PostgreSql query planning and tuning

PostgreSQL query planning and tuning
Federico Campoli
03 Mar 2016
Federico Campoli PostgreSQL query planning and tuning 03 Mar 2016 1 / 47

Table of contents
1 Jargon
2 I ﬁnd your lack of plan disturbing
3 I love it when a plan comes together
4 EXPLAIN! EXPLAIN!
5 I ﬁght for the users!
6 Wrap up

Jargon
Table of contents
1 Jargon
4 EXPLAIN! EXPLAIN!
6 Wrap up

Jargon
Jargon
source https://commons.wikimedia.org/wiki/File:Klingon Empire Flag.svg

Jargon
Jargon
Jargon
OID Object ID, 4 bytes integer. Used to identify the system objects (tables,
indices etc)
class any relational object, table, index, view, sequence...
attribute the table ﬁelds
execution plan the steps required for executing a query
plan nodes execution plan’s steps
CBO cost based optimizer
cost arbitrary value used to determine a score for the plan nodes

I ﬁnd your lack of plan disturbing
Table of contents
1 Jargon
4 EXPLAIN! EXPLAIN!
6 Wrap up

source http://apbialek.deviantart.com/art/Darth-Vader-171921375

Query stages
The query execution requires four stages
Syntax validation
Query tree generation
Plan estimation
Execution

I ﬁnd your lack of plan disturbing Syntax validation
Syntax validation
The query parser validates the query syntax using ﬁxed rules.
Any error at this step will cause the execution to stop, returning a syntax error.
This early stage doesn’t requires a system catalogue access.
The parser returns a normalised parse tree for the next step.

I ﬁnd your lack of plan disturbing Query tree generation
The query tree
The query parser in the second stage look up the system catalogue and translates
the parse tree into the query tree.
The query tree is the query’s logical representation where any object involved is
unique and described using the object id mapping.

The query tree
The query tree

The planner
The planner stage
The next stage is the query planner. The parser sends the generated query tree to
the planner. The query planner reads the tree and generates all the possible
execution plans. The planner, using the internal statistics,determines the
estimated costs for each plan.
The execution plan with the minimum estimated cost is sent to the executor.
Old or missing statistics will result not-optimal execution plans and therefore slow
queries.

The executor
The executor
The executor performs the plan nodes in the execution plan generated by the
planner.

The workﬂow
Figure : The query stages

I love it when a plan comes together
Table of contents
1 Jargon
4 EXPLAIN! EXPLAIN!
6 Wrap up

source http://clipperdata.com/blogpost/i-love-it-when-a-plan-comes-together/i-
love-it-when-a-plan-comes-together/

The plan nodes
Scan nodes used by the executor to retrieve data from the relations
Join nodes used by the executor to perform joins of the data streams

seq scan
seq scan: reads sequentially the table and discards the unmatched rows. The
output is a data stream. Returns unsorted rows.

index scan
index scan: read the index tree with random disk read and gets the heap blocks
pointed by the index. Returns sorted rows.

index only scan
index only scan: read the index tree with random disk read and returns the data
without accessing the heap page. Returns sorted rows.

bitmap index/heap scan
bitmap index/heap scan: read the index sequentially generating a bitmap used to
recheck on heap pages. It’s a good compromise between seq scan and a full index
scan. Returns unsorted rows.

sort
sort : reads the rows and returns them in an ordered way like in queries with an
ORDER BY

nested loop
nested loop join: The right relation is scanned once for every row found in the left
relation. This strategy is easy to implement but can be very time consuming.
However, if the right relation can be scanned with an index scan, this can be a
good strategy. It is possible to use values from the current row of the left relation
as keys for the index scan of the right.

hash join
hash join: the right relation is ﬁrst scanned and loaded into a hash table, using its
join attributes as hash keys. Next the left relation is scanned and the appropriate
values of every row found are used as hash keys to locate the matching rows in
the table.

merge join
merge join: Each relation is sorted on the join attributes before the join starts.
Then the two relations are scanned in parallel, and matching rows are combined to
form join rows. This kind of join is more attractive because each relation has to
be scanned only once. The required sorting might be achieved either by an explicit
sort step, or by scanning the relation in the proper order using an index on the
join key.

EXPLAIN! EXPLAIN!
Table of contents
1 Jargon
4 EXPLAIN! EXPLAIN!
6 Wrap up

EXPLAIN! EXPLAIN!
EXPLAIN! EXPLAIN!
source http://keepcalmandwatchdoctorwho.tumblr.com/post/5450959631/the-
daleks-personally-i-like-it-when-they-shout

EXPLAIN! EXPLAIN!
EXPLAIN
Prepending EXPLAIN to any query will display the query’s estimated
execution plan .

EXPLAIN! EXPLAIN!
EXPLAIN
execution plan .
The ANALYZE clause executes the query, discards the results and returns
the real execution plan.

