Oracle Scratchpad

October 28, 2018

Upgrades – again

Filed under: 12c,Histograms,Oracle,Statistics,Upgrades — Jonathan Lewis @ 12:39 pm GMT Oct 28,2018

I’ve got a data set which I’ve recreated in 11.2.0.4 and 12.2.0.1.

I’ve generated stats on the data set, and the stats are identical.

I don’t have any indexes or extended stats, or SQL Plan directives or SQL Plan Profiles, or SQL Plan Baselines, or SQL Patches to worry about.

I’m joining two tables, and the join column on one table has a frequency histogram while the join column on the other table has a height-balanced histogram.  The histograms were created with estimate_percent => 100%. (which explains why I’ve got a height-balanced histogram in 12c rather than a hybrid histogram.)

Here are the two execution plans, 11.2.0.4 first, pulled from memory by dbms_xplan.display_cursor():


SQL_ID  f8wj7karu0hhs, child number 0
-------------------------------------
select         count(*) from         t1, t2 where         t1.j1 = t2.j2

Plan hash value: 906334482

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation           | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |      1 |        |      1 |00:00:00.01 |      12 |       |       |          |
|   1 |  SORT AGGREGATE     |      |      1 |      1 |      1 |00:00:00.01 |      12 |       |       |          |
|*  2 |   HASH JOIN         |      |      1 |   1855 |   1327 |00:00:00.01 |      12 |  2440K|  2440K| 1357K (0)|
|   3 |    TABLE ACCESS FULL| T1   |      1 |    100 |    100 |00:00:00.01 |       6 |       |       |          |
|   4 |    TABLE ACCESS FULL| T2   |      1 |    800 |    800 |00:00:00.01 |       6 |       |       |          |
-----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("T1"."J1"="T2"."J2")



SQL_ID	f8wj7karu0hhs, child number 0
-------------------------------------
select	       count(*) from	     t1, t2 where	  t1.j1 = t2.j2

Plan hash value: 906334482

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation	    | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |	OMem |	1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |	   |	  1 |	     |	    1 |00:00:00.01 |	  41 |	     |	     |		|
|   1 |  SORT AGGREGATE     |	   |	  1 |	   1 |	    1 |00:00:00.01 |	  41 |	     |	     |		|
|*  2 |   HASH JOIN	    |	   |	  1 |	1893 |	 1327 |00:00:00.01 |	  41 |	2545K|	2545K| 1367K (0)|
|   3 |    TABLE ACCESS FULL| T1   |	  1 |	 100 |	  100 |00:00:00.01 |	   7 |	     |	     |		|
|   4 |    TABLE ACCESS FULL| T2   |	  1 |	 800 |	  800 |00:00:00.01 |	   7 |	     |	     |		|
-----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("T1"."J1"="T2"."J2")

The key point is the the difference between the two cardinality estimates. Why has that appeared, and what might the optimizer do in a more complex plan when a cardinality estimates changes?

The difference is only 2% but that was on a couple of data sets I just happened to run up to check something completely different, I wasn’t trying to break something, so who know how big the variation can get. Of course if you’re switching from 11g to 12c then Oracle (Corp.) expects you to be using auto_sample_size anyway so you shouldn’t be producing height-balanced histograms.

So does this difference really matter? Maybe not, but if you (like many sites I’ve seen) are still using fixed percentage sample sizes and are generating histograms it’s another reason (on top of the usual instability effects of height-balanced and hybrid histograms) why you might see plans change as you upgrade from 11g to 12c.

Footnote

It looks as if the difference comes mostly from a coding error in 11g that has been fixed in 12c – I couldn’t find an official bug or fix_control that matched, though. More on that later in the week.

Update

Chinar Aliyev has pointed out that there are three fix-controls that may be associated with this (and other ) changes. From v$system_fix_control these are:

14033181 1 QKSFM_CARDINALITY_14033181   correct ndv for non-popular values in join cardinality comp.         (12.1.0.1)
19230097 1 QKSFM_CARDINALITY_19230097   correct join card when popular value compared to non popular         (12.2.0.1)
22159570 1 QKSFM_CARDINALITY_22159570   correct non-popular region cardinality for hybrid histogram          (12.2.0.1)

I haven’t tested them yet, but with the code easily available in the article it won’t take long to see what the effects are when I have a few minutes. The first fix may also be why I had a final small discrepancy between 11g and 12c on the join on two columns with frequency histograms.

1 Comment »

  1. […] Chinar Aliyev, he suggested three fix-controls that might be relevant. I’ve added these to an earlier posting I did when I first hit the anomaly a few days ago but I’ll repeat them here. I will be […]

    Pingback by Join Cardinality – 5 | Oracle Scratchpad — November 1, 2018 @ 1:34 pm GMT Nov 1,2018 | Reply


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