def initialize(model, attribute, values)
+        @model = model
+        @attribute = attribute.to_s
+        @values = values
+      end
+
+      def to_sql
+        "unnest(#{serialized_values}::#{sql_type}[]) AS #{table_name}(#{column_name})"
+      end
+
+      def as_predicate
+        "#{model.table_name}.#{column_name} = #{table_name}.#{column_name}"
+      end
+
+      private
+
+      attr_reader :model, :attribute, :values
+
+      delegate :connection, :columns, :attribute_types, to: :model, private: true
+      delegate :quote, :quote_table_name, :quote_column_name, to: :connection
+
+      def table_name
+        quote_table_name(attribute.pluralize)
+      end
+
+      def column_name
+        quote_column_name(attribute)
+      end
+
+      def serialized_values
+        array_type.serialize(values)
+                  .then { |array| quote(array) }
+      end
+
+      def array_type
+        ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Array.new(attribute_types[attribute])
+      end
+
+      def sql_type
+        column.sql_type_metadata.sql_type
+      end
+
+      def column
+        columns.find { _1.name == attribute }
+      end
+    end
+
+    def initialize(relation)
+      @relation = relation
+    end
+
+    # Rewrites the given ActiveRecord::Relation object to
+    # utilize the DB indices efficiently.
+    #
+    # Example usage;
+    #
+    #   relation = Vulnerabilities::Read.where(state: [1, 4])
+    #   relation = relation.order(severity: :desc, vulnerability_id: :desc)
+    #
+    #   rewriter = UnnestedInFilters::Rewriter.new(relation)
+    #   optimized_relation = rewriter.rewrite
+    #
+    # In the above example. the `relation` object would produce the following SQL query;
+    #
+    #   SELECT
+    #     "vulnerability_reads".*
+    #   FROM
+    #     "vulnerability_reads"
+    #   WHERE
+    #     "vulnerability_reads"."state" IN (1, 4)
+    #   ORDER BY
+    #     "vulnerability_reads"."severity" DESC,
+    #     "vulnerability_reads"."vulnerability_id" DESC
+    #   LIMIT 20;
+    #
+    # And the `optimized_relation` object would would produce the following query to
+    # utilize the index on (state, severity, vulnerability_id);
+    #
+    #   SELECT
+    #     "vulnerability_reads".*
+    #   FROM
+    #     unnest('{1, 4}'::smallint[]) AS "states" ("state"),
+    #     LATERAL (
+    #       SELECT
+    #         "vulnerability_reads".*
+    #       FROM
+    #         "vulnerability_reads"
+    #       WHERE
+    #         (vulnerability_reads."state" = "states"."state")
+    #       ORDER BY
+    #         "vulnerability_reads"."severity" DESC,
+    #         "vulnerability_reads"."vulnerability_id" DESC
+    #       LIMIT 20) AS vulnerability_reads
+    #   ORDER BY
+    #     "vulnerability_reads"."severity" DESC,
+    #     "vulnerability_reads"."vulnerability_id" DESC
+    #   LIMIT 20
+    #
+    def rewrite
+      log_rewrite
+
+      model.from(from)
+           .limit(limit_value)
+           .order(order_values)
+           .includes(relation.includes_values)
+           .preload(relation.preload_values)
+           .eager_load(relation.eager_load_values)
+    end
+
+    def rewrite?
+      strong_memoize(:rewrite) do
+        in_filters.present? && has_index_coverage?
+      end
+    end
+
+    private
+
+    attr_reader :relation
+
+    delegate :model, :order_values, :limit_value, :where_values_hash, to: :relation, private: true
+
+    def log_rewrite
+      ::Gitlab::AppLogger.info(message: 'Query is being rewritten by `UnnestedInFilters`', model: model.name)
+    end
+
+    def from
+      [value_tables.map(&:to_sql) + [lateral]].join(', ')
+    end
+
+    def lateral
+      "LATERAL (#{join_relation.to_sql}) AS #{model.table_name}"
+    end
+
+    def join_relation
+      value_tables.reduce(unscoped_relation) do |memo, tmp_table|
+        memo.where(tmp_table.as_predicate)
+      end
+    end
+
+    def unscoped_relation
+      relation.unscope(where: in_filters.keys)
+    end
+
+    def in_filters
+      @in_filters ||= where_values_hash.select { _2.is_a?(Array) }
+    end
+
+    def has_index_coverage?
+      indices.any? do |index|
+        (filter_attributes - Array(index.columns)).empty? && # all the filter attributes are indexed
+          index.columns.last(order_attributes.length) == order_attributes && # index can be used in sorting
+          (index.columns - (filter_attributes + order_attributes)).empty? # there is no other columns in the index
+      end
+    end
+
+    def filter_attributes
+      @filter_attributes ||= where_values_hash.keys
+    end
+
+    def order_attributes
+      @order_attributes ||= order_values.flat_map(&method(:extract_column_name))
+    end
+
+    def extract_column_name(order_value)
+      case order_value
+      when Arel::Nodes::Ordering
+        order_value.expr.name
+      when ::Gitlab::Pagination::Keyset::Order
+        order_value.attribute_names
+      end
+    end
+
+    def indices
+      model.connection.schema_cache.indexes(model.table_name)
+    end
+
+    def value_tables
+      @value_tables ||= in_filters.map do |attribute, values|
+        ValueTable.new(model, attribute, values)
+      end
+    end
+  end
+end