Another bug with lateral

Posted on by Posted in 12c, bug, CBO, curious, oracle, troubleshooting 1,625 Page views Leave a comment

Compare the results of the following query with the clause “fetch first 2 rows only”

with 
 t1(a) as (select * from table(odcinumberlist(1,3)))
,t2(a,b) as (select * from table(ku$_objnumpairlist(
                                 sys.ku$_objnumpair(1,1),
                                 sys.ku$_objnumpair(1,2),
                                 sys.ku$_objnumpair(1,3),
                                 sys.ku$_objnumpair(3,1),
                                 sys.ku$_objnumpair(3,2),
                                 sys.ku$_objnumpair(3,3)
                                 )))
,t(id) as (select * from table(odcinumberlist(1,2,3,4,5,6,7)))
select
  *
from t,
     lateral(select t1.a,t2.b
             from t1,t2 
             where t1.a = t2.a 
               and t1.a = t.id
             order by t2.b
             fetch first 2 rows only
             )(+)
order by id;

        ID          A          B
---------- ---------- ----------
         1          1          1
         1          3          1
         2          1          1
         2          3          1
         3          1          1
         3          3          1
         4          1          1
         4          3          1
         5          1          1
         5          3          1
         6          1          1
         6          3          1
         7          1          1
         7          3          1

14 rows selected.

with this one (i’ve just commented out the line with “fetch-first-rows-only”:

with 
 t1(a) as (select * from table(odcinumberlist(1,3)))
,t2(a,b) as (select * from table(ku$_objnumpairlist(
                                 sys.ku$_objnumpair(1,1),
                                 sys.ku$_objnumpair(1,2),
                                 sys.ku$_objnumpair(1,3),
                                 sys.ku$_objnumpair(3,1),
                                 sys.ku$_objnumpair(3,2),
                                 sys.ku$_objnumpair(3,3)
                                 )))
,t(id) as (select * from table(odcinumberlist(1,2,3,4,5,6,7)))
select
  *
from t,
     lateral(select t1.a,t2.b
             from t1,t2 
             where t1.a = t2.a 
               and t1.a = t.id
             order by t2.b
--             fetch first 2 rows only
             )(+)
order by id;

        ID          A          B
---------- ---------- ----------
         1          1          2
         1          1          3
         1          1          1
         2
         3          3          2
         3          3          1
         3          3          3
         4
         5
         6
         7

11 rows selected.

Obviously, the first query should return less rows than second one, but we can see that it returned more rows and join predicate “and t1.a = t.id” was ignored, because A and B are not empty and “A” is not equal to t.ID.

Lateral view decorrelation(VW_DCL) causes wrong results with rownum

Posted on by Posted in 12c, bug, CBO, oracle, query optimizing, rownum, troubleshooting 1,945 Page views 3 Comments

Everyone knows that rownum in inline views blocks many query transformations, for example pushing/pulling predicates, scalar subquery unnesting, etc, and many people use it for such purposes as a workaround to avoid unwanted transformations(or even CBO bugs).

Obviously, the main reason of that is different calculation of rownum:

If we pull the predicate “column_value = 3″ from the following query to higher level
[sourcecode language=”sql” highlight=””]
select *
from (select * from table(odcinumberlist(1,1,1,2,2,2,3,3,3)) order by 1)
where rownum <= 2
and column_value = 3;

COLUMN_VALUE
————
3
3
[/sourcecode]
we will get different results:
[sourcecode language=”sql” highlight=”8″]
select *
from (select *
from (select * from table(odcinumberlist(1,1,1,2,2,2,3,3,3)) order by 1)
where rownum <= 2
)
where column_value = 3;

no rows selected
[/sourcecode]
Doc ID 62340.1

[collapse]

