One bitmap index works, one has no effect
Poor performance with following:
select a, b
from tablea
where c = 'Y'
and d = 'Y'
c and d are char(1) with NULLS or 'Y'. Bitmap index created for each (separately). Query takes 4.5s. If the restriction on d is removed - 4.3s. If the restriction on c is removed, and d left in - 0.2s. Why the disparity for seemingly identical attributes?
How many records are in the table?
How many records have C = 'Y'?
How many records have D = 'Y'?
If there are 8 million records, 7 million have C = 'Y' and only 1 have D = 'Y' the C part of the query will skew the numbers the way you described.
null
Similar Messages
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Unable to create a bitmap index
Hi,
I want to create a bitmap index on one of the colums on a table. I fired following query and it failed.
SQL> CREATE BITMAP INDEX TRANS_N_BITMAPIDX_VEH_PLATFORM ON TRANSACTION_NEW(VEH_PLATFORM) LOCAL;
Error starting at line 2 in command:
CREATE BITMAP INDEX TRANS_N_BITMAPIDX_VEH_PLATFORM ON TRANSACTION_NEW(VEH_PLATFORM) LOCAL
Error at Command Line:2 Column:70
Error report:
SQL Error: ORA-01408: such column list already indexed
01408. 00000 - "such column list already indexed"
*Cause:
*Action:I found there is a combined NORMAL index with VEH_PLATFORM column called ‘IDX$$_30AA00021’. It has following columns
• VEH_PLATFORM
• VEH_MODEL
• MODL_YR_NBR
• VEH_MAKE
• TXN_PROCS_DT
But I was able to create bitmap index on VEH_MODEL,MODL_YR_NBR and VEH_MAKE columns without any error.
Why does it error out for VEH_PLATFORM column?
Regards,
SamHi,
I am sure that BITMAP index is only created for
VEH_MAKE
VEH_MODEL
MODL_YR_NBR
BUILD_REGION
VEH_LINE_SERIES
TRANS_CTG_ID
and not for VEH_PLATFORM.
SQL> SELECT * FROM USER_INDEXES WHERE INDEX_TYPE='BITMAP' AND TABLE_NAME='TRANSACTION_NEW';
INDEX_NAME INDEX_TYPE TABLE_OWNER TABLE_NAME TABLE_TYPE UNIQUENESS COMPRESSION PREFIX_LENGTH TABLESPACE_NAME INI_TRANS MAX_TRANS INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS MAX_EXTENTS PCT_INCREASE PCT_THRESHOLD INCLUDE_COLUMN FREELISTS FREELIST_GROUPS PCT_FREE LOGGING BLEVEL LEAF_BLOCKS DISTINCT_KEYS AVG_LEAF_BLOCKS_PER_KEY AVG_DATA_BLOCKS_PER_KEY CLUSTERING_FACTOR STATUS NUM_ROWS SAMPLE_SIZE LAST_ANALYZED DEGREE INSTANCES PARTITIONED TEMPORARY GENERATED SECONDARY BUFFER_POOL FLASH_CACHE CELL_FLASH_CACHE USER_STATS DURATION PCT_DIRECT_ACCESS ITYP_OWNER ITYP_NAME PARAMETERS GLOBAL_STATS DOMIDX_STATUS DOMIDX_OPSTATUS FUNCIDX_STATUS JOIN_INDEX IOT_REDUNDANT_PKEY_ELIM DROPPED VISIBILITY DOMIDX_MANAGEMENT SEGMENT_CREATED
TRANS_N_BITMAPIDX_MAKE BITMAP VISUAL TRANSACTION_NEW TABLE NONUNIQUE DISABLED 2 6102 20 305 611 12226 N/A 12226 2208 11-MAY-11 1 1 YES N N N DEFAULT DEFAULT DEFAULT NO YES NO NO NO VISIBLE N/A
TRANS_N_BITMAPIDX_MODEL BITMAP VISUAL TRANSACTION_NEW TABLE NONUNIQUE DISABLED 2 11284 158 71 144 22813 N/A 22813 2321 11-MAY-11 1 1 YES N N N DEFAULT DEFAULT DEFAULT NO YES NO NO NO VISIBLE N/A
TRANS_N_BITMAPIDX_BUILD_REGION BITMAP VISUAL TRANSACTION_NEW TABLE NONUNIQUE DISABLED 2 2911 5 582 1161 5808 N/A 5808 5808 11-MAY-11 1 1 YES N N N DEFAULT DEFAULT DEFAULT NO YES NO NO NO VISIBLE N/A
TRANS_N_BITMAPIDX_MODL_YR_NBR BITMAP VISUAL TRANSACTION_NEW TABLE NONUNIQUE DISABLED 2 2911 5 582 1161 5808 N/A 5808 5808 11-MAY-11 1 1 YES N N N DEFAULT DEFAULT DEFAULT NO YES NO NO NO VISIBLE N/A
TRANS_N_BITMAPIDX_TRANS_CTG_ID BITMAP VISUAL TRANSACTION_NEW TABLE NONUNIQUE DISABLED 2 7547 43 175 359 15457 N/A 15457 2345 11-MAY-11 1 1 YES N N N DEFAULT DEFAULT DEFAULT NO YES NO NO NO VISIBLE N/A
TRANS_N_BITMAPIDX_VEH_LINE_S BITMAP VISUAL TRANSACTION_NEW TABLE NONUNIQUE DISABLED 2 10581 253 41 84 21477 N/A 21477 2247 11-MAY-11 1 1 YES N N N DEFAULT DEFAULT DEFAULT NO YES NO NO NO VISIBLE N/A
6 rows selectedRegards,
Sam -
Hi All,
We have one BITMAP index on a flag column (Y/N).Table is non partitioned with 3.4 million records.
