RE: Optimizer estimated cardinality very low
Date: Tue, 27 Oct 2020 13:03:52 +0000
Message-ID: <15909_1603803849_5F981AC9_15909_4502_1_9d6be7519f8e4c75a2902e1f2ba44bf1_at_vontobel.com>
Hi Daniel,
“NUM_DISTINCT for the column_group in July are lower than ORDR_RQST_NUMBER instead of being equal or higher.”
This is exactly the reason that sanity check kicks in.
“ This happens across other partitions but the estimation doesn't seem impacted by this alone.”
There is some tolerance range. Set num_distinct for SYS_STU4S$4GK1UDMC2M8$MI#W6QS_ of the June partition to a sufficiently low value for sanity check to kick in. Then you should get sel = 1.0 and a “good” cardinality estimate for your case.
“but the estimation doesn't seem impacted by this alone.”
Histogram on ORDR_RQST_DATE is apparently another prerequisite for the sanity check.
Best regards,
Nenad
https://nenadnoveljic.com/blog
From: Daniel Coello <coello.daniel_at_gmail.com>
Sent: Montag, 26. Oktober 2020 18:07
Hi Nenand.
For when the interval when the estimation is good:
...
To: Noveljic Nenad <nenad.noveljic_at_vontobel.com>
Cc: ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
Subject: Re: Optimizer estimated cardinality very low
Join ColGroups for ORDER_REQST[ORDER_REQST] and ORDER_DETL[ORDER_DETL] : Using cdn sanity check
Below is some information relevant to the columns: ORDER_REQST PARTITION_NAME COLUMN_NAME NUM_DISTINCT NUM_BUCKETS SAMPLE_SIZE HISTOGRAM NUM_ROW IN PARTITION JULY SYS_P971481 ORDR_RQST_DATE 3
3
26895392
FREQUENCY 26895392
SYS_P971481 SYS_STU4S$4GK1UDMC2M8$MI#W6QS_ 26636288
1
26895392
NONE 26895392
SYS_P971481 ORDR_RQST_NUMBER 26895392
1
26895392
NONE 26895392
JUNE SYS_P964661 ORDR_RQST_DATE 4
4
17506471
FREQUENCY 17506471
SYS_P964661 SYS_STU4S$4GK1UDMC2M8$MI#W6QS_ 15794176
1
17506471
NONE 17506471
SYS_P964661 ORDR_RQST_NUMBER 15649792
1
17506471
NONE 17506471
ORDER_DETL PARTITION_NAME COLUMN_NAME NUM_DISTINCT NUM_BUCKETS SAMPLE_SIZE HISTOGRAM NUM_ROW IN PARTITION JULY SYS_P971761 ORDR_RQST_DATE 3
3
32314200
FREQUENCY 32314200
SYS_P971761 SYS_STU4S$4GK1UDMC2M8$MI#W6QS_ 26636288
1
32314200
NONE 32314200
SYS_P971761 ORDR_RQST_NUMBER 26984448
1
32314200
NONE 32314200
JUNE SYS_P964866 ORDR_RQST_DATE 4
4
21814573
FREQUENCY 21814573
SYS_P964866 SYS_STU4S$4GK1UDMC2M8$MI#W6QS_ 15794176
1
21814573
NONE 21814573
SYS_P964866 ORDR_RQST_NUMBER 15649792
1
21814573
NONE 21814573
Things that have been pointed out to me or relevant to the env:
- NUM_DISTINCT for the column_group in July are lower than ORDR_RQST_NUMBER instead of being equal or higher. This happens across other partitions but the estimation doesn't seem impacted by this alone. - I added the NUM_ROW values from the partitions for comparison with the total number of rows in it. - ORDER_DETL columns' values should exist in ORDER_REQST, there shouldn't be an orphan row which in real counts you can see.
- The actual count for distincts are: July: ORDER_REQST ORDR_RQST_NUMBER: 26846523 COL_GROUP ( ORDR_RQST_NUMBER , ORDR_RQST_DATE ): 26846523
ORDR_RQST_NUMBER: 26846523
COL_GROUP ( ORDR_RQST_NUMBER , ORDR_RQST_DATE ) : 26846523 June:
ORDER_REQST
ORDR_RQST_NUMBER : 15683040
COL_GROUP ( ORDR_RQST_NUMBER , ORDR_RQST_DATE ) : 15683040 ORDER_DETL
ORDR_RQST_NUMBER : 15683040
COL_GROUP ( ORDR_RQST_NUMBER , ORDR_RQST_DATE ) : 15683040 Thanks,
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