But we recently encountered a bug with it: lateral view with ROWNUM returns wrong results in case of lateral view decorrelation.
Compare results of this query with and without no_decorrelation hint:

with 
 t1(a) as (select * from table(odcinumberlist(1,3)))
,t2(b) as (select * from table(odcinumberlist(1,1,3,3)))
,t(id) as (select * from table(odcinumberlist(1,2,3)))
select
  *
from t,
     lateral(select/*+ no_decorrelate */ rownum rn 
             from t1,t2 
             where t1.a=t2.b and t1.a = t.id
            )(+)
order by 1,2;

        ID         RN
---------- ----------
         1          1
         1          2
         2
         3          1
         3          2

with 
 t1(a) as (select * from table(odcinumberlist(1,3)))
,t2(b) as (select * from table(odcinumberlist(1,1,3,3)))
,t(id) as (select * from table(odcinumberlist(1,2,3)))
select
  *
from t,
     lateral(select rownum rn 
             from t1,t2 
             where t1.a=t2.b and t1.a = t.id
            )(+)
order by 1,2;

        ID         RN
---------- ----------
         1          1
         1          2
         2
         3          3
         3          4

Of course, we can draw conclusions even from these results: we can see that in case of decorrelation(query with hint) rownum was calculated before the join. But to be sure we can check optimizer’s trace 10053:

Final query after transformations:

[sourcecode language=”sql”]
******* UNPARSED QUERY IS *******
SELECT VALUE(KOKBF$2) "ID", "VW_DCL_76980902"."RN" "RN"
FROM TABLE("ODCINUMBERLIST"(1, 2, 3)) "KOKBF$2",
(SELECT ROWNUM "RN_0", VALUE(KOKBF$0) "ITEM_3"
FROM TABLE("ODCINUMBERLIST"(1, 3)) "KOKBF$0",
TABLE("ODCINUMBERLIST"(1, 1, 3, 3)) "KOKBF$1"
WHERE VALUE(KOKBF$0) = VALUE(KOKBF$1)
) "VW_DCL_76980902"
WHERE "VW_DCL_76980902"."ITEM_3"(+) = VALUE(KOKBF$2)
ORDER BY VALUE(KOKBF$2), "VW_DCL_76980902"."RN"

*************************
[/sourcecode]

[collapse]

I’ll modify it a bit just to make it more readable:
we can see that 

select
  *
from t,
     lateral(select rownum rn 
             from t1,t2 
             where t1.a=t2.b and t1.a = t.id)(+)
order by 1,2;

was transformed to 

select
  t.id, dcl.rn
from t,
     (select rownum rn 
      from t1,t2 
      where t1.a=t2.b) dcl
where dcl.a(+) = t.id
order by 1,2;

And it confirms that rownum was calculated on the different dataset (t1-t2 join) without join filter by table t.
I created SR with Severity 1 (SR #3-19117219271) more than a month ago, but unfortunately Oracle development doesn’t want to fix this bug and moreover they say that is not a bug. So I think this is a dangerous precedent and probably soon we will not be able to be sure in the calculation of rownum and old fixes…

Top-N again: fetch first N rows only vs rownum

Posted on by Posted in adaptive serial direct path reads, CBO, oracle, query optimizing, SQL, troubleshooting 7,409 Page views Leave a comment

Three interesting myths about rowlimiting clause vs rownum have recently been posted on our Russian forum:

  1. TopN query with rownum<=N is always faster than "fetch first N rows only" (ie. row_number()over(order by ...)<=N)
  2. “fetch first N rows only” is always faster than rownum<=N
  3. “SORT ORDER BY STOPKEY” stores just N top records during sorting, while “WINDOW SORT PUSHED RANK” sorts all input and stores all records sorted in memory.

Interestingly that after Vyacheslav posted first statement as an axiom and someone posted old tests(from 2009) and few people made own tests which showed that “fetch first N rows” is about 2-3 times faster than the query with rownum, the final decision was that “fetch first” is always faster.

