Category Archives: oracle

Create External Table as Select

Posted on by Posted in curious, oracle, SmartScan 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.ย 

Understanding External Table URowids

Posted on by Posted in External tables, oracle, SmartScan 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→

How to tell if the Exadata column cache is fully loaded

Posted on by Posted in oracle, SmartScan, statistics, troubleshooting 1 Comment

When a background activity is happening on the cell you typically can’t use RDBMS v$ views to monitor it in the same way. One such question is how to tell if a segment is fully loaded in the Exadata column cache since this does not appear in the equivalent In-Memory v$ views.

When a segment is scanned by Smart Scan sufficiently often to be eligible the AUTOKEEP pool (typically that means at least twice an hour), the eligible 1MB chunks are written to flash in 12.1.0.2 style format, and put on a background queue. Lower priority tasks pick up the queued 1MB 12.1.0.2 format chunks from the flash cache, run them though the In-Memory loader, and rewrite the pure columnar representation in place of the old 12.1.0.2 style column cache chunks.

The easiest way that I know of to tell when this completes is to monitor that background activity is to use the following query until it shows zero:

select name, sum(value) value from (
      select extractvalue(value(t),'/stat/@name') name,
            extractvalue(value(t),'/stat') value
      from v$cell_state cs,
           table(xmlsequence(extract(xmltype(cs.statistics_value),
                                     '//stats[@type="columnarcache"]/stat'))) t
     where statistics_type='CELL')
     where name in ('outstanding_imcpop_requests')
     group by name;

Top N biggest tables (with lobs, indexes and nested table)

Posted on by Posted in oracle Leave a comment

Script for SQL*Plus: https://github.com/xtender/xt_scripts/blob/master/tops/top_seg_by_size.sql

with 
  seg as (
     select 
       owner,segment_name
      ,segment_type
      ,tablespace_name
      ,sum(blocks) blocks
      ,sum(bytes)  bytes
     from dba_segments s
     where  segment_type not in (
       'TYPE2 UNDO'
      ,'ROLLBACK'
      ,'SYSTEM STATISTICS'
     )
     and segment_name not like 'BIN$%' --not in recyclebin
     and owner like '&owner_mask' -- you can specify schema here
     group by owner,segment_name,segment_type,tablespace_name
  )
 ,segs as (
     select 
       owner,segment_name
      ,case when segment_name like 'DR$%$%' then 'CTX INDEX' else segment_type end segment_type
      ,tablespace_name
      ,case 
         when segment_name like 'DR$%$%' 
           then (select table_owner||'.'||table_name from dba_indexes i where i.owner=s.owner and i.index_name = substr(segment_name,4,length(segment_name)-5))
         when segment_type in ('TABLE','TABLE PARTITION','TABLE SUBPARTITION')
            then owner||'.'||segment_name
         when segment_type in ('INDEX','INDEX PARTITION','INDEX SUBPARTITION')
            then (select i.table_owner||'.'||i.table_name from dba_indexes i where i.owner=s.owner and i.index_name=s.segment_name)
         when segment_type in ('LOBSEGMENT','LOB PARTITION','LOB SUBPARTITION')
            then (select l.owner||'.'||l.TABLE_NAME from dba_lobs l where l.segment_name = s.segment_name and l.owner = s.owner)
         when segment_type = 'LOBINDEX'
            then (select l.owner||'.'||l.TABLE_NAME from dba_lobs l where l.index_name = s.segment_name and l.owner = s.owner)
         when segment_type = 'NESTED TABLE'
            then (select nt.owner||'.'||nt.parent_table_name from dba_nested_tables nt where nt.owner=s.owner and nt.table_name=s.segment_name)
         when segment_type = 'CLUSTER'
            then (select min(owner||'.'||table_name) from dba_tables t where t.owner=s.owner and t.cluster_name=s.segment_name and rownum=1)
       end table_name
      ,blocks
      ,bytes
     from seg s
  )
 ,so as (
     select
       segs.owner
      ,substr(segs.table_name,instr(segs.table_name,'.')+1) TABLE_NAME
      ,sum(segs.bytes)  total_bytes
      ,sum(segs.blocks) total_blocks
      ,sum(case when segs.segment_type in ('TABLE','TABLE PARTITION','TABLE SUBPARTITION','NESTED TABLE','CLUSTER') then segs.bytes end) tab_size
      ,sum(case when segs.segment_type in ('INDEX','INDEX PARTITION','INDEX SUBPARTITION','CTX INDEX') then segs.bytes end) ind_size
      ,sum(case when segs.segment_type in ('CTX INDEX') then segs.bytes end) ctx_size
      ,sum(case when segs.segment_type in ('LOBSEGMENT','LOBINDEX','LOB PARTITION','LOB SUBPARTITION') then segs.bytes end) lob_size
     from segs
     group by owner,table_name
  )
 ,tops as (
     select 
           dense_rank()over (order by total_bytes desc) rnk
          ,so.*
     from so
  )
select *
from tops
where rnk<=50 -- top 50
/

