Tuning Slow Running SQLs in

PostgreSQL

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

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!

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

How to Read Explain Plan

Explain Analyze

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

Tips for Tuning the Query

Tips #1- Avoid select *

- Select only what you need- Everyone wants to use “select *” but no one needs it

Example

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

Example

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

Example

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_sourcefrom 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);

Example

Example – UPDATE FROMupdate update_target t1

set columne_to_update=t2.source_for_update ,t1.column_2_for_update=t2.column_2_sourcefrom update_source t2 where

t1.column_id=t2.column_id;

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

When Basic Tuning

Does not Help

Tip #1- Look for Usual SuspectsTry to Look for Costly Clauses in Explain Plan

Tip #2- Indexing can Help

Create Indexes and test them

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

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