PostgreSQL 9.6: Parallel Sequential Scan

For a long time, one of the most known shortcomings of PostgreSQL was the ability to parallelise queries. With the release of version 9.6, this will no longer be an issue. A great job has been done on this subject, starting from the commit 80558c1, the introduction of parallel sequential scan, which we will see in the course of this article.

First, you must take note: the development of this feature has been continuous and some parameters have changed names between a commit and another. This article has been written using a checkout taken on June 17 and some features here illustrated will be present only in the version 9.6 beta2.

Compared to the 9.5 release, new parameters have been introduced inside the configuration file. These are:

max_parallel_workers_per_gather: the number of workers that can assist a sequential scan of a table;
min_parallel_relation_size: the minimum size that a relation must have for the planner to consider the use of additional workers;
parallel_setup_cost: the planner parameter that estimates the cost of instantiate a worker;
parallel_tuple_cost: the planner parameter that estimates the cost of transferring a tuple from one worker to another;
force_parallel_mode: parameter useful for testing, strong parallelism and also a query in which the planner would operate in other ways.

Let’s see how the additional workers can be used to speed up our queries. We create a test table with an INT field and one hundred million records:

postgres=# CREATE TABLE test (i int);
CREATE TABLE
postgres=# INSERT INTO test SELECT generate_series(1,100000000);
INSERT 0 100000000
postgres=# ANALYSE test;
ANALYZE

PostgreSQL has max_parallel_workers_per_gather set to 2 by default, for which two workers will be activated during a sequential scan.

A simple sequential scan does not present any novelties:

postgres=# EXPLAIN ANALYSE SELECT * FROM test;
                                                       QUERY PLAN                         
------------------------------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..1442478.32 rows=100000032 width=4) (actual time=0.081..21051.918 rows=100000000 loops=1)
 Planning time: 0.077 ms
 Execution time: 28055.993 ms
(3 rows)

In fact, the presence of a WHERE clause is required for parallelisation:

postgres=# EXPLAIN ANALYZE SELECT * FROM test WHERE i=1;
                                                       QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..964311.60 rows=1 width=4) (actual time=3.381..9799.942 rows=1 loops=1)
   Workers Planned: 2
   Workers Launched: 2
   ->  Parallel Seq Scan on test  (cost=0.00..963311.50 rows=0 width=4) (actual time=6525.595..9791.066 rows=0 loops=3)
         Filter: (i = 1)
         Rows Removed by Filter: 33333333
 Planning time: 0.130 ms
 Execution time: 9804.484 ms
(8 rows)

We can go back to the previous action and observe the differences setting max_parallel_workers_per_gather to 0:

postgres=# SET max_parallel_workers_per_gather TO 0;
SET
postgres=# EXPLAIN ANALYZE SELECT * FROM test WHERE i=1;
                                               QUERY PLAN
--------------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..1692478.40 rows=1 width=4) (actual time=0.123..25003.221 rows=1 loops=1)
   Filter: (i = 1)
   Rows Removed by Filter: 99999999
 Planning time: 0.105 ms
 Execution time: 25003.263 ms
(5 rows)

A time 2.5 times greater.

The planner does not always consider a parallel sequential scan to be the best option. If a query is not selective enough and there are many tuples to transfer from worker to worker, it may prefer a “classic” sequential scan:

postgres=# SET max_parallel_workers_per_gather TO 2;
SET
postgres=# EXPLAIN ANALYZE SELECT * FROM test WHERE i<90000000;
                                                      QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..1692478.40 rows=90116088 width=4) (actual time=0.073..31410.276 rows=89999999 loops=1)
   Filter: (i < 90000000)
   Rows Removed by Filter: 10000001
 Planning time: 0.133 ms
 Execution time: 37939.401 ms
(5 rows)

In fact, if we try to force a parallel sequential scan, we get a worse result:

postgres=# SET parallel_tuple_cost TO 0;
SET
postgres=# EXPLAIN ANALYZE SELECT * FROM test WHERE i<90000000;
                                                             QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..964311.50 rows=90116088 width=4) (actual time=0.454..75546.078 rows=89999999 loops=1)
   Workers Planned: 2
   Workers Launched: 2
   ->  Parallel Seq Scan on test  (cost=0.00..1338795.20 rows=37548370 width=4) (actual time=0.088..20294.670 rows=30000000 loops=3)
         Filter: (i < 90000000)
         Rows Removed by Filter: 3333334
 Planning time: 0.128 ms
 Execution time: 83423.577 ms
(8 rows)

The number of workers can be increased up to max_worker_processes (default: 8). We restore the value of parallel_tuple_cost and we see what happens by increasing max_parallel_workers_per_gather to 8.

postgres=# SET parallel_tuple_cost TO DEFAULT ;
SET
postgres=# SET max_parallel_workers_per_gather TO 8;
SET
postgres=# EXPLAIN ANALYZE SELECT * FROM test WHERE i=1;
                                                       QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..651811.50 rows=1 width=4) (actual time=3.684..8248.307 rows=1 loops=1)
   Workers Planned: 6
   Workers Launched: 6
   ->  Parallel Seq Scan on test  (cost=0.00..650811.40 rows=0 width=4) (actual time=7053.761..8231.174 rows=0 loops=7)
         Filter: (i = 1)
         Rows Removed by Filter: 14285714
 Planning time: 0.124 ms
 Execution time: 8250.461 ms
(8 rows)

Even though PostgreSQL could use up to 8 workers, it has instantiated only six. This is because Postgres also optimises the number of workers according to size of the table and the min_parallel_relation_size. The number of workers made available by postgres is based on a geometric progression with 3 as common ratio 3 and min_parallel_relation_size as scale factor. Here is an example. Considering the 8MB of default parameter:

Size	Worker
<8MB	0
<24MB	1
<72MB	2
<216MB	3
<648MB	4
<1944MB	5
<5822MB	6
…	…

Our table size is 3458MB, so 6 is the maximum number of available workers.

postgres=# \dt+ test
                    List of relations
 Schema | Name | Type  |  Owner   |  Size   | Description
--------+------+-------+----------+---------+-------------
 public | test | table | postgres | 3458 MB |
(1 row)

Finally, I will give a brief demonstration of the improvements achieved by through this patch. Running our query with a growing number of growing workers, we obtain the following results:

Workers	Time
0	24767.848 ms
1	14855.961 ms
2	10415.661 ms
3	8041.187 ms
4	8090.855 ms
5	8082.937 ms
6	8061.939 ms

We can see that the times dramatically improve, until you reach a third of the initial value. It is also simple to explain the fact that we do not see improvements between the use of 3 and 6 workers: the machine on which the test was run has 4 CPUs, so the results are stable after having added 3 more workers to the original process.

Finally, PostgreSQL 9.6 has set the stage for query parallelisation, in which parallel sequential scan is only the first great result. We will also see that in 9.6, aggregations have been parallelised, but that is information for another article which will be released in the upcoming weeks!

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