RE: query from sql server linked server slow

Hi Kellyn,

Using extended events, I can see that the query that the SQL Server (2012) submits to the Oracle database via the OLE DB linked server is exactly the same as the query I run in the Oracle database (12.2) using SQL Developer. Both have the same sql id and hash plan value in the Oracle database. But the one submitted from SQL Server has a different (poor) execution plan in the Oracle database and takes forever to run. And the one run from SQL Developer has a good execution plan and runs in less than 20 seconds in the Oracle database. The stats are up to date in the Oracle database. And locking the good plan in Oracle doesn’t make a difference.

Any ideas on why the exact same SQL statement would generate 2 different execution plans in the same Oracle database?

Bob Stauffer
rstauffer_at_fult.com
(717) 327-2646 x10646

From: Kellyn Pot'Vin-Gorman [mailto:dbakevlar_at_gmail.com] Sent: Wednesday, December 26, 2018 14:06 To: Stauffer, Bob <RStauffer_at_fult.com> Cc: oracle-l_at_freelists.org
Subject: Re: query from sql server linked server slow

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Hi Bob,
Although I don't know what version of MSSQL, I'll assume a newer version...:)

From the SSMS side, set up an extended event, (tracing, to translate) and track down what the difference is. Identify plan migration due to links, views and distributed query. From there, once identified, lock the plan on the SQL Server side to ensure the most optimal plan.

https://docs.microsoft.com/en-us/sql/relational-databases/extended-events/extended-events?view=sql-server-2017

Kellyn

On Wed, Dec 26, 2018 at 10:47 AM Stauffer, Bob <RStauffer_at_fult.com<mailto:RStauffer_at_fult.com>> wrote: Hi all,

Oracle 12.2, Red Hat 7, SQL Server 2012

I know, this isn’t a SQL Server forum, but I’m running out of ideas on how to troubleshoot the following. When running a specific query from SQL Developer or SQLPlus on my Windows 7 PC in and Oracle database, that query is relatively fast. It returns 16 records in less than 20 seconds. But when run in a SQL Server instance using SSMS and using OPENQUERYlinked server, it never completes – it will actually run for days if we don’t kill it. I’ve tracked it down to a difference in execution plans on the Oracle database side. Of course, when run in the Oracle database using SQL Developer, it generates an efficient execution plan. But when run via OPENQUERY/linked server in SQL Server, the execution plan in Oracle is horrendous. I’ve tried loading a better plan for the query using dbms_spm.alter_sql_plan_baseline, etc., but it ignores the new plan. And I’ve tried creating a view for the query, but that makes no difference either.

First, can anyone explain why 2 different execution plans are used in the Oracle db for the same query? Supposedly OPENQUERY/linked server are just submitting the query to Oracle for optimization without doing any “pre-optimization”. And any thoughts on how I can troubleshoot/figure out how to fix or get around the problem?

Thanks much.

Bob Stauffer
rstauffer_at_fult.com<mailto:rstauffer_at_fult.com>

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