An Oracle Distributed Query CBO Bug Finally Fixed (After 7 Years)

April 9, 2014

Optimizing distributed queries in Oracle is inherently more difficult. The CBO not only has to account for the additional resources related with distributed processing in Oracle, such as networking, but also has to get reliable table/column statistics for the remote objects.

It is well documented that Oracle has(d) trouble passing information about histograms for distributed queries( http://jonathanlewis.wordpress.com/2013/08/19/distributed-queries-3/ ).

In addition, Oracle was not able to pass selectivity information for “IS NULL/NOT NULL” filters via a DB link, even though the value of records with NULL is already written to NUM_NULLS column in DBA_TAB_COLUMNS…
As a result of this bug, every query that has IS NULL against a remote table ended up with cardinality of 1, even if there were many NULL records in the table.

PLAN_TABLE_OUTPUT
SQL_ID  djpaw3d54d5uq, child number 0
-------------------------------------
select 
       * 
from 
       tab1@db_link_loop a , dual  
where 
       a.num_nullable is null

Plan hash value: 3027949496

-------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |       |       |     8 (100)|          |        |      |
|   1 |  NESTED LOOPS      |      |     1 |    11 |     8   (0)| 00:00:01 |        |      |
|   2 |   TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |        |      |
|   3 |   REMOTE           | TAB1 |     1 |     9 |     6   (0)| 00:00:01 | DB_LI~ | R->S |
-------------------------------------------------------------------------------------------

Remote SQL Information (identified by operation id):
----------------------------------------------------

   3 - SELECT ID,NUM,NUM_NULLABLE FROM TAB1 A 
WHERE NUM_NULLABLE IS  NULL (accessing 'DB_LINK_LOOP')

The behavior was due to MOS Bug 5702977 Wrong cardinality estimation for “is NULL” predicate on a remote table

Fortunatly, the bug is fixed in 12c and 11.2.0.4. A patch is available for 11.2.0.3 on certain platforms.


When Oracle would Choose an Adaptive Execution Plan – General Thoughts

March 31, 2014

Adaptive Execution Plans is one of the most existing new features in Oracle 12c.
This post is not about how this feature works or its benefits, but rather about when Oracle would choose to use it.

In general, the Oracle CBO would use Adaptive Execution Plans if it is not sure which standard join (NL or HJ) is better:

  • If at SQL parse time, the Oracle CBO estimated that one of the sets to join is “significantly” smaller the other, where “significantly” is defined internally by the CBO, and there are appropriate indexes, then Oracle would opt for Nested Loops. Oracle CBO probably figured out that that the cost of NL is so much better than the cost of HJ, so it is not worth the effort of using an adaptive execution plan.
  • If one of the sets is only “slightly” smaller than the other, where “slightly” is defined internally by the CBO, then the performance of the two standard join types would be similar, so Oracle would typically decide to go with an Adaptive Plan and postpone the decision until run time. Oracle CBO probably saw that that that the cost of NL is “close” to the cost of HJ, so it is worth the effort of using an adaptive execution plan.
  • Finally, when the two sets have “similar” sizes, where “similar” is defined internally by the CBO, then Oracle would go with Hash join. Oracle CBO probably figured out that that the cost of HJ is so much better than the cost of NL, so it is not worth the effort of using an adaptive execution plan.

The figure below illustrates that behavior:

adaptive_exec_plans


RMOUG 2014

February 11, 2014

I was very excited to present at RMOUG 2014 – my first time at that conference.

Unfortunately, I got sick and I had to cancel.

The name of the presentation was:
Working with Confidence: How Sure Is the Oracle CBO About Its Cardinality Estimates, and Why Does It Matter?

Here are the Powerpoint and the White paper.


More on Dynamic Sampling Level 11 (AUTO) in Oracle 12c

January 29, 2014

In a previous post I showed an example of how the new AUTO dynamic sampling level (11) consumed significant resources for a very simple SQL statement.

Here, I’ll try to find your why.
First surprise!
10053 trace does not capture information about dynamic sampling (DS) level 11. It works quite fine for other levels (0-10) of dynamic sampling though.
Here is how a dynamic sampling section of 10053 file for levels 0 to 10 looks like:

————————————————————————————

SINGLE TABLE ACCESS PATH
Single Table Cardinality Estimation for TAB3[TAB3]
SPD: Return code in qosdDSDirSetup: NOCTX, estType = TABLE

*** 2014-01-29 11:41:02.678
** Performing dynamic sampling initial checks. **
** Dynamic sampling initial checks returning TRUE (level = 3).

