Re: Bad execution plan after migrating to exadata ( 12c) from non-exadata (11g)

From: Jack van Zanen <>
Date: Mon, 26 Aug 2019 13:20:05 +1000
Message-ID: <>

Sorry for Hijacking this thread

I have a DW where I see queries using index and Nested loops even after I gathered system stats using EXADATA and bumping up the parameter MBR for session to 256/512.
However a fts hint on the biggest table results in a much faster execution plan and almost 100% saving on I/O through the exadata smart scan. This code is created by reporting tool so not sure if we can tune it this way going forward. I would really like the Optimizer to become more aware.

Is there anything else that can make the optimizer more aware and make better decisions?
I have analyzed the tables with the 12C defaults as well as created histograms (for all columns size auto & for all columns size 254)

I can always make the index invisible for this query but that may make other processes slow so requires quite a bit of investigation.Plus the database objects are maintained by vendor so we can only suggest and than they will have to "hotfix" it

Jack van Zanen

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On Tue, Aug 13, 2019 at 7:25 PM Jonathan Lewis <> wrote:

> By gathering stats with the EXADATA option you've told the optimizer that
> a typical single block read will take about 10ms while a 1MB multiblock
> read will take about 15ms, so it's not surprising that you will have
> execution plans that switch from indexed access and nested loop to full
> tablescan and hash joins.
> Unfortunately the cost of I/O to temp becomes highly visible on EXADATA
> when a hash join spills to disc, and the implied speed of tablescan is only
> helpful if you can push predicates down to storage and take advantage of
> storage indexes. (Or if you have purchase the IN-Memory option and have
> the right sort of queries that can do vector transformations).
> Generic strategy point 1:
> Look at the "table_cached_blocks" preference for gathering table stats.
> This can help to give Oracle a much better idea of the quality of an index
> by allowing it do derive a better value for the clustering_factor.
> Generic strategy point 2:
> Look for places where the optimizer's estimate of cardinality is HIGHER
> than it ought to be and find out why - as higher cardinalities tend to push
> the optimizer away from indexes/nested loops.
> Generic strategy point 3:
> If Exadata is wasting a lot of resources on tablescans that clearly be
> indexed access paths you consider deleting system stats, or faking some
> system stats that promise less about tablescan speed.
> I note you said you had an 11g outline - presumably this means that left
> to itself on 11g this query picked the "wrong" path - which means nothing
> has really changed.
> Regards
> Jonathan Lewis
> ________________________________________
> From: <> on
> behalf of kunwar singh <>
> Sent: 13 August 2019 07:13
> Subject: Bad execution plan after migrating to exadata ( 12c) from
> non-exadata (11g)
> Hi Listers,
> How to approach this? Looking for a approach in general when it comes to
> check plan issues when migrating to exadata and not something to this query
> ( but wont mind any insights into it either ;) )
> Issue:
> with outline data from 11g(in 12c exa DB)
> - cost ~90k, fast, elapsed time about 15 ms.
> - doing index range scan on a index on a 2GB table .
> 12c exadata
> - cost ~6k , slower , elapsed time about 4 seconds.
> - FTS on the 2GB table and from sql monitor report time is spent on
> reading it only/processing the hash join on it.
> - execution plan is having a view VW_NSO_1
> Few details:
> 1. I have already gathered stats on all tables/indexes
> 2. Have gathered system statistics with 'EXADATA'
> 3. Don't have the access to source 11g DB . getting it will take some time.
> Will post redacted version of the SQL & the execution plan ( if you prefer
> to look at it ) as account is very strict about security.
> --
> Cheers,
> Kunwar
> --

Received on Mon Aug 26 2019 - 05:20:05 CEST

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