Following are index stats.
select INDEX_NAME,INDEX_TYPE,BLEVEL,LEAF_BLOCKS,DISTINCT_KEYS,CLUSTERING_FACTOR,NUM_ROWS,SAMPLE_SIZE,DEGREE FROM DBA_INDEXES WHERE
INDEX_NAME='LATE_VER_FLG_CAX_IDX' AND OWNER='EQHUB'
INDEX_NAME INDEX_TYPE BLEVEL LEAF_BLOCKS DISTINCT_KEYS CLUSTERING_FACTOR NUM_ROWS SAMPLE_SIZE DEGREE
LATE_VER_FLG_CAX_IDX BITMAP 11 716 2 72783 72783 72783 1
select last_analyzed,sample_size,num_rows from dba_indexes where index_name='LATE_VER_FLG_CAX_IDX';
LAST_ANALYZED SAMPLE_SIZE NUM_ROWS
31.Jan.09/00:29:41 72783 72783
NAME HEIGHT BLOCKS LF_ROWS LF_BLKS LF_ROWS_LEN BR_ROWS BR_BLKS DEL_LF_ROWS DISTINCT_KEYS BTREE_SPACE USED_SPACE PCT_USED
LATE_VER_FLG_CAX_IDX 12 4352 0 0 1 1 0 0 8028
CREATE BITMAP INDEX "EQHUB"."LATE_VER_FLG_CAX_IDX" ON "EQHUB"."BBO_CORPORATE_ACTIONS" ("LATEST_VERSION_FLAG")
PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 2097152 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "ETDHUB_EQHUB_INDEX"
SQL> select LATEST_VERSION_FLAG,count(*) from BBO_CORPORATE_ACTIONS group by LATEST_VERSION_FLAG;
L COUNT(*)
N 2265303
Y 1189116I am wondering
1.What can be done to reduce index BLEVEL/HEIGHT? (i assume COALESCE will not help)..Should i rebuild the index?
2.Why BTREE_SPACE is only showing 8028 where dba_segments is showing index size =34 MB?
3.Why PCT_USED is NULL?
Is that so BLEVEL/PCT_USED not relative for BITMAP Indexes?
I found that no query is using this index for last 60 days and performance is getting degraded due to FTS.
Should i REBUILD the index so that i can reduce BLEVEL/HEIGHT/INDEX_SIZE and improve query performance with INDEX ACCESS.
Regards,
Bhavik DesaiBhavik Desai wrote:
You perfectely caught the DML pattern.There are 4 daily ELT jobs which INSERT 300 and UPDATE 250 records/execution.
May i ask you,upon which observation,you point this?Also it would be greate if you tell me how did you calculate 67% PCT_FREE.
This was based on the fact that an update to a single row in a table could result in a copy of an entire bitmap index chunk being made; so you need to leave a lot of space free in the leaf blocks to allow for these copies because you really don't want to cause an index leaf block split to occur - especially since that would tend to cascade up the index and cause a branch block split as well. On this basis, pctfree 51 should be adequate for most cases, but setting it to 67 (a) allows you to do a couple of updates/clones to the same chunk before you hit a problem, and (b) helps to keep the chunks smaller which helps to reduce the undo and redo a bit.
This has just reminded me of a little detail - I am a little surprised by the fact that you got an index into this state in 10.2, because that release of Oracle seems to avoid creating a long chain of clones of the same index chunk when you do a series of small updates, and it is the chain of clones that results in branch blocks split running up the index and exploding it to an enormous height. Has this database been upgraded from 9i (or even 10.1) some time in the fairly recent past ?
