RE: Statistics Problem on partitioned table

From: Ric Van Dyke <ric.van.dyke_at_hotsos.com>
Date: Tue, 1 Oct 2013 09:35:20 -0500
Message-ID: <C970F08BBE1E164AA8063E01502A71CF021C0D02_at_WIN02.hotsos.com>

I suspect that the real issue for the optimizer is the fact that the index is a non-prefixed local index. I can only presume that the estimated rows you see are a "pre partition" based number, it's still wrong should be in the near 4000 not less than 100. (given the total number for rows returned is 6 million) Since it's not prefixed the optimizer has no idea which partitions it will go once it retrieves a row from the index. What at the global level stats for the per_ind column? Especially the number for distinct values.

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Mohamed Houri Sent: Tuesday, October 01, 2013 10:07 AM To: ORACLE-L
Subject: Statistics Problem on partitioned table

Dear list
I have a performance problem that I have narrowed to a statistics problem where the CBO is not doing good estimations on a partitioned table

The culprit select looks like

select

    a

   ,b

   ,c

   ,per_ind

from XXX_PER_YYY

where per_ind = 0;

XXX_PER_YYY is a table range partitioned by a date. There are 1493 partitions.

AND there is an index XXX_PER_IND on (per_ind) local (note that it is local non prefixed)

The execution plan with Estimations and Actuals looks like

---

---

| Id | Operation | Name | Starts | E-Rows
|

A-Rows |

---

---

| 0 | SELECT STATEMENT | | 1 |
|550K |

| 1 | PARTITION RANGE ALL | | 1 | 69
|550K |

| 2 | TABLE ACCESS BY LOCAL INDEX ROWID| XXX_PER_YYY | 111 | 69
|550K |

|* 3 | INDEX RANGE SCAN | XXX_PER_IND | 111 | 69
|550K |

---

---

Predicate Information (identified by operation id):

---

    3 - access("PER_IND"=0)

And the execution plan showing the number of partition looks like

---

---

| Id | Operation | Name | Rows | Bytes
|

Pstart| Pstop |

---

---

| 0 | SELECT STATEMENT | | 69 | 1173
| | |

| 1 | PARTITION RANGE ALL | | 69 | 1173
| 1 | 1493 |

| 2 | TABLE ACCESS BY LOCAL INDEX ROWID| XXX_PER_YYY | 69 | 1173
| 1 | 1493 |

|* 3 | INDEX RANGE SCAN | XXX_PER_IND | 69 |
| 1 | 1493 |

---

---

 Predicate Information (identified by operation id):

---

    3 - access("PER_IND"=0)

There is only one distinct value of per_ind (per_ind =0)

select per_ind, count(1) from XXX_PER_YYY group by per_ind;

*per_ind cout(1)*

0 6,118,184

And I have the following partitions num_rows distribution

758 partitions with num_rows = 0;

60 partitions with num_rows <= 5000

295 partitions with num_rows between 5000 and 10000

315 partitions with num_rows > 10,000 and num_rows <50,000

65 partitions with num_rows > 100,000;

Statistics are calculated at a global level

How to make the CBO having correct estimations and hence an optimal execution plan?

Thanks in advance

--
Bien Respectueusement
Mohamed Houri
www.hourim.wordpress.com


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Received on Tue Oct 01 2013 - 16:35:20 CEST