How BITMAP INDEX SINGLE VALUE returns rows ?

Hi guys,
here is piece from serial execution plan (display_cursor)
| Id | Operation                                        | Name                         | Starts | E-Rows | A-Rows |
| 48 |                  PARTITION LIST ALL              |                              |   3712 |        |   4089K|
| 49 |                   BITMAP CONVERSION TO ROWIDS    |                              | 40832 |        |   4089K|
| 50 |                    BITMAP AND                    |                              | 40832 |        | 23708 |
|* 51 |                     BITMAP INDEX SINGLE VALUE    | IDX1                         | 40832 |        | 26014 |
|* 52 |                     BITMAP INDEX SINGLE VALUE    | IDX2                         | 40832 |        | 28076 |
...What does mean such difference in A-Rows in steps 50->49, from 23708 to 4089K.
Do steps 51-52 return not regular rows but rows with bitmaps, which consist many regular rows?

853118 wrote:
here is piece from serial execution plan (display_cursor)
What does mean such difference in A-Rows in steps 50->49, from 23708 to 4089K.
Do steps 51-52 return not regular rows but rows with bitmaps, which consist many regular rows?First of all have you noticed that you have more than 40,000 starts of these operations accounted for? That explains why you see more than 1 "row" accounted for the "single value" operation. But to answer your question: Yes, a single value in a bitmap index will have one or more bitmap pieces, and each bitmap piece covers a range of ROWIDs with the bitmap (0 or 1) pattern that tells whether the value is stored in that row or not.
So a single value of a bitmap can potentially return many, if not all rows of a table. The "rows" output in the A-Rows column for bitmap indexes is probably the number of bitmap pieces (sometimes also called "fragments" I think) and therefore can look pretty small in comparison to the rows actually accessed in the table based on the information extracted from those bitmap pieces.
For more info, I recommend to have a look at Julian Dyke's presentation about bitmap index internals: http://www.juliandyke.com/Presentations/BitmapIndexInternals.ppt
Also Greg Rahn has an interesting blog post that also covers the discrepancy between "rows" of a bitmap operation and "rows" of the corresponding table access: http://structureddata.org/2008/05/29/using-bitmap-indexes-effectively/
Hope that helps,
Randolf
Oracle related stuff blog:
http://oracle-randolf.blogspot.com/
Co-author of the "OakTable Expert Oracle Practices" book:
http://www.apress.com/book/view/1430226684
http://www.amazon.com/Expert-Oracle-Practices-Database-Administration/dp/1430226684

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Thanks

793769 wrote:
Jonathan Lewis wrote:
I think this means there's a hole in the optimizer's legal strategies that you might have to fill by other methods.Do I right understand that it is impossible to combine bitmap non-one-column indexes?No, you're generalising from the particular - thus are myths created.
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|   0 | SELECT STATEMENT             |         |    10 | 1250 |     6 |
|   1 | TABLE ACCESS BY INDEX ROWID | T1      |    10 | 1250 |     6 |
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|   3 |    BITMAP AND                |         |       |       |       |
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|   5 |     BITMAP MERGE             |         |       |       |       |
|* 6 |      BITMAP INDEX RANGE SCAN | T1_B3B1 |       |       |       |
Predicate Information (identified by operation id):
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Regards
Jonathan Lewis

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Thanks,

<quote>I clearly tell optimizer to use the bitmap index</quote>
You are clearly not doing it right (see bellow). But anyway
1. For frequently modified tables (OLTP type application) you may want to rethink the applicability of bitmap indexes
low cardinality in itself is not enough justification for using bitmap indexes.
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(if there were one they wouldnt have bothered with coding the rest).
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DIRECTORY                  32
EVALUATION CONTEXT         16
FUNCTION                 1648
INDEX                   23152
INDEX PARTITION          2048
INDEXTYPE                 128
JAVA CLASS             163024
JAVA RESOURCE            3120
LIBRARY                   224
LOB                        16
MATERIALIZED VIEW          32
OPERATOR                  464
PACKAGE                  5488
PACKAGE BODY               32
PROCEDURE                 640
SEQUENCE                  144
SYNONYM                202512
TABLE                   18816
TABLE PARTITION           880
TRIGGER                  4768
TYPE                    10640
TYPE BODY                  16
VIEW                    42816
                       480688
flip@FLOP> set autotrace on explain
update few rows CBO goes with the index no hinting
flip@FLOP> update t d set object_id=object_id-1 where object_type in ('INDEX','PACKAGE','PACKAGE BODY','TABLE');
47488 rows updated.
Elapsed: 00:00:09.02
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=536 Card=47488 Bytes
          =1044736)
   1    0   UPDATE OF 'T'
   2    1     INLIST ITERATOR
   3    2       BITMAP CONVERSION (TO ROWIDS)
   4    3         BITMAP INDEX (SINGLE VALUE) OF 'TX'
update lots of rows CBO goes with the ft no hinting
flip@FLOP> update t d set object_id=object_id-1 where object_type in ('JAVA CLASS','SYNONYM');
365536 rows updated.
Elapsed: 00:00:25.04
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=638 Card=365536 Byte
          s=8041792)
   1    0   UPDATE OF 'T'
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=638 Card=365536 Bytes=8
          041792)
update lots of rows wrong hint syntax CBO goes with the ft
flip@FLOP> update /*+ index_combine(tx) */ t d set object_id=object_id-1 where object_type in ('JAVA CLASS','SYNONYM');
365536 rows updated.
Elapsed: 00:00:21.00
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=638 Card=365536 Byte
          s=8041792)
   1    0   UPDATE OF 'T'
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=638 Card=365536 Bytes=8
          041792)
update lots of rows correct hint syntax CBO goes with the index but was it better than the ft?
flip@FLOP> update /*+ index_combine(d tx) */ t d set object_id=object_id-1 where object_type in ('JAVA CLASS','SYNONYM')
365536 rows updated.
Elapsed: 00:00:25.01
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=1665 Card=365536 Byt
          es=8041792)
   1    0   UPDATE OF 'T'
   2    1     INLIST ITERATOR
   3    2       BITMAP CONVERSION (TO ROWIDS)
   4    3         BITMAP INDEX (SINGLE VALUE) OF 'TX'
flip@FLOP>

