analyzing executing and Indexes - B-Tree, Bitmap, Hash

Author: ptyadana

Advanced SQL for Query Tuning and Performance Optimization/01.Analyzing_Execution_Plan.sql

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+--------------------- Analyzing Execution Plan --------------------
+
+/* get explaination 
+EXPLAIN won't actually execute query. Instead it try to make estimation.
+*/
+EXPLAIN SELECT * FROM staff;
+
+/* get details time to build the execution plan and explaination */
+EXPLAIN ANALYZE SELECT * FROM staff;
+
+/* now width is 7, which is lower than previous one*/
+-- NOTE: if we are working with large amount data, we can use rows and width as a guide to understand the amount of data returned.
+EXPLAIN ANALYZE SELECT last_name FROM staff;
+
+
+/* As EXPLAIN just make estimation, rows=xxx sometimes a bit off by 1 or 2 
+when we run the actual query, the rows returned are 717. but using EXPLAIN, it said 715.
+*/
+EXPLAIN SELECT *
+FROM staff
+WHERE salary > 75000;
+
+
+EXPLAIN ANALYZE SELECT *
+FROM staff
+WHERE salary > 75000;
+
+
+--------------------- Indexes --------------------
+
+/* create index on salary column*/
+CREATE INDEX idx_staff_salary ON staff(salary);
+
+
+/* list all indexes*/
+SELECT
+    tablename,
+    indexname,
+    indexdef
+FROM
+    pg_indexes
+WHERE
+    schemaname = 'public'
+ORDER BY
+    tablename,
+    indexname;
+
+
+/* compare against full table scan (without index) and using indexes */
+
+EXPLAIN SELECT * FROM staff;
+
+/*  
+when we check the query plan, we can see full table scan is used instead of using index
+the reason is the criteria is fulfilled on many rows and system decided to use full table scan instead of using index.
+*/
+EXPLAIN ANALYZE SELECT * FROM staff WHERE salary > 75000;
+
+/* here index is used because salary cut off criteria is much more selective */
+EXPLAIN ANALYZE SELECT * FROM staff WHERE salary > 150000;

Advanced SQL for Query Tuning and Performance Optimization/02.Types_of_Indexes.sql

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+/***************** Types of Indexes ************************/
+/*
+B-tree indexes
+Bitmap indexes
+Hash indexes
+PostgreSQL specific indexes
+*/
+
+
+
+/************** B-tree Indexing (Binary Tree Indexing) ******************/
+
+-- check on full table scan
+EXPLAIN SELECT * FROM staff
+WHERE email = '[email protected]';
+
+
+-- create index
+CREATE INDEX idx_email ON staff(email);
+
+EXPLAIN SELECT * FROM staff
+WHERE email = '[email protected]';
+
+
+/******************** Bit Map Indexing *********************/
+
+-- get all unique job titles
+SELECT DISTINCT(job_title)
+FROM staff
+ORDER BY job_title;
+
+
+EXPLAIN SELECT *
+FROM staff
+WHERE job_title = 'Operator';
+
+-- create index
+CREATE INDEX idx_staff_job_title ON staff(job_title);
+
+
+/* 
+now we can see query plan is used Bitmap Heap Scan using Bitmap index 
+In postgreSQL, we don't need to explicty create bitmap index. 
+postgreSQL will create on the fly if it finds situation if bitmap index is needed.
+*/
+EXPLAIN SELECT *
+FROM staff
+WHERE job_title = 'Operator';
+
+
+/************** Hash Index ******************/
+
+-- create index
+CREATE INDEX idx_staff_email ON staff USING HASH(email);
+
+EXPLAIN SELECT * FROM staff
+WHERE email = '[email protected]';
+
+
+/************** PostgreSQL specific indexes ******************/
+/*
+GIST
+SP-GIST
+GIN
+BRIN
+*/