“Collection iterator pickler fetch”: pipelined vs simple table functions

Posted on by Posted in oracle, PL/SQL, PL/SQL optimization, query optimizing, SQL, troubleshooting 2 Comments

Alex R recently discovered interesting thing: in SQL pipelined functions work much faster than simple non-pipelined table functions, so if you already have simple non-pipelined table function and want to get its results in sql (select * from table(fff)), it’s much better to create another pipelined function which will get and return its results through PIPE ROW().

A bit more details:

Assume we need to return collection “RESULT” from PL/SQL function into SQL query “select * from table(function_F(…))”.
If we create 2 similar functions: pipelined f_pipe and simple non-pipelined f_non_pipe, 

create or replace function f_pipe(n int) return tt_id_value pipelined 
as
  result tt_id_value;
begin
  ...
  for i in 1..n loop
    pipe row (result(i));
  end loop;
end f_pipe;
/
create or replace function f_non_pipe(n int) return tt_id_value 
as
  result tt_id_value;
begin
  ...
  return result;
end f_non_pipe;
/

Full functions definitions 

create or replace type to_id_value as object (id int, value int)
/
create or replace type tt_id_value as table of to_id_value
/
create or replace function f_pipe(n int) return tt_id_value pipelined 
as
  result tt_id_value;
  
  procedure gen is
  begin
     result:=tt_id_value();
     result.extend(n);
     for i in 1..n loop
        result(i):=to_id_value(i, 1);
     end loop;
  end;    
begin
  gen();
  for i in 1..n loop
    pipe row (result(i));
  end loop;
end f_pipe;
/
create or replace function f_non_pipe(n int) return tt_id_value 
as
  result tt_id_value;
  
  procedure gen is
  begin
     result:=tt_id_value();
     result.extend(n);
     for i in 1..n loop
        result(i):=to_id_value(i, 1);
     end loop;
  end;    
begin
  gen();
  return result;
end f_non_pipe;
/
create or replace function f_pipe_for_nonpipe(n int) return tt_id_value pipelined 
as
  result tt_id_value;
begin
  result:=f_non_pipe(n);
  for i in 1..result.count loop
    pipe row (result(i));
  end loop;
end;
/
create or replace function f_udf_pipe(n int) return tt_id_value pipelined 
as
  result tt_id_value;
  
  procedure gen is
  begin
     result:=tt_id_value();
     result.extend(n);
     for i in 1..n loop
        result(i):=to_id_value(i, 1);
     end loop;
  end;    
begin
  gen();
  for i in 1..n loop
    pipe row (result(i));
  end loop;
end;
/
create or replace function f_udf_non_pipe(n int) return tt_id_value 
as
  result tt_id_value;
  
  procedure gen is
  begin
     result:=tt_id_value();
     result.extend(n);
     for i in 1..n loop
        result(i):=to_id_value(i, 1);
     end loop;
  end;    
begin
  gen();
  return result;
end;
/

