Module: WtActiverecordIndexSpy::QueryAnalyser::Postgres

Extended by:
Postgres
Included in:
Postgres
Defined in:
lib/wt_activerecord_index_spy/query_analyser/postgres.rb

Overview

It analyses the result of an EXPLAIN query to see if any index is missing.

Instance Method Summary collapse

Instance Method Details

#analyse(results) ⇒ Object 



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# File 'lib/wt_activerecord_index_spy/query_analyser/postgres.rb', line 10

def analyse(results)
  WtActiverecordIndexSpy.logger.debug("results:\n#{results.rows.join("\n")}")

  full_results = results.rows.join(", ").downcase

  # rubocop:disable Layout/LineLength
  # Postgres sometimes uses a "seq scan" even for queries that could use an index.
  # So it's almost impossible to be certain if an index is missing!
  # The result of the EXPLAIN query varies depending on the state of the database
  # because Postgres collects statistics from tables and decide if it's better
  # using an index or not based on that.
  # This is an example in a real application:
  #
  # [1] pry(main)> Feature.where(plan_id: 312312).explain
  #   Feature Load (4.0ms)  SELECT "features".* FROM "features" WHERE "features"."plan_id" = $1  [["plan_id", 312312]]
  # => EXPLAIN for: SELECT "features".* FROM "features" WHERE "features"."plan_id" = $1 [["plan_id", 312312]]
  #                        QUERY PLAN
  # ---------------------------------------------------------
  #  Seq Scan on features  (cost=0.00..1.06 rows=1 width=72)
  #    Filter: (plan_id = 312312)
  # (2 rows)
  #
  # [2] pry(main)> Feature.count
  #    (2.8ms)  SELECT COUNT(*) FROM "features"
  # => 5
  # [3] pry(main)> Plan.count
  #    (2.7ms)  SELECT COUNT(*) FROM "plans"
  # => 2
  #
  ####################################################################################################################
  #
  # [1] pry(main)> Feature.where(plan_id: 312312).explain
  #   Feature Load (2.3ms)  SELECT "features".* FROM "features" WHERE "features"."plan_id" = $1  [["plan_id", 312312]]
  # => EXPLAIN for: SELECT "features".* FROM "features" WHERE "features"."plan_id" = $1 [["plan_id", 312312]]
  #                                        QUERY PLAN
  # ----------------------------------------------------------------------------------------
  #  Bitmap Heap Scan on features  (cost=4.18..12.64 rows=4 width=72)
  #    Recheck Cond: (plan_id = 312312)
  #    ->  Bitmap Index Scan on index_features_on_plan_id  (cost=0.00..4.18 rows=4 width=0)
  #          Index Cond: (plan_id = 312312)
  # rubocop:enable Layout/LineLength
  return :uncertain if full_results.include?("seq scan on")
end