First of all I want to show that statement #3 is wrong and “WINDOW SORT PUSHED RANK” with row_number works similarly as “SORT ORDER BY STOPKEY”:
It’s pretty easy to show using sort trace:
Let’s create simple small table Tests1 with 1000 rows where A is in range 1-1000 (just 1 block):

create table test1(a not null, b) as
  select level, level from dual connect by level<=1000;

alter session set max_dump_file_size=unlimited;
ALTER SESSION SET EVENTS '10032 trace name context forever, level 10';

ALTER SESSION SET tracefile_identifier = 'rownum';
select * from (select * from test1 order by a) where rownum<=10;

ALTER SESSION SET tracefile_identifier = 'rownumber';
select * from test1 order by a fetch first 10 rows only;

And we can see from the trace files that both queries did the same number of comparisons:

rownum:

[sourcecode language=”sql” highlight=”7″]
—– Current SQL Statement for this session (sql_id=bbg66rcbt76zt) —–
select * from (select * from test1 order by a) where rownum<=10

—- Sort Statistics ——————————
Input records 1000
Output records 10
Total number of comparisons performed 999
Comparisons performed by in-memory sort 999
Total amount of memory used 2048
Uses version 1 sort
—- End of Sort Statistics ———————–
[/sourcecode]

[collapse]
row_number

[sourcecode language=”sql” highlight=”7″]
—– Current SQL Statement for this session (sql_id=duuy4bvaz3d0q) —–
select * from test1 order by a fetch first 10 rows only

—- Sort Statistics ——————————
Input records 1000
Output records 10
Total number of comparisons performed 999
Comparisons performed by in-memory sort 999
Total amount of memory used 2048
Uses version 1 sort
—- End of Sort Statistics ———————–
[/sourcecode]

[collapse]

Ie. each row (except first one) was compared with the biggest value from top 10 values and since they were bigger than top 10 value, oracle doesn’t compare it with other TopN values.

And if we change the order of rows in the table both of these queries will do the same number of comparisons again:

from 999 to 0

[sourcecode language=”sql”]
create table test1(a not null, b) as
select 1000-level, level from dual connect by level<=1000;

alter session set max_dump_file_size=unlimited;
ALTER SESSION SET EVENTS ‘10032 trace name context forever, level 10’;

ALTER SESSION SET tracefile_identifier = ‘rownum’;
select * from (select * from test1 order by a) where rownum<=10;

ALTER SESSION SET tracefile_identifier = ‘rownumber’;
select * from test1 order by a fetch first 10 rows only;
[/sourcecode]

[collapse]
rownum

[sourcecode language=”sql” highlight=”7″]
—– Current SQL Statement for this session (sql_id=bbg66rcbt76zt) —–
select * from (select * from test1 order by a) where rownum<=10

—- Sort Statistics ——————————
Input records 1000
Output records 1000
Total number of comparisons performed 4976
Comparisons performed by in-memory sort 4976
Total amount of memory used 2048
Uses version 1 sort
—- End of Sort Statistics ———————–
[/sourcecode]

[collapse]
row_number

[sourcecode language=”sql” highlight=”7″]
—– Current SQL Statement for this session (sql_id=duuy4bvaz3d0q) —–
select * from test1 order by a fetch first 10 rows only

—- Sort Statistics ——————————
Input records 1000
Output records 1000
Total number of comparisons performed 4976
Comparisons performed by in-memory sort 4976
Total amount of memory used 2048
Uses version 1 sort
—- End of Sort Statistics ———————–
[/sourcecode]

[collapse]

We can see that both queries required much more comparisons(4976) here, that’s because each new value is smaller than the biggest value from the topN and even smaller than lowest value, so oracle should get right position for it and it requires 5 comparisons for that (it compares with 10th value, then with 6th, 3rd, 2nd and 1st values from top10). Obviously it makes less comparisons for the first 10 rows.

