RE: [External] Slow query accessing remote database tables

From: Mark W. Farnham <>
Date: Wed, 23 Dec 2020 10:32:23 -0500
Message-ID: <26b001d6d940$d0230a60$70691f20$>

Since your query involves a possibly remote UNION, the first thing I would try is to deliver the pieces of the UNION into two temporary tables with the appropriate column definitions and then do the UNION locally between the two temporary tables.  

After that, possibly you will need driving site hints, but on the basis of least human work to get a tolerable result, I would try the local pieces of the UNION first in the hope that the CBO does the correct thing with the separate pieces of the execution plan all by itself. Other than first executing the query both ways and then comparing the results, I’m not sure how Oracle would be able to develop statistics about where to do the mutual sorting of the query pieces to minimize total network transmission of resolving the UNION.  

Now, if you know LOGICALLY that your data is such that duplicated rows cannot be produced by the separate pieces of the UNION, you might also substitute a UNION ALL for the UNION. But that verification of non-duplication is then on you, not the database engine. Quite often UNIONs can be converted to UNION ALL via knowledge of logical constraints that are not actually present in the database.  

Good luck. It is entirely possible this will not help at all, but the effort required to test it is tiny compared to possible improvements and even compared to reading your plan. Without looking at the plan or even the query except that it contains a UNION, I wild ass guess your odds at better than 50-50.  

It is possible you will need to evaluate the types produced by the query halves to instantiate the temp table. An easy way to do that is CTAS of the query piece with rownum < 1.  

Good luck,  


From: [] On Behalf Of Laurentiu Oprea Sent: Tuesday, December 22, 2020 3:38 PM To: Beckstrom, Jeffrey
Cc:; ORACLE-L ( Subject: Re: [External] Slow query accessing remote database tables  

In theory the hint should be enough to instruct oracle to use spp_to_ebs database to process all the joins localy and call remotely for XXSPP01W_SPFST_STG.It might be the case that we still need to see how table XXSPP01W_SPFST_STG is integrated in the execution plan. Based on that fact that all tables except one are located in the remote DB it might be possible to run faster but in reality I have no idea is a matter of trying and let us know  

În mar., 22 dec. 2020 la 22:08, Beckstrom, Jeffrey <> a scris:

You could try taking your select and converting it into a view on the remote database. Then select from that view. That has worked for us in the past when doing joins on remote tables.  

From: <> On Behalf Of Amit Saroha Sent: Tuesday, December 22, 2020 3:02 PM To: Laurentiu Oprea <> Cc: ORACLE-L ( <> Subject: Re: [External] Slow query accessing remote database tables  

Thank you, Laurentiu. I'll try with the hint you have suggested. However, because there are multiple remote tables used in the query, will it take care of the whole join itself with other remote tables by adding a hint in only one table in this case PO_HEADERS_ALL?  

Best Regards,

AMIT SAROHA     On Tue, Dec 22, 2020 at 2:43 PM Laurentiu Oprea <> wrote:

You can try using DRIVING_SITE hint:  



        SELECT  /*+ DRIVING_SITE(poh)*/ 
            trunc(to_date(sysdate))                                                     issue_date,
            mp.organization_code                                                        ship_to_location,


) > xss.max_update_date )
UNION SELECT /*+ DRIVING_SITE(poh)*/ trunc(to_date(sysdate)) issue_date, mp.organization_code ship_to_location, ...........................................

În mar., 22 dec. 2020 la 20:29, Amit Saroha <> a scris:

Hi All,  

I apologize but I couldn't find a way to send the execution plan details in an email because every time I send my post didn't show up in the forum. I have posted the detail at PASTEBIN at the below location. It seems there is some character limit for posting in the forum.  

I am looking for your expert advice to improve the performance of the below query which is running nearly 3 hours every day. This query is accessing data from multiple tables from the remote database over the DB link and with my minimum understanding of the execution plan, I surmise PO_HEADERS_ALL table data is coming into the local database and is the main issue. I have also studied that the REMOTE hint can be used in such scenarios but I couldn't figure out how and where to place the hint in the below query.

Do let me know if it can be re-written any other way to run time and the below information is enough for you to comment on the issue?  

Database details -  

Oracle Database 12c Enterprise Edition Release - 64bit Production PL/SQL Release - Production
"CORE Production"
TNS for Linux: Version - Production NLSRTL Version - Production  

Thank you in advance for all your inputs in this regard.      



Received on Wed Dec 23 2020 - 16:32:23 CET

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