Bitmap indexes and group by

I'm trying to understand why Oracle 8.1.6 sometimes uses bitmap indexes sometimes not.
Of course I have all the statistics, my bitmap indexes are valid and so on.
The problem is this:
- I have a customer table very very large
- I have many columns with bitmap indexes
- I run this statement:
select education_key, count(*)
from customer
group by education_key
and obtain a correct execution plan:
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=235 Card=5 Bytes=10)
1 0 SORT (GROUP BY NOSORT) (Cost=235 Card=5 Bytes=10)
2 1 BITMAP CONVERSION (COUNT)
3 2 BITMAP INDEX (FULL SCAN) OF 'CL_EDU'
with the use of the bitmap indexes (only five different values)
- now I want to put a condition on a column
which has a bitmap index.
The new statement is:
select education_key, count(*)
from customer
where age_key = 30
group by education_key
No join, only a scan of a sort of
fact table. In this case Oracle
doesn't use the bitmap index on
education_key:
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=3458 Card=5 Bytes=20
1 0 SORT (GROUP BY) (Cost=3458 Card=5 Bytes=20)
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'CUSTOMER' (Cost=3395 C
ard=18296 Bytes=73184)
3 2 BITMAP CONVERSION (TO ROWIDS)
4 3 BITMAP INDEX (SINGLE VALUE) OF 'CL_AGE'
With this execution plan Oracle has to
read some blocks from the table when I
think that using both the bitmap indexes
the query is faster.
Someone knows why Oracle has this
behaviour? I'm trying to change
many things (also altering statistics
value), but it seems that with a
condition Oracle doesn'use the bitmap
index on the group by column.
null

Hi Michael, what is the address for the lasagne? ....
In init.ora.parms I had 32Mb for bitmap index merge, I suppose it's enough, hovewer I have tried to increase it and nothing changes.
I already use STAR_TRASFORMATION_ENABLED=TRUE.
I have tried with different level of statistics (estimate, compute, also with very precise histograms), always the same
behaviour. I have also tried to change, in a manual way, the statistics in order to change the Oracle behaviour but nothing ...
For this particular statement even if I use the INDEX_COMBINE hint, I mantain the same execution plan.
I use CHOOSE, FIRST_ROWS, ALL_ROWS: always the same behaviour.
I think that Oracle is not able to use a bitmap index on a column involved in a group by when there is another condition in the select.
Hovewer with bitmap indexes, I have many other problems (to read 1% of the table Oracle decides to make a full table scan even if there are the correct bitmap indexes ...).
Many of them are solved using BITMAP_INDEX, but it's difficult to use it in a query tool like Discover or Business Objects because I have to fix the use of INDEX_COMBINE also in situation in which this use is not correct from a performance point of view.
The life with the query optimizer is very hard!
Thank you for your guesses

How to find dynamic value(screen value) in table control for current row .

hi to all,
i used table control in my screen. for column no 2 field i was used serrch help. and for column number 3 i used a dynamic help.
in change mode you can change any row for table control.
when i was using search help for a row which was already entered in column no 3 i cannot get any value.
how i can get value of row no 3 and column no 2 value.
thanks

Try using like index for the serarch the TC-current_line

Bitmap index Vs B-Tree index

Hi All,
Could some one please let me know how Bitmap indexes are useful compared to B-Tree indexes on low-cardinality columns ?.
Thanks,
-Kumar.

>>
As Re: why oracle db 9i optimizer can't choose to use the bitmap index? there are a number of issues with bitmap indexes. Your best bet is to read these three articles by Jonathan Lewis.
It does pay us to understand how the optimizer works with bitmap indexes. I posted some Re: Cost-based optimizer behavior to show how indexes on even very low valued columns can be useful in certain circumstances.
Cheers, APC
Blog : http://radiofreetooting.blogspot.com/

How to display 7 values of single field in row wise.

hi,
how to display 7 values of single field in row wise.
thankx in advance.

hi ,
do it like this :
1 Place ur UI element in tranparent container with Layout as Row Data and Layout Data as Row Head Data
2 Ur first UI , which contains the first value as Row Head Data ,
3 Others as Row Data
u can do it with Matrix Data as well
if u want to give space , u can use HORIZONTAL GUTTER and set its width to medium / large / Xlarge
also there is a UI element "INVISIBLE ELEMENT" ,
1 u can use this UI element to provide space between ur other UI elements in the view
2 u can insert a text element as well , and in the Text property of the element press ALT + 0160.
u can give the space bw UIs as desired.
regards,
amit

How BITMAP INDEX SINGLE VALUE returns rows ?

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