Structure et exécution des requêtes

Améliorer la performance des requêtes dans PostgreSQL

Amy McCarty

Instructor

Sous-requêtes et jointures

-- SOUS-REQUÊTE
SELECT COUNT(athlete_id)
FROM athletes
WHERE country IN 
  (SELECT country FROM climate 
    WHERE temp_annual > 22)

-- JOINTURE
SELECT COUNT(athlete_id)
FROM athletes a
INNER JOIN climate c
  ON a.country = c.country
  AND c.temp_annual > 22

Plan de requête

Aggregate  ()
  ->  Hash Join  ()
        Hash Cond: (athletes.country = climate.country)
        ->  Seq Scan on athletes  ()
        ->  Hash  ()
              ->  Seq Scan on climate  ()
                    Filter: (temp_annual > '22'::numeric)

Expressions de table communes et tables temporaires

-- CTE
WITH celsius AS 
(
  SELECT country 
  FROM climate 
  WHERE temp_annual > 22 -- Celsius
)
SELECT count(athlete_id)
FROM athletes a
INNER JOIN celsius c
  ON a.country = c.country

-- TABLE TEMPORAIRE
CREATE TEMPORARY TABLE celsius AS 
  SELECT country 
  FROM climate 
  WHERE temp_annual > 22; -- Celsius

SELECT count(athlete_id)
FROM athletes a
INNER JOIN celsius c
  ON a.country = c.country

Plan de requête

  Aggregate  ()
  CTE celsius
    ->  Seq Scan on climate  ()
          Filter: (temp_annual > '22'::numeric)
  ->  Hash Join  ()
        Hash Cond: (a.country_code = c.country_code)
        ->  Seq Scan on athletes a  ()
        ->  Hash  ()
              ->  CTE Scan on celsius c  ()

Limiter les données

  SELECT country_code
  , COUNT(athlete_id) as athletes
  FROM athletes
  WHERE year in (2014, 2010) -- Colonne indexée
  GROUP BY country_code

Limiter les données

  SELECT country_code
  , COUNT(athlete_id) as athletes
  FROM athletes
  WHERE year in (2014, 2010) -- Colonne indexée
  GROUP BY country_code

Sans index	Avec index
Planning Time: 3.370 ms	Planning Time: 0.163 ms
Execution Time: 0.143 ms	Execution Time: 0.062 ms

Agrégations — granularités différentes

SELECT r.country
  , COUNT(a.athlete_id) as athletes
FROM regions r -- niveau pays
INNER JOIN athletes a -- niveau athlètes
  ON r.country = a.country
GROUP BY r.country

Execution Time : 0.267 ms

Agrégations — changer la granularité

WITH olympians AS ( -- niveau pays
  SELECT country
  , COUNT(athlete_id) as athletes
  FROM athletes -- niveau athlètes
  GROUP BY country
)
SELECT country, athletes
FROM regions r -- niveau pays
INNER JOIN olympians o
  ON r.country = o.country

	Execution Time
Joindre d'abord	0.267 ms
Agréger d'abord	0.192 ms

Passons à la pratique !

Améliorer la performance des requêtes dans PostgreSQL