EXPLAIN! EXPLAIN!
EXPLAIN
execution plan .
The ANALYZE clause executes the query, discards the results and returns
the real execution plan.
Using EXPLAIN ANALYZE with the DML queries will change the data. It is
safe to wrap the EXPLAIN ANALYZE between BEGIN; and ROLLBACK;

EXPLAIN! EXPLAIN!
Explain in action
For our example we’ll create a test table with two ﬁelds, a serial and character
varying.
test =# CREATE TABLE t_test
(
i_id serial ,
v_value character varying (50)
)
;
CREATE TABLE

EXPLAIN! EXPLAIN!
Explain in action
Now let’s add some rows to our table
Listing 1: Insert in table
test =# INSERT INTO t_test
(v_value)
SELECT
v_value
FROM
(
SELECT
generate_series (1 ,1000) as i_cnt ,
md5(random ():: text) as v_value
) t_gen
;
INSERT 0 1000

EXPLAIN! EXPLAIN!
Explain in action
Let’s generate the estimated plan for one row result
test =# EXPLAIN SELECT * FROM t_test WHERE i_id =20;
QUERY PLAN
-- -------------------------------------------------------
Seq Scan on t_test (cost =0.00..16.62 rows =3 width =122)
Filter: (i_id = 20)
(2 rows)
The cost is just an arbitrary value
The values after the cost are the the startup and total cost
The start up cost is the cost to deliver the ﬁrst row to the next step
The total cost is cost to deliver all the rows to the next step
The value in rows is the planner’s estimation for the total rows returned by
the plan node
The value in width is the estimated average row width in bytes

EXPLAIN! EXPLAIN!
EXPLAIN ANALYZE
Now let’s generate the real execution plan for one row result
test =# EXPLAIN ANALYZE SELECT * FROM t_test WHERE i_id =20;
QUERY PLAN
--
---------------------------------------------------------------------------
Seq Scan on t_test (cost =0.00..21.50 rows =1 width =37) (actual time
=0.022..0.262 rows =1 loops =1)
Filter: (i_id = 20)
Rows Removed by Filter: 999
Planning time: 0.066 ms
Execution time: 0.286 ms
(5 rows)
The values in actual time are the time, in milliseconds, required for the
startup and total cost
The value in rows is the number of rows returned by the step
The value loops value is the number of times the step is executed
On the bottom we have the planning time and the total execution time

EXPLAIN! EXPLAIN!
Indices
Let’s add an index on the i id ﬁeld
test =# CREATE INDEX idx_i_id ON t_test (i_id);
CREATE INDEX

EXPLAIN! EXPLAIN!
Indices
test =# EXPLAIN ANALYZE SELECT * FROM t_test WHERE i_id =20;
QUERY PLAN
--
---------------------------------------------------------------------------
Index Scan using idx_i_id on t_test (cost =0.28..8.29 rows =1 width =37) (
actual time =0.035..0.036 rows =1 loops =1)
Index Cond: (i_id = 20)
(4 rows)
The query is several times faster.

EXPLAIN! EXPLAIN!
Controlling the planner
The cost based optimiser becomes cleverer for each major release. For example, if
the query’s ﬁlter returns almost the entire table, the database will choose the
sequential scan which is by default 4 times cheaper than a full index scan.
test =# EXPLAIN ANALYZE SELECT * FROM t_test WHERE i_id >2;
QUERY PLAN
--
---------------------------------------------------------------------------
Seq Scan on t_test (cost =0.00..21.50 rows =999 width =37) (actual time
=0.012..0.467 rows =998 loops =1)
Filter: (i_id > 2)
Rows Removed by Filter: 2
(5 rows)

EXPLAIN! EXPLAIN!
test =# SET enable_seqscan =’off’;
SET
test =# EXPLAIN ANALYZE SELECT * FROM t_test WHERE i_id >2;
QUERY PLAN
--
---------------------------------------------------------------------------
Index Scan using idx_i_id on t_test (cost =0.28..49.76 rows =999 width =37) (
actual time =0.029..0.544 rows =998 loops =1)
Index Cond: (i_id > 2)
(4 rows)

I ﬁght for the users!
Table of contents
1 Jargon
4 EXPLAIN! EXPLAIN!
6 Wrap up

source http://tron.wikia.com/wiki/Tron

ANALYZE
ANALYZE
ANALYZE gather statistics runs random reads on the tables
The statistics are stored in the pg statistic system table
The view pg stats translates the statistics in human readable format
The parameter default statistics target sets the limit for the random read
The statistic target can be ﬁne tuned per column

enable bitmapscan
enable hashagg
enable hashjoin
enable indexscan
enable indexonlyscan
enable material
enable mergejoin
enable nestloop
enable seqscan
enable sort
enable tidscan

Wrap up
Table of contents
1 Jargon
4 EXPLAIN! EXPLAIN!
6 Wrap up

Wrap up
Questions
Questions?

Wrap up
Boring legal stuﬀ
The join nodes descriptions are excerpts from the PostgreSQL 9.4 on line manual.
Copyright by PostgreSQL Global Development Group.
http://www.postgresql.org/
The scan node images are derived from the pgadmin3 scan nodes. Copyright by
pgadmin development group. http://www.pgadmin.org/
All the images copyright is owned by the respective authors. The sources the
author’s attribution is provided with a link alongside with image.

Wrap up
Contacts and license
Twitter: 4thdoctor scarf
Blog:http://www.pgdba.co.uk
Brighton PostgreSQL Meetup:
http://www.meetup.com/Brighton-PostgreSQL-Meetup/
This document is distributed under the terms of the Creative Commons

Wrap up
PostgreSQL query planning and tuning
Federico Campoli
03 Mar 2016

PostgreSql query planning and tuning

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