Re: query from sql server linked server slow

From: "Kellyn Pot'Vin-Gorman" <dbakevlar@xxxxxxxxx>
To: "Stauffer, Bob" <RStauffer@xxxxxxxx>
Date: Wed, 26 Dec 2018 14:07:52 -0800

First off, apologies- reading this on a larger screen, I now see the
versions. So even if you creat a sql brofile or use a baseline, the query
uses a different plan when executed from SSMS?

And you’ve confirmed it’s the same sql that’s issued once it is presented
to the oracle database, (sql_id, hash plan) as when you execute it from sql
developer?

Can you paste the plans for both here? Maybe there’s something amiss one
of us will notice.

Thanks,
Kellyn

On Wed, Dec 26, 2018 at 1:35 PM Stauffer, Bob <RStauffer@xxxxxxxx> wrote:

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@xxxxxxxx

(717) 327-2646 x10646

*From:* Kellyn Pot'Vin-Gorman [mailto:dbakevlar@xxxxxxxxx]
*Sent:* Wednesday, December 26, 2018 14:06
*To:* Stauffer, Bob <RStauffer@xxxxxxxx>
*Cc:* oracle-l@xxxxxxxxxxxxx
*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@xxxxxxxx> 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@xxxxxxxx

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References:
- query from sql server linked server slow
  - From: Stauffer, Bob
- Re: query from sql server linked server slow
  - From: Kellyn Pot'Vin-Gorman
- RE: query from sql server linked server slow
  - From: Stauffer, Bob

Re: query from sql server linked server slow

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