[collapse]
Test queries 

set echo on feed only timing on;
--alter session set optimizer_adaptive_plans=false;
--alter session set "_optimizer_use_feedback"=false;

select sum(id * value) s from table(f_pipe(&1));
select sum(id * value) s from table(f_non_pipe(&1));
select sum(id * value) s from table(f_pipe_for_nonpipe(&1));
select sum(id * value) s from table(f_udf_pipe(&1));
select sum(id * value) s from table(f_udf_non_pipe(&1));
with function f_inline_non_pipe(n int) return tt_id_value 
as
  result tt_id_value;
begin
     result:=tt_id_value();
     result.extend(n);
     for i in 1..n loop
        result(i):=to_id_value(i, 1);
     end loop;
     return result;
end;
select sum(id * value) s from table(f_inline_non_pipe(&1));
/
set timing off echo off feed on;

[collapse]

we’ll find that the function with simple “return result” works at least twice slower than pipelined function:

Function 1 000 000 elements 100 000 elements
F_PIPE 2.46 0.20
F_NON_PIPE 4.39 0.44
F_PIPE_FOR_NONPIPE 2.61 0.26
F_UDF_PIPE 2.06 0.20
F_UDF_NON_PIPE 4.46 0.44

I was really surprised that even “COLLECTION ITERATOR PICKLER FETCH” with F_PIPE_FOR_NONPIPE that gets result of F_NON_PIPE and returns it through PIPE ROW() works almost twice faster than F_NON_PIPE, so I decided to analyze it using stapflame by Frits Hoogland.

I added “dbms_lock.sleep(1)” into both of these function after collection generation, to compare the difference only between “pipe row” in loop and “return result”:

Modified functions 

create or replace function f_pipe(n int) return tt_id_value pipelined 
as
  result tt_id_value;
  
  procedure gen is
  begin
     result:=tt_id_value();
     result.extend(n);
     for i in 1..n loop
        result(i):=to_id_value(i, 1);
     end loop;
  end;    
begin
  gen();
  dbms_lock.sleep(1);
  for i in 1..n loop
    pipe row (result(i));
  end loop;
end f_pipe;
/
create or replace function f_non_pipe(n int) return tt_id_value 
as
  result tt_id_value;
  
  procedure gen is
  begin
     result:=tt_id_value();
     result.extend(n);
     for i in 1..n loop
        result(i):=to_id_value(i, 1);
     end loop;
  end;    
begin
  gen();
  dbms_lock.sleep(1);
  return result;
end f_non_pipe;
/

[collapse]

And stapflame showed that almost all overhead was consumed by the function “kgmpoa_Assign_Out_Arguments”:

I don’t know what this function is doing exactly, but we can see that oracle assign collection a bit later.
From other functions in this stack(pmucpkl, kopp2isize, kopp2colsize, kopp2atsize(attribute?), kopuadt) I suspect that is some type of preprocessiong of return arguments.
What do you think about it?

Full stapflame output:
stapflame_nonpipe
stapflame_pipe

SQL*Plus tips #8: How to read the output of dbms_output without “serveroutput on”

Posted on by Posted in oracle, SQL*Plus, SQL*PLus tips 1 Comment

When “serveroutput” is enabled, SQL*Plus executes “BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;” after each command.
That’s why I don’t like when it is always enabled: it adds extra calls and round-trips and it is inconvenient when I want to get a plan of the last executed query:

SQL> set serverout on;
SQL> select * from dual;

D
-
X

SQL> select * from table(dbms_xplan.display_cursor('','','allstats last'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
SQL_ID  9babjv8yq8ru3, child number 0

BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;

NOTE: cannot fetch plan for SQL_ID: 9babjv8yq8ru3, CHILD_NUMBER: 0
      Please verify value of SQL_ID and CHILD_NUMBER;
      It could also be that the plan is no longer in cursor cache (check v$sql_plan)

So usually I switch “serveroutput” on only if needed, but sometimes I can forget to enable it. In such cases I use very simple script that reads the output using dbms_output.get_lines and prints it using refcursor:
https://github.com/xtender/xt_scripts/blob/master/output_print.sql

When you set “serveroutput on“, SQL*Plus also executes “dbms_output.enable” and if you set “serverout off” it executes “dbms_output.disable”, that’s why my glogin.sql contains “call dbms_output.enable(1e6);” and you need to execute it after each “set serverout off” if you want to use this script.