Now let’s talk about statements #1 and #2:
We know that rownum forces optimizer_mode to switch to “first K rows”, because of the parameter “_optimizer_rownum_pred_based_fkr”

SQL> @param_ rownum

NAME                               VALUE  DEFLT  TYPE      DESCRIPTION
---------------------------------- ------ ------ --------- ------------------------------------------------------
_optimizer_rownum_bind_default     10     TRUE   number    Default value to use for rownum bind
_optimizer_rownum_pred_based_fkr   TRUE   TRUE   boolean   enable the use of first K rows due to rownum predicate
_px_rownum_pd                      TRUE   TRUE   boolean   turn off/on parallel rownum pushdown optimization

while fetch first/row_number doesn’t (it will be changed after the patch #22174392) and it leads to the following consequences:
1. first_rows disables serial direct reads optimization(or smartscan on Exadata), that’s why the tests with big tables showed that “fetch first” were much faster than the query with rownum.
So if we set “_serial_direct_read”=always, we get the same performance in both tests (within the margin of error).

2. In cases when index access (index full scan/index range scan) is better, CBO differently calculates the cardinality of underlying INDEX FULL(range) SCAN:
the query with rownum is optimized for first_k_rows and the cardinality of index access is equal to K rows, but CBO doesn’t reduce cardinality for “fetch first”, so the cost of index access is much higher, compare them:

rownum

[sourcecode language=”sql” highlight=”13″]
SQL> explain plan for
2 select *
3 from (select * from test order by a,b)
4 where rownum<=10;

——————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
——————————————————————————————–
| 0 | SELECT STATEMENT | | 10 | 390 | 4 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | VIEW | | 10 | 390 | 4 (0)| 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| TEST | 1000K| 12M| 4 (0)| 00:00:01 |
| 4 | INDEX FULL SCAN | IX_TEST_AB | 10 | | 3 (0)| 00:00:01 |
——————————————————————————————–

Predicate Information (identified by operation id):
—————————————————

1 – filter(ROWNUM<=10)
[/sourcecode]

[collapse]
fetch first

[sourcecode language=”sql” highlight=”13″]
SQL> explain plan for
2 select *
3 from test
4 order by a,b
5 fetch first 10 rows only;

—————————————————————————————–
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
—————————————————————————————–
| 0 | SELECT STATEMENT | | 10 | 780 | | 5438 (1)| 00:00:01 |
|* 1 | VIEW | | 10 | 780 | | 5438 (1)| 00:00:01 |
|* 2 | WINDOW SORT PUSHED RANK| | 1000K| 12M| 22M| 5438 (1)| 00:00:01 |
| 3 | TABLE ACCESS FULL | TEST | 1000K| 12M| | 690 (1)| 00:00:01 |
—————————————————————————————–

Predicate Information (identified by operation id):
—————————————————

1 – filter("from$_subquery$_002"."rowlimit_$$_rownumber"<=10)
2 – filter(ROW_NUMBER() OVER ( ORDER BY "TEST"."A","TEST"."B")<=10)
[/sourcecode]

[collapse]
fetch first + first_rows

[sourcecode language=”sql” highlight=”14″]
SQL> explain plan for
2 select/*+ first_rows */ *
3 from test
4 order by a,b
5 fetch first 10 rows only;

——————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
——————————————————————————————–
| 0 | SELECT STATEMENT | | 10 | 780 | 27376 (1)| 00:00:02 |
|* 1 | VIEW | | 10 | 780 | 27376 (1)| 00:00:02 |
|* 2 | WINDOW NOSORT STOPKEY | | 1000K| 12M| 27376 (1)| 00:00:02 |
| 3 | TABLE ACCESS BY INDEX ROWID| TEST | 1000K| 12M| 27376 (1)| 00:00:02 |
| 4 | INDEX FULL SCAN | IX_TEST_AB | 1000K| | 2637 (1)| 00:00:01 |
——————————————————————————————–

Predicate Information (identified by operation id):
—————————————————

1 – filter("from$_subquery$_002"."rowlimit_$$_rownumber"<=10)
2 – filter(ROW_NUMBER() OVER ( ORDER BY "TEST"."A","TEST"."B")<=10)
[/sourcecode]

