Performance issue with order by clause on

Hi,

I would like to overcome an issue which occurs only in case with *order by *
clause.

Details:
I am trying to insert into a temporary table 50 rows from a joined table
ordered by a modification time column which is inserted by the current time
so it is ordered ascending.

Each table has index on the following columns: PRIMARY KEY(SystemID,
ObjectID, ElementID, ModificationTime)

Statement:

*sqlString := 'INSERT INTO ResultTable (*

*SELECT * FROM "TABLE" a LEFT OUTER JOIN "TABLE_Text" l1031 ON
a.ModificationTime = l1031.ModificationTime AND a.SystemID = l1031.SystemID
AND a.ObjectID = l1031.ObjectID AND a.ElementID = l1031.ElementID AND
l1031.LCID = 1031 LEFT OUTER JOIN ( SELECT * AS CommentNumber FROM
"TABLE_Comment" v1 GROUP BY v1.ModificationTime, v1.SystemID, v1.ObjectID,
v1.ElementID ) c ON a.ModificationTime = c.ModificationTime AND a.SystemID
= c.SystemID AND a.ObjectID = c.ObjectID AND a.ElementID = c.ElementID
WHERE a.ModificationTime BETWEEN $1 AND $2 AND ( a.Enabled = 1 ) ORDER BY
a.ModificationTime DESC LIMIT 50));*

*EXECUTE sqlString USING StartTime,EndTime; *

node typecountsum of times% of query
Hash 1 8.844 ms 10.0 %
Hash Left Join 1 33.715 ms 38.0 %
Insert 1 0.734 ms 0.8 %
Limit 1 0.003 ms 0.0 %
Seq Scan 2 22.735 ms 25.6 %
Sort 1 22.571 ms 25.5 %
Subquery Scan 1 0.046 ms 0.1 %

Execution Plan: https://explain.depesz.com/s/S96g (Obfuscated)

If I remove the order by clause I get the following results:

node type

count

sum of times

% of query

*Index Scan*

2

27.632 ms

94.9 %

Insert

1

0.848 ms

2.9 %

Limit

1

0.023 ms

0.1 %

Merge Left Join

1

0.423 ms

1.5 %

Result

1

0.000 ms

0.0 %

Subquery Scan

1

0.186 ms

0.6 %

Which is pointing me to a problem with the sorting. Is there any way that I
could improve the performance with order by clause?

To make the problem more transparent I ran a long run test where you can
see that with order by clause the performance is linearly getting worse:

[image: image.png]

Postgresql version: "PostgreSQL 11.1, compiled by Visual C++ build 1914,
64-bit"

Istalled by: With EnterpriseDB One-click installer from EDB's offical site.

Postgresql.conf changes: Used pgtune suggestions:
# DB Version: 11
# OS Type: windows
# DB Type: desktop
# Total Memory (RAM): 8 GB
# CPUs num: 4
# Connections num: 25
# Data Storage: hdd
max_connections = 25
shared_buffers = 512MB
effective_cache_size = 2GB
maintenance_work_mem = 512MB
checkpoint_completion_target = 0.5
wal_buffers = 16MB
default_statistics_target = 100
random_page_cost = 4
work_mem = 8738kB
min_wal_size = 100MB
max_wal_size = 1GB
max_worker_processes = 4
max_parallel_workers_per_gather = 2
max_parallel_workers = 4

Operating System: Windows 10 x64, Version: 1607

Thanks in advance,
Best Regards,
Tom Nay