Bug with integer literals in PL/SQL

Posted on by Posted in curious, oracle, PL/SQL, PL/SQL optimization, undocumented Leave a comment

This interesting question was posted on our russian forum yesterday:

We have a huge PL/SQL package and this simple function returns wrong result when it’s located at the end of package body:

create or replace package body PKGXXX as
  ...
  function ffff return number is
  nRes number;
  begin        
    nRes :=  268435456;
    return nRes;
  end;
end;
/

But it works fine in any of the following cases:
* replace 268435456 with power(2, 28), or
* replace 268435456 with small literal like 268, or
* move this function to the beginning of package body

The one of the interesting findings was that the returned value is equal to the one of literals in another function.
We can reproduce this bug even with an anonymous pl/sql block. The following test case uses 32768 integer literals from 1000001 to 1032768 and prints 5 other integers:

declare n number;
begin
  n:=1000001; -- this part
  n:=1000002; -- creates
  n:=1000003; -- 32768 
   ...        -- integer
  n:=1032768; -- literals
    dbms_output.put_line('100000='||100000); -- it should print: 100000=100000
    dbms_output.put_line('32766 ='||32766);
    dbms_output.put_line('32767 ='||32767);    
    dbms_output.put_line('32768 ='||32768);
    dbms_output.put_line('32769 ='||32769);
end;

Test code 

declare
   c clob:='declare n number;begin'||chr(10);
   f varchar2(100):='n:=%s;'||chr(10);
   v varchar2(32767);
   n number:=32768;
begin
   for i in 1..n loop
      v:=v||utl_lms.format_message(f,to_char(1e7+i));
      if length(v)>30000 then
         c:=c||v;
         v:='';
      end if;
   end loop;
   v:=v||q'[
    dbms_output.put_line('100000='||100000);
    dbms_output.put_line('32766 ='||32766);
    dbms_output.put_line('32767 ='||32767);    
    dbms_output.put_line('32768 ='||32768);
    dbms_output.put_line('32769 ='||32769);
   end;
   ]';
   c:=c||v;
   execute immediate c;
end;
/

[collapse]
It produces the following output:

100000=10000001
32766 =32766
32767 =32767
32768 =10000002
32769 =10000003

This test case well demonstrates wrong results:
* instead of 100000 we get 10000001, which is the value from first line after “begin”, ie 1st integer literal in the code,
* for 32766 and 32767 oracle returns right values
* instead of 32768 (==32767+1) it returns 10000002, which is the integer from 2nd line, ie 2nd integer literal in the code,
* instead of 32769 (==32767+2) it returns 10000003, which is the integer from 3rd line, ie 3rd integer literal in the code
After several tests I can make a conclusion:

  • It doesn’t matter what plsql_optimize_level or plsql_code_type you set, was debug enabled or not, the behaviour is the same.
  • It seems that this is a kind of PL/SQL optimization: during parsing, oracle leaves integer literal in place if its value is in range -32768..32767 (16bit signed int), but if its value is out of this range, oracle adds this value into array of integers’ constants and replaces the value with the index of this element in this array. But because of index value overflow in cases when a count of such integer literals becomes larger than 32768, instead of Nth element of this array, oracle returns Mth element, where M is mod(N,32767).

