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 type | count | sum 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:
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
