RE: CBO Friday Fun!
- From: Iggy Fernandez <iggy_fernandez@xxxxxxxxxxx>
- To: "vxsmimmcp@xxxxxxxxxx" <vxsmimmcp@xxxxxxxxxx>
- Date: Fri, 6 Jun 2014 18:58:02 -0700
Also, I wonder whether the CROSS JOIN clause is blindsiding the optimizer. The
developer has chosen to express the INNER JOIN ON clause in CROSS JOIN WHERE
fashion.
Iggy
From: iggy_fernandez@xxxxxxxxxxx
To: vxsmimmcp@xxxxxxxxxx
CC: oracle-l@xxxxxxxxxxxxx
Subject: RE: CBO Friday Fun!
Date: Fri, 6 Jun 2014 15:25:29 -0700
re: does the fact that the cost is lower on the hinted version mean that CBO is
simply not considering the join order?
That sounds logical enough. The 10053 trace lists all the permutations that
were considered so you have confirmation. The optimizer does not consider all
join orders even if _optimizer_max_permutations has not been exceeded. The
optimizer only considers all join permutations if the number of tables is not
greater than _optimizer_search_limit whose default is 5. You could try
increasing the value to 6.
From: vxsmimmcp@xxxxxxxxxx
To: oratune@xxxxxxxxx; oracle-l@xxxxxxxxxxxxx
Subject: RE: CBO Friday Fun!
Date: Fri, 6 Jun 2014 21:33:36 +0000
>>
There are only 5 tables in this query – 120 permutations. It seems that CBO
should be finding this plan on its own.
Miscounted – 6 tables. Still…?
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of McPeak, Matt
Sent: Friday, June 06, 2014 5:30 PM
To: David Fitzjarrell; oracle-l@xxxxxxxxxxxxx
Subject: RE: CBO Friday Fun!
11.2.0.2
STATS are current. Histograms, maybe…. SO2.CANCELLED_FLAG could benefit from
one, maybe.
But that’s all irrelevant! With an ORDERED hint, CBO is computing a cost a
lower cost *with the stats and histograms that it has*. Why would CBO skip
a plan that is obviously legal and lower cost? I say “obviously legal”
because, if it were not legal, the ORDERED hint wouldn’t give me the lower cost
plan.
There are only 5 tables in this query – 120 permutations. It seems that CBO
should be finding this plan on its own.
Thanks,
Matt
From: "" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "oratune@xxxxxxxxx" for
DMARC)
Sent: Friday, June 06, 2014 5:21 PM
To: McPeak, Matt; oracle-l@xxxxxxxxxxxxx
Subject: Re: CBO Friday Fun!
Oracle version? Are stats current? Are histograms representative of the data?
More information would be appreciated.
David Fitzjarrell
Principal author, "Oracle Exadata Survival Guide"
On Friday, June 6, 2014 3:17 PM, "McPeak, Matt" <vxsmimmcp@xxxxxxxxxx> wrote:
Hi, Gurus!
I have a query that someone gave me. I’ve been looking at a 10053 event trace
for it, trying to figure out why CBO will not come up with the “obvious”
plan without a hint.
I’ve boiled the query down to a relatively simple form, and here is what I find:
·
If I explain plan the query, the cost is 1,918
·
If I put an /*+ ORDERED */ hint in the query and explain plan that one, the
cost is 959
I’ll paste the two queries below, for reference.
My question is: does the fact that the cost is lower on the hinted version mean
that CBO is simply not considering the join order?
In the 10053 trace, all the join orders considered want to go to table vvmi2
before oel2 (joining on the car_line column instead of the stars_model_item_id
which has a unique index). That move makes that cardinality going into the
CROSS JOIN on so2 much higher than 1, making that cross join too expensive.
Anyway, at this point, I guess I’m wondering why the CBO doesn’t look at and
choose the plan with the 959 cost without needing me to specify /*+ ORDERED
*/.
Thanks, in advance, for your thoughts. The queries are below. (The purpose of
the query is to find orders that are potential duplicates with a given
order).
Thanks,
Matt
*** This query explains with a cost of 1,918 ***
SELECT
MIN
(so2.order_number)
dup_order
FROM
verp_om_sold_orders so1
CROSS
JOIN
verp_om_sold_orders so2
INNER
JOIN
oe_order_lines_all oel1
ON
oel1.header_id
=
so1.order_header_id
AND
oel1.item_type_code
=
'MODEL'
INNER
JOIN
verp_vps_model_items vvmi1
ON
oel1.inventory_item_id
=
vvmi1.stars_model_item_id
INNER
JOIN
oe_order_lines_all oel2
ON
oel2.header_id
=
so2.order_header_id
AND
oel2.item_type_code
=
'MODEL'
INNER
JOIN
verp_vps_model_items vvmi2
ON
vvmi2.stars_model_item_id
=
oel2.inventory_item_id
WHERE
1
=
1
AND
so1.order_header_id
=
41356864
AND
so2.order_header_id
!=
41356864
AND
so2.entered_date
>
to_date('06-JAN-2014')
AND
NVL
(so2.cancelled_flag,
'N') =
'N'
-- Is duplicate
AND
(so1.email_address
=
so2.email_address
OR
(so1.person_last_name
=
so2.person_last_name
AND
so1.postal_code
=
so2.postal_code))
AND
vvmi1.car_line
=
vvmi2.car_line
*** This query explains with a cost of 959 ***
SELECT
/*+ ORDERED */
MIN
(so2.order_number)
dup_order
FROM
verp_om_sold_orders so1
CROSS
JOIN
verp_om_sold_orders so2
INNER
JOIN
oe_order_lines_all oel1
ON
oel1.header_id
=
so1.order_header_id
AND
oel1.item_type_code
=
'MODEL'
INNER
JOIN
verp_vps_model_items vvmi1
ON
oel1.inventory_item_id
=
vvmi1.stars_model_item_id
INNER
JOIN
oe_order_lines_all oel2
ON
oel2.header_id
=
so2.order_header_id
AND
oel2.item_type_code
=
'MODEL'
INNER
JOIN
verp_vps_model_items vvmi2
ON
vvmi2.stars_model_item_id
=
oel2.inventory_item_id
WHERE
1
=
1
AND
so1.order_header_id
=
41356864
AND
so2.order_header_id
!=
41356864
AND
so2.entered_date
>
to_date('06-JAN-2014')
AND
NVL
(so2.cancelled_flag,
'N') =
'N'
-- Is duplicate
AND
(so1.email_address
=
so2.email_address
OR
(so1.person_last_name
=
so2.person_last_name
AND
so1.postal_code
=
so2.postal_code))
AND
vvmi1.car_line
=
vvmi2.car_line
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