So we can describe this behaviour using first test case:

declare n number;
begin
  n:=1000001; -- this part
  n:=1000002; -- creates
  n:=1000003; -- 32768 
   ...        -- integer
  n:=1032768; -- literals
    dbms_output.put_line('100000='||100000); -- it should print 100000, ie 32768th element of array, but prints 10000001
                                             -- where 10000001 is the 1st element of array (1==mod(32768,32767))
    dbms_output.put_line('32766 ='||32766);  -- these 2 lines print right values,
    dbms_output.put_line('32767 ='||32767);  -- because their values are in the range of -32768..32767
    dbms_output.put_line('32768 ='||32768);  -- this line contains 32769th element and prints 2nd element of array (2==mod(32769,32767))
    dbms_output.put_line('32769 ='||32769);  -- this line contains 32770th element and prints 3nd element of array (3==mod(32770,32767))
end;

The following query can help you to find objects which can potentially have this problem:

select
  s.owner,s.name,s.type
 ,sum(regexp_count(text,'(\W|^)3\d{4,}([^.0-9]|$)')) nums_count -- this regexp counts integer literals >= 30000
from dba_source s 
where 
    owner='&owner'
and type in ('FUNCTION','PROCEDURE','PACKAGE','PACKAGE BODY')
group by s.owner,s.name,s.type
having sum(regexp_count(text,'(\W|^)3\d{4,}([^.0-9]|$)'))>32767 -- filter only objects which have >=32767 integer literal

Workaround:
You may noticed that I wrote about INTEGER literals only, so the easiest workaround is to make them FLOAT – just add “.” to the end of each literal:

declare n number;
begin
  n:=1000001.;
  n:=1000002.;
  n:=1000003.;
   ...       
  n:=1032768.;
    dbms_output.put_line('100000='||100000.);
    dbms_output.put_line('32766 ='||32766.);
    dbms_output.put_line('32767 ='||32767.);    
    dbms_output.put_line('32768 ='||32768.);
    dbms_output.put_line('32769 ='||32769.);
end;

Fixed test cases 

declare
   c clob:='declare n number;begin'||chr(10);
   f varchar2(100):='n:=%s.;'||chr(10); -- I've added here "."
   v varchar2(32767);
   n number:=32768;
begin
   for i in 1..n loop
      v:=v||utl_lms.format_message(f,to_char(1e7+i));
      if length(v)>30000 then
         c:=c||v;
         v:='';
      end if;
   end loop;
   v:=v||q'[
    dbms_output.put_line('100000='||100000.); -- .
    dbms_output.put_line('32766 ='||32766.);
    dbms_output.put_line('32767 ='||32767.);    
    dbms_output.put_line('32768 ='||32768.);
    dbms_output.put_line('32769 ='||32769.);
   end;
   ]';
   c:=c||v;
   execute immediate c;
end;
/

[collapse]

Oracle issues after upgrade to 12.2

Posted on by Posted in 12.2, bug, oracle 2 Comments

Sometimes it’s really hard even to create reproducible test case to send it to oracle support, especially in case of intermittent errors.
In such cases, I think it would be really great to have access to similar service requests or bugs of other oracle clients.
So while my poll about knowledge sharing is still active, I want to share a couple of bugs we have faced after upgrade to 12.2 (and one bug from Eric van Roon). I’m going to remove the bugs from this list when they become “public” or “fixed”.
If you want to add own findings into this list, you can add them into comments. To make this process easier, you can provide just symptomps, short description and the link to own post with details – I’ll add it just as a link.
Continue reading→

Triggers and Redo: changes on 12.2

Posted on by Posted in 12c, oracle, trace 1 Comment

In one of the previous posts I showed How even empty trigger increases redo generation, but running the test from that post, I have found that this behaviour a bit changed on 12.2:
In my old test case, values of column A were equal to values of B, and on previous oracle versions including 12.1.0.2 we can see that even though “update … set B=A” doesn’t change really “B”, even empty trigger greatly increases redo generation.
But on 12.2.0.1 in case of equal values, the trigger doesn’t increase redo, so we can see small optimization here, though in case of different values, the trigger still increases reado generation greatly.

