Re: Query question

I should have mentioned which version I am using. We are at version 9.2 and the Lateral was not introduced until version 9.3. 

The partitioning worked Thanks. 

stanton schmidt 
Database Administrator 
direct. [ callto:920.884.1281 | 920. ] 471.4495 cell 920.660.1828 

RGL 
GO AHEAD. ASK WHAT IF. 
[ http://www.rgllogistics.com/ | www.RGLlogistics.co m ] 


From: "David G. Johnston" <[email protected]> 
To: "Jeff Fletcher" <[email protected]> 
Cc: "Stanton Schmidt" <[email protected]>, "Martin Stöcker" <[email protected]>, "pgsql-sql" <[email protected]> 
Sent: Thursday, March 8, 2018 1:28:31 PM 
Subject: Re: Query question 

We prefer to avoid top-posting on these lists but I'll go with the flow here... 

<not tested, syntax is close but might have errors> 
select * 
from (select distinct equipment_id from log_table) a, 
lateral get_log_data_for_equipment(equipment_id, 5) 

then write the get_log_data_for_equipment function as dynamic sql substituting the desired limit value. 

Not saying the above is going to be well performing but in terms of simplicity of the top-level query it scores high. 

If you have an "equipment" table, which you should, replacing the select distinct with select should be considered. The presence of "distinct" (without 'on') is a code smell for me. 

David J. 


On Thu, Mar 8, 2018 at 12:20 PM, Jeff Fletcher < [ mailto:[email protected] | [email protected] ] > wrote: 



Partition by is your friend... 

[ https://urldefense.proofpoint.com/v2/url?u=https-3A__stackoverflow.com_questions_1124603_grouped-2Dlimit-2Din-2Dpostgresql-2Dshow-2Dthe-2Dfirst-2Dn-2Drows-2Dfor-2Deach-2Dgroup&d=DwMFaQ&c=eLkx2stMcShI0L6xvxICXHnFB9zmDvZmvnhsCd8Gf8M&r=EWbtHQpXIg2XbQduIylzyXrAFdbjWaMyy-p_LkyfjhQ&m=DEdLcsFrDyuVKOvYidkUgnvb5VlWOA5TPoLM_4cEMbI&s=PGsyF58Wm2BowXszKbPpE1wx40C7KcKhewcEIXUILas&e= | https://stackoverflow.com/questions/1124603/grouped-limit-in-postgresql-show-the-first-n-rows-for-each-group [stackoverflow.com] ] 


On Thu, Mar 8, 2018 at 1:17 PM, Stanton Schmidt < [ mailto:[email protected] | [email protected] ] > wrote: 

BQ_BEGIN

So far I have been unable to figure out how to do that. 

I tried: 
select a.equipment_id, b.log_date, b.log_time, b.event_desc 
from (select distinct equipment_id from log_table) a 
, (select equipment_id, log_date, log_time, event_desc from log_table order by log_date desc, log_time desc limit 5) b 
where a.equipment_id = b.equipment_id 

but all I end up with is 5 total records. 

Stanton 

From: "Martin Stöcker" < [ mailto:[email protected] | [email protected] ] > 
To: "pgsql-sql" < [ mailto:[email protected] | [email protected] ] > 
Sent: Thursday, March 8, 2018 1:07:30 PM 
Subject: Re: Query question 

My first idea is to select all equipments and lateral join them to the 5 most recent events 

Regards Martin 

Am 08.03.2018 um 19:58 schrieb Stanton Schmidt: 

BQ_BEGIN

Hi, 
I am new to the list so feel free to let me know if I am out of line. 

My question is: 
I have a table that has log events for pieces of equipment. For each piece of equipment this table may contain 1 or more (hundreds potentially). 
I need to write a query that will return only the last 5 log events for each (and every) piece of equipment. 

log_table ( 
equipment_id character(30), 
log_date date, 
log_time time, 
event_desc text 
) 

Thanks for your help. 

stanton schmidt 
Database Administrator 
direct. [ callto:920.884.1281 | 920. ] 471.4495 cell [ tel:(920)%20660-1828 | 920.660.1828 ] 

RGL 
GO AHEAD. ASK WHAT IF. 
[ http://www.rgllogistics.com/ | www.RGLlogistics.co
                          m ] 





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