Oracle SQL

Tag Archives: materialization

Materialization in subquery factoring without hint “materialize” can be considered only when exists at least one predicate

Posted on by Posted in CBO, oracle, query optimizing, undocumented 6 Comments

I found just now that materialization can not be considered by optimizer if there are no predicates in subquery factoring clause. Of course, i mean cases without forcing materialization through hint “materialize”.
Simple example:

Spoiler 


SQL> create table tt1 as select 1 id from dual;

Table created.

SQL> exec dbms_stats.gather_table_stats('','TT1');

PL/SQL procedure successfully completed.

SQL> explain plan for
  2  with gen as (select * from tt1)
  3  select * from gen,gen g2;

Explained.

SQL> @xplan

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------
Plan hash value: 486748850

-----------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |     1 |     6 |     6   (0)| 00:00:01 |
|   1 |  MERGE JOIN CARTESIAN|      |     1 |     6 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL  | TT1  |     1 |     3 |     3   (0)| 00:00:01 |
|   3 |   BUFFER SORT        |      |     1 |     3 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL | TT1  |     1 |     3 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------

SQL> explain plan for
  2  with gen as (select * from tt1 where 1=1)
  3  select * from gen,gen g2;

Explained.

SQL> @xplan

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------
Plan hash value: 2673059801

---------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name                       | Rows  | Bytes | Cost (%CPU)| Time  |
---------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |                            |     1 |    26 |     7   (0)| 00:00:01 |
|   1 |  TEMP TABLE TRANSFORMATION |                            |       |       |            |       |
|   2 |   LOAD AS SELECT           | SYS_TEMP_0FD9D6610_6641830 |       |       |            |       |
|   3 |    TABLE ACCESS FULL       | TT1                        |     1 |     3 |     3   (0)| 00:00:01 |
|   4 |   MERGE JOIN CARTESIAN     |                            |     1 |    26 |     4   (0)| 00:00:01 |
|   5 |    VIEW                    |                            |     1 |    13 |     2   (0)| 00:00:01 |
|   6 |     TABLE ACCESS FULL      | SYS_TEMP_0FD9D6610_6641830 |     1 |     3 |     2   (0)| 00:00:01 |
|   7 |    BUFFER SORT             |                            |     1 |    13 |     4   (0)| 00:00:01 |
|   8 |     VIEW                   |                            |     1 |    13 |     2   (0)| 00:00:01 |
|   9 |      TABLE ACCESS FULL     | SYS_TEMP_0FD9D6610_6641830 |     1 |     3 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------------

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Update: I did some additional tests and found:

  1. with “table()” but without “xmltable” materialization occurs always regardless of existence of predicates or another tables in subquery factoring clause
  2. with “xmltable” behavior is very strange – decision about materialization depends on ‘columns …’ clause: when it exists – materialization occurs, if not – not occurs.
  3. with “selects a subset of table columns” as David Aldridge said – decision still depends from predicates existence
Tests with table() were like this: 
with t as (select * from table(cast(:a as sys.ku$_vcnt)) /*where 1=0*/ )
select count(*) from t,t t2;
with t as (select * from t10,table(cast(:a as sys.ku$_vcnt)) /*where 1=0*/ )
select count(*) from t,t t2;

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Tests with xmltable() were like this: 

with t as (select * from t10,xmltable(:a ) ttt where 1=1)
select count(*)
from t, t t1;
with t as (select * from t10,xmltable(:a columns n int) ttt where 1=0)
select count(*)
from t, t t1;
with t as (select/*+ no_merge */ * from table(cast(:a as sys.ku$_vcnt)),xmltable(:a) where 1=0 )
select count(*) from t,t t2;

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Test with 2 from 1000 columns 

declare 
  c  varchar2(32767):='create table t_1000_cols as select ';
  c2 varchar2(32767);
begin
  for i in 1..1000 loop
    c2:=c2||',lpad(1,4000,1) c'||i;
  end loop;
  c:=c||ltrim(c2,',')||' from dual connect by level<=100';
  execute immediate c;
end;
/
exec dbms_stats.gather_table_stats('','T_1000_COLS');
alter session set tracefile_identifier = mat1000;
alter session set events='10053 trace name context forever, level 1';

with t as (select c1,c2 from t_1000_cols)
select count(*)
from t, t t2;

with t as (select c1,c2 from t_1000_cols where 1=1)
select count(*)
from t, t t2;

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