same_dumpredo.sql 

set feed on;
drop table xt_curr1 purge;
drop table xt_curr2 purge;
-- simple table:
create table xt_curr1 as select '2' a, '2' b from dual connect by level<=1000;
-- same table but with empty trigger:
create table xt_curr2 as select '2' a, '2' b from dual connect by level<=1000;
create or replace trigger tr_xt_curr2 before update on xt_curr2 for each row
begin
  null;
end;
/
-- objectID and SCN:
col obj1 new_val obj1;
col obj2 new_val obj2;
col scn  new_val scn;
select 
  (select o.OBJECT_ID from user_objects o where o.object_name='XT_CURR1') obj1
 ,(select o.OBJECT_ID from user_objects o where o.object_name='XT_CURR2') obj2
 ,d.CURRENT_SCN scn
from v$database d
/
-- logfile1:
alter system switch logfile;
col member new_val logfile;
SELECT member
FROM v$logfile
WHERE 
     is_recovery_dest_file='NO'
 and group#=(SELECT group# FROM v$log WHERE status = 'CURRENT')
 and rownum=1;
-- update1:
set autot trace stat;
update xt_curr1 set b=a;
set autot off;
commit;
-- dump logfile1:
alter session set tracefile_identifier='log1_same';
ALTER SYSTEM DUMP LOGFILE '&logfile' SCN MIN &scn OBJNO &obj1;

-- logfile2:
alter system switch logfile;
col member new_val logfile;
SELECT member
FROM v$logfile
WHERE 
     is_recovery_dest_file='NO'
 and group#=(SELECT group# FROM v$log WHERE status = 'CURRENT')
 and rownum=1;
-- update2:
set autot trace stat;
update xt_curr2 set b=a;
set autot off;
commit;
-- dump logfile2:
alter session set tracefile_identifier='log2_same';
ALTER SYSTEM DUMP LOGFILE '&logfile' OBJNO &obj2;
alter session set tracefile_identifier='off';
disc;

[collapse]

diff_dumpredo.sql 

set feed on;
drop table xt_curr1 purge;
drop table xt_curr2 purge;
-- simple table:
create table xt_curr1 as select '1' a, '2' b from dual connect by level<=1000;
-- same table but with empty trigger:
create table xt_curr2 as select '1' a, '2' b from dual connect by level<=1000;
create or replace trigger tr_xt_curr2 before update on xt_curr2 for each row
begin
  null;
end;
/
-- objectID and SCN:
col obj1 new_val obj1;
col obj2 new_val obj2;
col scn  new_val scn;
select 
  (select o.OBJECT_ID from user_objects o where o.object_name='XT_CURR1') obj1
 ,(select o.OBJECT_ID from user_objects o where o.object_name='XT_CURR2') obj2
 ,d.CURRENT_SCN scn
from v$database d
/
-- logfile1:
alter system switch logfile;
col member new_val logfile;
SELECT member
FROM v$logfile
WHERE 
     is_recovery_dest_file='NO'
 and group#=(SELECT group# FROM v$log WHERE status = 'CURRENT')
 and rownum=1;
-- update1:
set autot trace stat;
update xt_curr1 set b=a;
set autot off;
commit;
-- dump logfile1:
alter session set tracefile_identifier='log1_diff';
ALTER SYSTEM DUMP LOGFILE '&logfile' SCN MIN &scn OBJNO &obj1;

-- logfile2:
alter system switch logfile;
col member new_val logfile;
SELECT member
FROM v$logfile
WHERE 
     is_recovery_dest_file='NO'
 and group#=(SELECT group# FROM v$log WHERE status = 'CURRENT')
 and rownum=1;
-- update2:
set autot trace stat;
update xt_curr2 set b=a;
set autot off;
commit;
-- dump logfile2:
alter session set tracefile_identifier='log2_diff';
ALTER SYSTEM DUMP LOGFILE '&logfile' OBJNO &obj2;
alter session set tracefile_identifier='off';
disc;

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Equal values:
12.1.0.2:

12.2.0.1:

Different values:
12.1.0.2:

12.2.0.1:

We can easily find that trigger disables batched “Array update”:

Adaptive serial direct path read decision ignores object statistics since 12.1

Posted on by Posted in 12c, adaptive serial direct path reads, oracle 1 Comment

On versions 11.2.0.2 – 11.2.0.4 Oracle uses objects’ statistics to make direct path reads decision (of course, if “_direct_read_decision_statistics_driven” haven’t changed it to “false”), and we can force serial direct reads on statement level using sql profiles with hints INDEX_STATS/TABLES_STATS, but since at least 12.1.0.2 this decision ignores statistics.

