How Can We Optimize Query Performance by Reducing Join Count?

Reducing Join Count for Optimized Query Performance

In database optimization, minimizing join operations is crucial to improve query speed. This principle dictates that each join multiplies the query cost rather than merely adding to it. To illustrate this concept, let's delve into a specific problem and explore an alternative solution that employs fewer joins.

Query to Count Join Results

Consider a scenario where you need to count the number of rows that result from three separate joins:

• album to pays_album
• album to pers_album
• album to juron_album

Typically, the straightforward approach would involve three subqueries, each performing a join and a count:

-- Subquery 1
SELECT COUNT(*)
FROM album alb
LEFT JOIN pays_album payalb USING ( idAlb )
WHERE alb.titreAlb = "LES CIGARES DU PHARAON"

-- Subquery 2
SELECT COUNT(*)
FROM album alb
LEFT JOIN pers_album peralb USING ( idAlb )
WHERE alb.titreAlb = "LES CIGARES DU PHARAON"

-- Subquery 3
SELECT COUNT(*)
FROM album alb
LEFT JOIN juron_album juralb USING ( idAlb )
WHERE alb.titreAlb = "LES CIGARES DU PHARAON"

However, it is possible to achieve the same result with a single query:

SELECT alb.titreAlb AS "Titre",
       COUNT(DISTINCT payalb.idAlb, payalb.PrimaryKeyFields) AS "Pays",
       COUNT(DISTINCT peralb.idAlb, peralb.PrimaryKeyFields) AS "Personnages",
       COUNT(DISTINCT juralb.idAlb, juralb.PrimaryKeyFields) AS "Jurons"
FROM album alb
LEFT JOIN pays_album payalb USING ( idAlb )
LEFT JOIN pers_album peralb USING ( idAlb )
LEFT JOIN juron_album juralb USING ( idAlb )
WHERE alb.titreAlb = "LES CIGARES DU PHARAON"
GROUP BY alb.titreAlb

By utilizing the DISTINCT keyword, the effect of additional joins on the count is negated. This approach effectively mimics the original notion of minimizing joins, although its performance advantage is contingent on optimal index availability. Nonetheless, it provides a viable alternative worth consideration.

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