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XTended Oracle SQL
Updated: 17 hours 1 min ago

12c: Little test of “TABLE ACCESS INMEMORY FULL” with count stopkey

Mon, 2015-03-16 17:02

The table has 9M rows:

SQL> with function f return int is
  2       begin
  3          for r in (select value from v$mystat natural join v$statname where name like 'IM scan rows') loop
  4             dbms_output.put_line(r.value);
  5             return r.value;
  6          end loop;
  7       end;
  8  select f() from t_inmemory where rownum<=1
  9  ;
 10  /

       F()
----------
         0

1 row selected.

SQL> /

       F()
----------
    491436

1 row selected.

SQL> /

       F()
----------
    982872

1 row selected.
DDL and Plan
create table t_inmemory inmemory
as 
with gen as (select 0 id from dual connect by level<=3e3)
select 0 n from gen,gen;

SQL_ID  cpgrrfv9h6m52, child number 0
-------------------------------------
with function f return int is      begin         for r in (select value
from v$mystat natural join v$statname where name like 'IM scan rows')
loop            dbms_output.put_line(r.value);            return
r.value;         end loop;      end; select f() from t_inmemory where
rownum<=1

Plan hash value: 3697881339

----------------------------------------------------------------------------------
| Id  | Operation                   | Name       | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |            |       |     3 (100)|          |
|*  1 |  COUNT STOPKEY              |            |       |            |          |
|   2 |   TABLE ACCESS INMEMORY FULL| T_INMEMORY |     1 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------

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

   1 - filter(ROWNUM<=1)

[collapse]

Categories: Development

Easy quiz: rownum < NaN

Tue, 2015-02-10 15:25

As you know, NaN is a “Not a Number”.
How do you think, what would be the result of the following query? (0f/0 == NaN)

select count(*) cnt from dual where rownum < 0f/0;

Answer
SQL> select count(*) cnt, 0f/0 from dual where rownum < 0f/0;

       CNT       0F/0
---------- ----------
         1        Nan

1 row selected.

[collapse]
Ok, when you know the result, try to guess what will return this query:

select count(*) cnt from dual where 1f/0 < 0f/0;
Answer
SQL> select count(*) cnt from dual where 1f/0 < 0f/0;

       CNT
----------
         1

1 row selected.

[collapse] PS

1 * X < 0 * Y

So we know now which non-negative values we should to substitute for X and Y in this expression, to make it true.

[collapse]
Categories: Development

select * from table where rownum=1

Sun, 2015-02-08 17:40

I never thought I would have to optimize so simple query as

select col1, col2, col4, col7 from table where rownum=1

(even though I read recently “SELECT * FROM TABLE” Runs Out Of TEMP Space)
But a few days ago frequent executions of this query caused big problems on the one of our databases(11.2.0.3) because of adaptive serial direct path reads.

I don’t know why, but I felt intuitively that full table scan with “First K rows” optimization (“_optimizer_rownum_pred_based_fkr“=true) should turn off adaptive serial direct path reads. It seems quite logical to me.

PS. Unfortunately I had a little time, so I didn’t investigate what process and why it was doing that, I just created profile with “index full scan” access, and it completely solved the problem.

Categories: Development

INDEX FULL SCAN (MIN/MAX) with two identical MIN()

Wed, 2015-02-04 09:39

I’ve just noticed an interesting thing:

Assume, that we have a simple query with “MIN(ID)” that works through “Index full scan(MIN/MAX)”:

SQL> explain plan for
  2  select
  3     min(ID)      as x
  4  from tab1
  5  where ID is not null;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
Plan hash value: 4170136576

---------------------------------------------------------------------------------------
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |     1 |     4 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE             |         |     1 |     4 |            |          |
|   2 |   FIRST ROW                 |         |     1 |     4 |     3   (0)| 00:00:01 |
|*  3 |    INDEX FULL SCAN (MIN/MAX)| IX_TAB1 |     1 |     4 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------------------

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

   3 - filter("ID" IS NOT NULL)

Test tables
create table tab1(id, x, padding)
as 
   with gen as (select level n from dual connect by level<=1000)
   select g1.n, g2.n, rpad(rownum,10,'x')
   from gen g1,gen g2;
create index ix_tab1 on tab1(id, x);
exec dbms_stats.gather_table_stats('','TAB1');

[collapse]
But look what will happen if we add one more “MIN(ID)”:

SQL> explain plan for
  2  select
  3     min(ID)      as x
  4   , min(ID)+1000 as x1000
  5  from tab1
  6  where ID is not null;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------
Plan hash value: 3397888171

---------------------------------------------------------------------------------
| Id  | Operation             | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |         |     1 |     4 |  3075  (17)| 00:00:02 |
|   1 |  SORT AGGREGATE       |         |     1 |     4 |            |          |
|*  2 |   INDEX FAST FULL SCAN| IX_TAB1 |   999K|  3906K|  3075  (17)| 00:00:02 |
---------------------------------------------------------------------------------

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

   2 - filter("ID" IS NOT NULL)

At first I thought it was the old limitation from “MIN/MAX index access is not used if query has multiple MIN/MAX functions (Doc ID 316467.1)”, but we can get same plan by using hint:

SQL> explain plan for
  2  select/*+ index(tab1) */
  3     min(ID)      as x
  4   , min(ID)+1000 as x1000
  5  from tab1
  6  where ID is not null;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
Plan hash value: 4170136576

---------------------------------------------------------------------------------------
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |     1 |     4 |  3433  (22)| 00:00:02 |
|   1 |  SORT AGGREGATE             |         |     1 |     4 |            |          |
|   2 |   FIRST ROW                 |         |   999K|  3906K|  3433  (22)| 00:00:02 |
|*  3 |    INDEX FULL SCAN (MIN/MAX)| IX_TAB1 |   999K|  3906K|  3433  (22)| 00:00:02 |
---------------------------------------------------------------------------------------

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

   3 - filter("ID" IS NOT NULL)

So, we can see that the real problem lies in the wrong cardinality calculation.

Of course, we can avoid this problem using the old method – with simple inner view:

SQL> explain plan for
  2  select x, x+1000
  3  from (
  4        select
  5           min(ID)      as x
  6        from tab1
  7        where ID is not null
  8       );

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------
Plan hash value: 2347179087

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |     1 |    13 |     3   (0)| 00:00:01 |
|   1 |  VIEW                        |         |     1 |    13 |     3   (0)| 00:00:01 |
|   2 |   SORT AGGREGATE             |         |     1 |     4 |            |          |
|   3 |    FIRST ROW                 |         |     1 |     4 |     3   (0)| 00:00:01 |
|*  4 |     INDEX FULL SCAN (MIN/MAX)| IX_TAB1 |     1 |     4 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

   4 - filter("ID" IS NOT NULL)
Categories: Development

Simple Android Oracle client

Mon, 2014-12-29 18:54

I am happy to announce, that I’ve just published my first android app – Simple oracle client for android!
Since this is only the first version, I’m sure that it contains various UI bugs, so I’ll wait for reviews and bug reports!

Several screenshots:


Screenshot_qe_activity

Screenshot_fullscreen_results

Screenshot_db_list_phone

Screenshot_phone2


Get it on Google Play

Categories: Development