Btw, thanks to Jure Bratina, we know now, that we need to repeat hint TABLE_STATS at least twice to make it work ๐Ÿ™‚ And from the following test case we know that it takes parameters from second one ๐Ÿ™‚

Compare trace files of the following test cases:

table_stats2.sql 

drop table t1;

create table t1 as select * from dual;
----------------------------
pro ######################################;
exec dbms_stats.gather_table_stats('','T1');
exec dbms_stats.set_table_stats(user,'T1',numblks => 33333333);

col value new_val oldval noprint;
select value from v$statname n, v$mystat s
where n.statistic#=s.statistic# and n.name ='physical reads direct';

----------------------------
alter session set tracefile_identifier='table_stats2';
--alter session set events '10053 trace name context forever, level 1';
alter session set events 'trace[nsmtio] disk highest';
select/*+ table_stats(t1, scale, blocks=66666666 rows=2222222222) 
          table_stats(t1, scale, blocks=99999999 rows=4444444444)
       */ 
       * 
from t1;
select value-&oldval directreads, value from v$statname n, v$mystat s
where n.statistic#=s.statistic# and n.name ='physical reads direct';

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You can see that our hint successfully changed number of blocks and forced direct path reads on 11.2.0.4:
Oracle 11.2.0.4: 

NSMTIO: qertbFetch:DirectRead:[OBJECT_SIZE>VLOT]
NSMTIO: Additional Info: VLOT=797445
Object# = 78376, Object_Size = 66666666 blocks
SqlId = 7naawntkc57yx, plan_hash_value = 3617692013, Partition# = 0

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But on 12.1.0.2 and 12.2.0.1 we can see 2 lines with “NSMTIO: kcbism” with the different types(2 and 3) and different number of blocks, and the direct path read decision was based on second one – from segment header:

Oracle 12.1.0.2: 

NSMTIO: kcbism: islarge 1 next 0 nblks 66666666 type 3, bpid 65535, kcbisdbfc 0 kcbnhl 4096 kcbstt 983 keep_nb 0 kcbnbh 59010 kcbnwp 1
NSMTIO: kcbism: islarge 0 next 0 nblks 4 type 2, bpid 3, kcbisdbfc 0 kcbnhl 4096 kcbstt 983 keep_nb 0 kcbnbh 59010 kcbnwp 1
NSMTIO: qertbFetch:NoDirectRead:[- STT < OBJECT_SIZE < MTT]:Obect's size: 4 (blocks), Threshold: MTT(4917 blocks),
_object_statistics: enabled, Sage: enabled, 
Direct Read for serial qry: enabled(::::::), Ascending SCN table scan: FALSE 
flashback_table_scan: FALSE, Row Versions Query: FALSE
SqlId: 7naawntkc57yx, plan_hash_value: 3617692013, Object#: 302342, Parition#: 0 DW_scan: disabled

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Oracle 12.2.0.1: 

NSMTIO: kcbism: islarge 1 next 0 nblks 66666666 type 3, bpid 65535, kcbisdbfc 0 kcbnhl 4096 kcbstt 1214 keep_nb 0 kcbnbh 45026 kcbnwp 1
NSMTIO: kcbism: islarge 0 next 0 nblks 4 type 2, bpid 3, kcbisdbfc 0 kcbnhl 4096 kcbstt 1214 keep_nb 0 kcbnbh 45026 kcbnwp 1
NSMTIO: qertbFetch:NoDirectRead:[- STT < OBJECT_SIZE < MTT]:Obect's size: 4 (blocks), Threshold: MTT(6072 blocks),
_object_statistics: enabled, Sage: enabled, 
Direct Read for serial qry: enabled(:::::::), Ascending SCN table scan: FALSE 
flashback_table_scan: FALSE, Row Versions Query: FALSE
SqlId: 7naawntkc57yx, plan_hash_value: 3617692013, Object#: 174411, Parition#: 0 DW_scan: disabled