*** 2014-01-29 11:41:02.678
** Generated dynamic sampling query:
query text :
SELECT /* OPT_DYN_SAMP */ /*+ ALL_ROWS IGNORE_WHERE_CLAUSE NO_PARALLEL(SAMPLESUB) opt_param('parallel_execution_enabled', 'false') NO_PARALLEL_INDEX(SAMPLESUB) NO_SQL_TUNE */ NVL(SUM(C1),0), NVL(SUM(C2),0), COUNT(DISTINCT C3) FROM (SELECT /*+ IGNORE_WHERE_CLAUSE NO_PARALLEL("TAB3") FULL("TAB3") NO_PARALLEL_INDEX("TAB3") */ 1 AS C1, CASE WHEN DECODE("TAB3"."ID",0,1)=567 THEN 1 ELSE 0 END AS C2, DECODE("TAB3"."ID",0,1) AS C3 FROM "JORDAN"."TAB3" SAMPLE BLOCK (0.829542 , 1) SEED (1) "TAB3") SAMPLESUB

*** 2014-01-29 11:41:03.597
** Executed dynamic sampling query:
level : 3
sample pct. : 0.829542
actual sample size : 9783
filtered sample card. : 0
orig. card. : 999999
block cnt. table stat. : 3737
block cnt. for sampling: 3737
max. sample block cnt. : 32
sample block cnt. : 31
unique cnt. C3 : 0
min. sel. est. : 0.01000000
** Using single table dynamic sel. est. : 0.00007085
Table: TAB3 Alias: TAB3
Card: Original: 999999.000000 Rounded: 71 
                             Computed: 70.85 Non Adjusted: 70.85

————————————————————————————

The section includes the query issues by the dynamic sampling along with lots of valuable information.

The same section for a query with dynamic sampling level 11 looks strikingly different:
————————————————————————————

SINGLE TABLE ACCESS PATH
Single Table Cardinality Estimation for TAB3[TAB3]
SPD: Return code in qosdDSDirSetup: NOCTX, estType = TABLE
Table: TAB3 Alias: TAB3
Card: Original: 999999.000000 >> Single Tab Card adjusted from:9999.990000 to:1.000000
Rounded: 1 Computed: 1.00 Non Adjusted: 9999.99

————————————————————————————
Even though we can see that the cardinality was adjusted from 9999.99 to 1 as a result of the dynamic sampling, there are no details about the DS queries that did the actual sampling. I was not able to see DS summary information either.

Since 10053 trace file did not give me the information I needed, I decided to look elsewhere – in the V$ tables.
After I flushed the shared pool and ran the query from the previous post, I issued the following SQL to get the DS SQL related to the statement:

select * from v$sql where sql_text like '%TAB3%'

Second surprise! The query returned a few records related to DS.
——————————————————————–

SELECT /* DS_SVC */ /*+ cursor_sharing_exact dynamic_sampling(0) 
...
FROM "TAB3" SAMPLE BLOCK(21.4075, 8) SEED(1) "TAB3" WHERE ...


SELECT /* DS_SVC */ /*+ cursor_sharing_exact dynamic_sampling(0) 
...
FROM "TAB3" SAMPLE BLOCK(42.8151, 8) SEED(2) "TAB3" WHERE ...

SELECT /* DS_SVC */ /*+ cursor_sharing_exact dynamic_sampling(0) 
...
FROM "TAB3" SAMPLE BLOCK(85.6302, 8) SEED(3) "TAB3" WHERE ....

———————————————————————

They are similar with the exception of SAMPLE BLOCK and SEED arguments. It appears that the optimizer tried to get sampling data using small sample block argument, but it failed. Them it tried again with larger sample size, but failed again. Finally, in the third attempt, with 85% sampling, it succeeded.

Generally speaking, the new policy to retry sampling until the desired result is achieved is good. The algorithm needs to be tweaked a bit to account for situations like the one I described in a previous post though.


What is the Deal with AVG_LEAF_BLOCKS_PER_KEY (DBA_INDEXES) !?!

December 17, 2013

AVG_LEAF_BLOCKS_PER_KEY column in DBA_INDEXES should show the average number of leaf blocks in which each distinct value in the index appears. Common sense and Christian’s Antognini’s book Troubleshooting Oracle Performance (pg.136) suggest that the value of AVG_LEAF_BLOCKS_PER_KEY should by derived from the values of LEAF_BLOCKS and DISTINCT_KEYS .

Well, that does not appear to be the case in Oracle 11g!

This query can help you find the indexes that do not conform to the rule:

select 
       * 
from 
       dba_indexes
where 
       DISTINCT_KEYS > 0
and 
       AVG_LEAF_BLOCKS_PER_KEY  > 3*LEAF_BLOCKS/DISTINCT_KEYS
and 
       LEAF_BLOCKS > DISTINCT_KEYS

The discrepancy is certainly not a rounding error. I have an index where the value of the AVG_LEAF_BLOCKS_PER_KEY is 150 times larger than LEAF_BLOCKS/DISTINCT_KEYS.

The problem seems to be happens only when stats are gathered (DBMS_STATS) with AUTO sampling. Computing stats ( estimate_percent = NULL) resolves the issue.

