How to tune slow running SQLs in PostgreSQL? See this to know (with screenshots) - 1. See the explain plan and analyze the slow running query 2. Some basic tips for tuning the query

1. Tuning Slow Running
SQLs in PostgreSQL

2. A quick recap!
Earlier we saw:
- How you can find session and details of slow
running query
- How you can cancel a slow running query
- How you can log a slow running query

3. What are we going to do today
- See the explain plan and analyze the slow
running query
- Some basic tips for tuning the query
- Headsup for the next session!

4. Explain Plan
- Syntax: explain [analyze] <query>
- Importance-
- Gives you important information about
query performance and plan
- See steps involved in execution
- Exposes the statistics assumed by your
query for planning the execution

5. How to Read Explain Plan

6. Explain Analyze

7. Tips for Tuning
- These are all incremental step  I prefer implementing in same order
- Tune the query first and optimize it to remove costly clauses
- Create required indexes if they are missing
- Feed the Optimizer with correct info- Tune the optimizer parameters if
required
- e.g seq_page_cost, random_page_cost, effective_cache_size etc
- Provision more resources for hungry (costly) queries- Set some
session/transaction/user level parameters for certain costly queries
- e.g. work_mem
- Consider other changes
- e.g application architecture, application logic, hardware upgrade

8. Tips for Tuning the Query

9. Tips #1- Avoid select *
- Select only what you need
- Everyone wants to use “select *” but no one needs it

10. Example

11. Tip #2- Filter the rows
- Neither you need all the rows in one go nor
your application can handle them all
- Avoid filtering in application
- Fetch only what you need- use WHERE clause
- Limit huge recordsets- use LIMIT clause
- Iterate using OFFSET clause

12. Example

13. Tip #3- Avoid Sorting
- Avoid Sorting if you don’t actually need it
- ORDER BY Clause uses sorting memory
(work_mem)
- If not enough memory then it spills over to disk

14. Example

15. Tip #4- UPDATE from a Source Table
- In lot of cases one has to update data in one table based
on data from another table or with data from another
table
- Example
update update_target t1
set columne_to_update=t2.source_for_update ,
t1.column_2_for_update=t2.column_2_source
from update_source t2 where t1.column_id=t2.column_id;
update update_target t1
set columne_to_update =(select source_for_update from update_source t2
where t1.column_id=t2.column_id),
column_2_for_update=(select column_2_source from update_source t2
where t1.column_id=t2.column_id)
where exists (select 1 from update_source t3 where
t3.column_id=t1.column_id);

16. Example

17. Example – UPDATE FROM
update update_target t1
set columne_to_update=t2.source_for_update ,
t1.column_2_for_update=t2.column_2_source
from update_source t2 where
t1.column_id=t2.column_id;

18. Tip #5- Use High School Mathematics
- Shift the Calculation from RHS to LHS
- Instead of using expressions on Columns use the inverse
expression with literals
- Examples
- select * from emp where salary+200=1200;
select * from emp where salary=1200-200;
- select * from emp where date_trunc(‘d’,hiredatetime)=‘2014-01-03’::date
select * from emp where hiredatetime>=‘2014-01-03 00:00:00’::timezone and
hiredatetime<‘2014-01-03 00:00:00’::timezone +’1 day’::interval

19. When Basic Tuning
Does not Help

20. Tip #1- Look for Usual Suspects
Try to Look for Costly Clauses in Explain Plan

21. Tip #2- Indexing can Help
Create Indexes and test them

22. Some facts to remember while Indexing
- PostgreSQL can do “index-only-scans”
- PostgreSQL can do intersection between result of two indexes
- You can use filter indexes
- Pattern searches may use indexes
- e.g. select empid from emp where empname like ‘SA%’
- Pattern searches may not use indexes
- e.g. select empid from emp where empname like ‘%ER’
- PostgreSQL has expression based indexes
- Indexes can not replace a flawed schema design and poorly
written query

23. Heads-up for the next Hangout!
- We will be discussing some basic configuration
tuning techniques