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;
Monthly Archives: February 2015
select * from table where rownum=1
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.
INDEX FULL SCAN (MIN/MAX) with two identical MIN()
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)