[collapse]
fetch first + index

[sourcecode language=”sql” highlight=”14″]
SQL> explain plan for
2 select/*+ index(test (a,b)) */ *
3 from test
4 order by a,b
5 fetch first 10 rows only;

——————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
——————————————————————————————–
| 0 | SELECT STATEMENT | | 10 | 780 | 27376 (1)| 00:00:02 |
|* 1 | VIEW | | 10 | 780 | 27376 (1)| 00:00:02 |
|* 2 | WINDOW NOSORT STOPKEY | | 1000K| 12M| 27376 (1)| 00:00:02 |
| 3 | TABLE ACCESS BY INDEX ROWID| TEST | 1000K| 12M| 27376 (1)| 00:00:02 |
| 4 | INDEX FULL SCAN | IX_TEST_AB | 1000K| | 2637 (1)| 00:00:01 |
——————————————————————————————–

Predicate Information (identified by operation id):
—————————————————

1 – filter("from$_subquery$_002"."rowlimit_$$_rownumber"<=10)
2 – filter(ROW_NUMBER() OVER ( ORDER BY "TEST"."A","TEST"."B")<=10)
[/sourcecode]

[collapse]

So in this case we can add hints “first_rows” or “index”, or install the patch #22174392.

ps. I thought to post this note later, since I hadn’t time enough to add other interesting details about the different TopN variants, including “with tie”, rank(), etc, so I’ll post another note with more details later.

Docker with Oracle database: install patches automatically

Posted on by Posted in Docker, oracle 1,934 Page views Leave a comment

Recently I had to install the patch for fixing cross-platform PDB transport bug onto the docker images with Oracle, so these are easy way how to do it:

1. create directory “patches” and create “install_patches.sh”: 

#!/bin/bash

unzip  -u ./*.zip
CURDIR=`pwd`

for D in *; do
    if [ -d "${D}" ]; then
        echo =================================================
        echo " *** Processing patch # ${D}... "   # your processing here
        cd "${D}"
        opatch apply -silent
    fi
    cd $CURDIR
done

2. add the following commands into the dockerfile:

USER root

# Copy DB install file
COPY --chown=oracle:dba patches $INSTALL_DIR/patches

# Install DB software binaries
USER oracle
RUN chmod ug+x $INSTALL_DIR/patches/*.sh && \
    sync && \
    cd $INSTALL_DIR/patches && \
    ./install_patches.sh

3. put downloaded patches into the “patches” directory and build image.

For example, dockerfile for 18.3:

FROM oracle/database:18.3.0-ee

MAINTAINER Sayan Malakshinov <sayan@orasql.org>

USER root

# Copy patches:
COPY --chown=oracle:dba patches $INSTALL_DIR/patches

# Install patches:
USER oracle
RUN chmod ug+x $INSTALL_DIR/patches/*.sh && \
    sync && \
    cd $INSTALL_DIR/patches && \
    ./install_patches.sh

ps. I’ve also create the request for that in the official Docker github: https://github.com/oracle/docker-images/issues/1070

Compression in a well-balanced system

Posted on by Posted in oracle, SmartScan 1,759 Page views Leave a comment

Exadata is designed to present a well-balanced system between disk scan rates, cell CPU processing rates, network bandwidth, and RDBMS CPU such that on average across a lot of workloads and a lot of applications there should not be no one obvious bottleneck. Good performance engineering means avoiding a design like this:

because that would be nuts. If you change one component in a car e.g. the engine you need to rework the other components such as the chassis and the brakes. When we rev the Exadata hardware we go through the same exercise to make sure the components match to avoid bottlenecks. 

One of the components in a well-balanced system that people often leave off this list is compression algorithm. With compression you are balancing storage costs as well as the disk scan rates against the CPU’s decompression rate. In a disk bound system you can alleviate the bottleneck by compressing the data down to the point that it is being scanned at the same rate that the CPUs can decompress and process it. Squeeze it too far, say with HCC “Compress For Archive High” which uses BZ2, the CPUs become a major bottleneck because the decompression costs unbalance the system (but it’s wonderful if you think you’re unlikely to ever need the data again).

