tuning joins

Author: ptyadana

Advanced SQL for Query Tuning and Performance Optimization/03.Tuning_Joins.sql

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+/***************** Tuning Joins *******************/
+
+--------------- Nested Loop --------------------------------------------------
+
+-- force query plan builder to use nested loop first, just for testing purpose
+set enable_nestloop = true;
+set enable_hashjoin = false;
+set enable_mergejoin = false;
+
+EXPLAIN SELECT s.id,s.last_name,s.job_title,cr.country
+FROM staff s
+JOIN company_regions cr
+ON cr.region_id = s.region_id;
+
+
+/*
+We can see that builder use Nested Lop then Index Scan using primary key (region_id).
+As postgreSQL automatically created index on primary key, we will delete this key (for testing purpose)
+and see how Nest Loop will full table scan will look like in performance wise.
+*/
+
+
+EXPLAIN SELECT s.id,s.last_name,s.job_title,cr.country
+FROM staff s
+JOIN company_regions cr
+ON cr.region_id = s.region_id;
+
+/* we can see that Cost got increased.
+
+So main take away is when we are using any kind of joins (especially, Nested Loop), it helps to ensure foreign keys
+columns and the columns you are trying to matching on are indexed properly.
+*/
+
+
+--------------- Hash Join --------------------------------------------------
+set enable_nestloop = false;
+set enable_hashjoin = true;
+set enable_mergejoin = false;
+
+
+EXPLAIN SELECT s.id,s.last_name,s.job_title,cr.country
+FROM staff s
+JOIN company_regions cr
+ON cr.region_id = s.region_id;
+
+/*
+We can see that in row 3, scanning on staff table.
+in row 4, hash table is being buit.
+*/
+
+--------------- Merge Join --------------------------------------------------
+
+set enable_nestloop = false;
+set enable_hashjoin = false;
+set enable_mergejoin = true;
+
+
+EXPLAIN SELECT s.id,s.last_name,s.job_title,cr.country
+FROM staff s
+JOIN company_regions cr
+ON cr.region_id = s.region_id;
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