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And similar example, but for IFFS(index fast full scan):
nb: I set the number of index blocks using dbms_stats to 33333000 and hinted the query with 77777700

index_stats2.sql 

drop table t2 purge;
ALTER SESSION SET optimizer_dynamic_sampling = 0;
ALTER SESSION SET "_optimizer_use_feedback" = FALSE;
ALTER SESSION SET optimizer_adaptive_features = FALSE;
ALTER SESSION SET optimizer_adaptive_plans=FALSE;

create table t2(x) as select level from dual connect by level<=1000;
create index t2_ix on t2(1,x,rpad(x,100));

begin
   dbms_stats.gather_table_stats('','T2',cascade => true);
   dbms_stats.set_table_stats(user,'T2'   ,numblks => 33333333);
   dbms_stats.set_index_stats(user,'T2_IX',numlblks => 33333000);
end;
/

col value new_val oldval noprint;
select value from v$statname n, v$mystat s
where n.statistic#=s.statistic# and n.name ='physical reads direct';

alter session set tracefile_identifier='index_stats2';
alter session set events 'trace[nsmtio] disk highest';
select/*+ index_stats(t2, t2_ix, scale, blocks=7777700)
          index_ffs(t2 t2_ix)
          dynamic_sampling(0)
       */ 
       count(*) cnt2
from t2;
select value-&oldval directreads, value from v$statname n, v$mystat s
where n.statistic#=s.statistic# and n.name ='physical reads direct';

disc;

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You can see that on 11.2.0.4 oracle gets number of blocks from the hint (7777700)
Oracle 11.2.0.4 - index_stats: 

NSMTIO: qerixFetchFastFullScan:DirectRead[OBJECT_SIZE > VLOT]:
NSMTIO: AdditionalInfo: Object_size: 7777700 (blocks), vlot=797445
SqlId=by2zv0k566hj5, plan_hash_value=3419274230,Object#=78375

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From the first line we can see that kcbism takes the hinted number of blocks, but later kcbivlo rewrites it with the number from segment header:

Oracle 12.1.0.2 - index_stats: 

NSMTIO: kcbism: islarge 1 next 0 nblks 7777700 type 3, bpid 65535, kcbisdbfc 0 kcbnhl 4096 kcbstt 983 keep_nb 0 kcbnbh 59010 kcbnwp 1
NSMTIO: kcbivlo: nblks 22 vlot 500 pnb 49175 kcbisdbfc 0 is_large 0
NSMTIO: qerixFetchFastFullScan:[MTT < OBJECT_SIZE < VLOT]:NSMTIO: AdditionalInfo: Object_size: 22 (blocks), vlot=245875
SqlId=by2zv0k566hj5, plan_hash_value=3419274230,Object#=302347

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Oracle 12.2.0.1 - index_stats: 

NSMTIO: kcbism: islarge 1 next 0 nblks 7777700 type 3, bpid 65535, kcbisdbfc 0 kcbnhl 4096 kcbstt 1214 keep_nb 0 kcbnbh 45026 kcbnwp 1
NSMTIO: kcbivlo: nblks 22 vlot 500 pnb 60729 kcbisdbfc 0 is_large 0
NSMTIO: qerixFetchFastFullScan:[MTT < OBJECT_SIZE < VLOT]:NSMTIO: AdditionalInfo: Object_size: 22 (blocks), vlot=303645
SqlId=by2zv0k566hj5, plan_hash_value=3419274230,Object#=174409

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So we have 2 options to force direct reads:
1. to execute alter session set “_serial_direct_read”=’ALWAYS’;
2. or to force parallel plan to get parallel direct path reads (we can do it with even with dop=1)