Continue reading→

Create Quarantine

Posted on by Posted in cell_offload, oracle, SmartScan 1,704 Page views Leave a comment

First if you want don’t know what an Exadata Quarantine is read this.

Someone asked whether you can create your own Exadata Cell quarantine and, if you can, why you might ever want to do it? 

The first step when you don’t know how to do something is try HELP in cellcli

CellCLI> HELP
...
ALTER QUARANTINE
...
CREATE QUARANTINE
...
DROP QUARANTINE
...
LIST QUARANTINE

So we see we can create a quarantine, so we use HELP again:

Continue reading→

Shining some light on Database In-Memory vs the Exadata Columnar Cache in 12.1.0.2

Posted on by Posted in cell_offload, inmemory, oracle, SmartScan, trace 1,558 Page views Leave a comment

I posted a while back on how to use Tracing Hybrid Columnar Compression in an offload server so this is a quick follow up.

  1. I have trouble remembering the syntax for setting a regular parameter in an offload server without bouncing it. Since I need to keep this written down somewhere I thought it might be use to support folks and dbas.
  2. I forgot to show you how to specify which offload group to set the trace event

So this example should do both: 

CellCLI > alter cell offloadGroupEvents = "immediate cellsrv.cellsrv_setparam('my_parameter, 'TRUE')", offloadGroupName = "SYS_122110_160621"

this will, of course, set a parameter temporarily until the next time the offload server is bounced, but also adding it to the offload group’s init.ora will take care of that.

Create External Table as Select

Posted on by Posted in curious, oracle, SmartScan 2,034 Page views Leave a comment

I was looking through a test script and saw something I didn’t know you could do in Oracle. I mentioned it to an Oracle ACE and he didn’t know it either. I then said to the External Table engineers “Oh I see you’ve added this cool new feature” and he replied dryly – “Yes, we added it in Oracle 10.1”. Ouch! So just in case you also didn’t know, you can create an External Table using a CTAS and the ORACLE_DATAPUMP driver.

This feature only work with the ORACLE_DATAPUMP access driver (it does NOT work with with the LOADER, HIVE, or HDFS drivers) and we can use it like this:

SQL> create table cet_test organization external
  2  (
  3    type ORACLE_DATAPUMP
  4    default directory T_WORK
  5    location ('xt_test01.dmp','xt_test02.dmp')
  6  ) parallel 2
  7  as select * from lineitem
 
Table created.

Checking the results shows us

-rw-rw---- ... 786554880 Mar 9 10:48 xt_test01.dmp 
-rw-rw---- ... 760041472 Mar 9 10:48 xt_test02.dmp

This can be a great way of creating a (redacted) sample of data to give to a developer to test or for a bug repro to give to support or to move between systems. 

Understanding External Table URowids

Posted on by Posted in External tables, oracle, SmartScan 1,746 Page views Leave a comment

I finally found time to get back to External Tables and have a list of blog posts I need to write on this topic. Here’s a brief one.

DBMS_ROWID will nicely break down a heap table’s rowid for you into file number, block number, and row number but it doesn’t handle the rowids coming from External Tables. So let’s look at how to make sense of them. They fall under the datatype UROWID which is a nominally opaque rowid defined by the data source. The first byte of a UROWID tells you which data source it came from and consequently how to deconstruct it.

The easiest way to see what is happening is via the SQL Dump function:

SQL> column xtrowid format a55
SQL> select c_custkey, dump(rowid,16) "XTROWID" from c_et 
  2> where c_custkey < 10;

 C_CUSTKEY XTROWID
---------- -------------------------------------------------------
         1 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,1
         2 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,2
         3 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,3
         4 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,4
         5 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,5
         6 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,6
         7 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,7
         8 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,8
         9 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,9

9 rows selected.
Continue reading→