Ironically, DBMS_STATS AUTO sampling delivers good estimates for LEAF_BLOCKS and DISTINCT_KEYS. If only Oracle computed AVG_LEAF_BLOCKS_PER_KEY by using LEAF_BLOCKS and DISTINCT_KEYS, everything would have been great…


Dynamic sampling in Oracle 12c – it goes up to 11. Really!

December 13, 2013

For the first time in many years, Oracle introduced a new option for dynamic sampling, and a major one for that – http://docs.oracle.com/cd/E11882_01/server.112/e16638/stats.htm#PFGRF95292  .

The new dynamic sampling level 11 is the all-encompassing “automatic” switch, which leaves all decisions to Oracle.
The big question now is when would the new auto dynamic sampling kick-in? Maria Colgan (http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-optimizer-with-oracledb-12c-1963236.pdf)  indicates that the auto option would fire more frequently that the default.
To begin filling in the blanks, I started with a really simple test – retrieval by primary key.

Here are the create scripts:

create table tab3 as
with v1 as 
     (select rownum n from dual connect by level <= 10000)
select
     rownum id , 
     dbms_random.string('u',dbms_random.value*29 + 1) str
from
     v1, v1
where
     rownum < 1000000;

alter table tab3 add primary key (id)

execute dbms_stats.gather_table_stats('JORDAN','TAB3')

Now let see when happens when set the optimizer_dynamic_sampling =11 and issue the search by primary key:

SQL> alter session set optimizer_dynamic_sampling =11 ;

Session altered.

SQL> set autotrace on
SQL> select * from tab3 where id = 123 ;

…output truncated…

Note
-----
- dynamic statistics used: dynamic sampling (level=AUTO)
Statistics
----------------------------------------------------------
15 recursive calls
0 db block gets
14 consistent gets
0 physical reads
0 redo size
468 bytes sent via SQL*Net to client
532 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

We see that that Oracle decided to do dynamic sampling. There is no guesswork in those types of queries. The cardinality of the statement is one, so dynamic sampling was not necessary.
But that’s not the worst part. The statement required 14 consistent gets and 15 recursive calls for a simple scan. Without dynamic sampling, the same statement requires 4 consistent gets and 1 recursive call.

SQL> select * from tab3 where id = 345 ;

…output truncated…

Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
468 bytes sent via SQL*Net to client
532 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

The idea behind automatic dynamic sampling is excellent. Starting dynamic sampling only when the Oracle CBO cannot get reliable cardinalities would be a giant step towards query self-tuning.

The implementation of the feature in Oracle version 12.1.0.1, however, needs some more work. Hopefully, the logic would get more precise in the newer versions….


The Cost of Adaptive Execution Plans in Oracle (a Small Study) Part 3

November 26, 2013

My previous two posts, The Cost of Adaptive Execution Plans in Oracle (a Small Study) Part 1 and The Cost of Adaptive Execution Plans in Oracle (a Small Study) Part 2,  focused on estimating the impact of OPTIMIZER_ADAPTIVE_REPORTING_ONLY parameter in Oracle 12c. The parameter controls if the adaptive plan would be acted upon. The adaptive plan, however, is always generated.

The post will try to estimate the impact of adaptive execution plans, while taking into account the resources needed for generating the adaptive execution plans. Since I was not able to find a way to suppress the generation of adaptive plans in Oracle 12c, I decided to just use an older version of the optimizer. I did that by utilizing “optimizer_features_enable” parameter.

Here is the table/view setup script, and here is the test script.

We can see the difference in elapsed time using this query:

select avg(r), avg(a) , STATS_T_TEST_PAIRED (R,A) FROM track_v

avg(r) = 1.5880118 avg(a) = 2.5002121 STATS_T_TEST_PAIRED (R,A) /p/ = 0

The adaptive (12c)  plan takes almost 1 millisecond more than the non-adaptive, “old-style” 11gR2 plan.

This query can show where the difference comes from:

select c.name , avg (a.val) a , avg (r.val) r , STATS_T_TEST_PAIRED (r.val,A.val)
from stats_a a , stats_r r , v$statname c
where a.id = r.id
and a.stat = r.stat
and a.stat = c.statistic#
group by c.name

The only statistics that are difference are:
CCursor + sql area evicted – the SQL with AEP(12c) on consumed more that the SQL run with 11gR2
CPU used by this session – the SQL with AEP(12c) on consumed more that the SQL run with 11gR2
parse time cpu – the SQL with AEP(12c) on consumed more that the SQL run with 11gR2
parse time elapsed – the SQL with AEP(12c) on consumed more that the SQL run with 11gR2
recursive cpu usage – the SQL with AEP(12c) on consumed more that the SQL run with 11gR2
sql area evicted – the SQL with AEP(12c) on consumed more that the SQL run with 11gR2

In conclusion, “adaptive execution plans” is a wonderful feature. The resource it needs are typically negligible, except for very light (fast) OLTP type queries.


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