>
I still dont understand,
1.Why there is no difference in BTREE_SPACE?
2.Why PCT_USED is NULL?
I've just run a quick test on 10.2.0.1 and 10.2.0.3 creating a table with a y/n/null column and creating a bitmap index on it. The results from index_stats didn't match across versions, and the 10.2.0.3 stats were clearly garbage - very similar to yours. For the moment I would assume that something has been broken in 10.2.0.3 with this use of the validate command.
>
If we consider Franck Pachot's observation,then it would be better to convert the BITMAP to BTREE.Is this a normal bitmap behavior?Randolf's observation about the numbers of rows involved is most relevant: it doesn't look like an index that the optimizer should consider using for anything other than select flag, count(*), and changing from a bitmap to a btree won't make any difference. In other words, unless you can point to an important piece of code and say "it makes tis bit run much better" maybe the index shouldn't exist at all.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
"The temptation to form premature theories upon insufficient data is the bane of our profession."
Sherlock Holmes (Sir Arthur Conan Doyle) in "The Valley of Fear". -
Multi-column BITMAP index vs. multiple BITMAP indices?
Given the table (simple, made-up example):
CREATE TABLE applicant_diversity_info (
applicant_diversity_id NUMBER(12), PRIMARY KEY(applicant_diversity_id),
apply_date DATE,
ssn_salted_md5 RAW(16),
gender CHAR(1), CHECK ( (gender IS NULL OR gender IN ('M','F')) ),
racial_continent VARCHAR2(30), CHECK ( (racial_continent IS NULL
OR racial_continent IN ('Europe','Africa','America','Asia_Pacific')) ),
ethnic_supergroup VARCHAR2(30), CHECK ( (ethnic_supergroup IS NULL OR ethnic_supergroup IN ('Latin American','Other')) ),
hire_salary NUMBER(11,2),
hire_month DATE,
termination_salary NUMBER(11,2),
termination_month DATE,
termination_cause VARCHAR2(30), CHECK ( (termination_cause IS NULL
OR termination_cause IN ('Resigned','Leave of Absence','Laid Off','Performance','Cause')) )
Oracle (syntactically) allows me to create either one BITMAP index over all four small-cardinality columns
CREATE BITMAP INDEX applicant_diversity_diversity_idx ON applicant_diversity_info (
gender, racial_continent, ethnic_supergroup, termination_reason );
or four independent indexes
CREATE BITMAP INDEX applicant_diversity_gender_idx ON applicant_diversity_info ( gender );
CREATE BITMAP INDEX applicant_diversity_race_idx ON applicant_diversity_info ( raceial_continent );
etc.
What is the difference between the two approaches; is there any meaningful difference in disk-space between the one multi-colum index and the four single-column indexes? Does it make a difference in what the query-planner will consider?
And, if I define one multi-column BITMAP index, does the order of columns matter?>
What is the difference between the two approaches; is there any meaningful difference in disk-space between the one multi-colum index and the four single-column indexes? Does it make a difference in what the query-planner will consider?
And, if I define one multi-column BITMAP index, does the order of columns matter?
>
You may want to read this two-part blog, that answers that exact question, by recognized expert Richard Foote
http://richardfoote.wordpress.com/2010/05/06/concatenated-bitmap-indexes-part-i-two-of-us/
http://richardfoote.wordpress.com/2010/05/12/concatenated-bitmap-indexes-part-ii-everybodys-got-to-learn-sometime/
As with many things Oracle the answer is 'it depends'.
In short the same considerations apply for a concatenated index whether it is bitmap or b-tree: 1) will the leading column usually be in the predicate and 2) will most or all of the index columns be specified in the queries.
Here are some quotes from part 1
>
Many of the same issues and factors in deciding to create a single, multi-column index vs. several, single column indexes apply to Bitmap indexes as they do with B-Tree indexes, although there are a number of key differences to consider as well.
Another thing to note regarding a concatenated Bitmap index is that the potential number of index entries is a product of distinct combinations of data of the indexed columns.
A concatenated Bitmap index can potentially use less or more space than corresponding single column indexes, it depends on the number of index entries that are derived and the distribution of the data with the table.
>
Here is the lead quote from part 2
>
The issues regarding whether to go for single column indexes vs. concatenated indexes are similar for Bitmap indexes as they are for B-Tree indexes.
It’s generally more efficient to access a concatenated index as it’s only the one index with less processing and less throwaway rowids/rows to contend with. However it’s more flexible to have single column indexes, especially for Bitmap indexes that are kinda designed to be used concurrently, as concatenated indexes are heavily dependant on the leading column being known in queries. -
I am not getting an example for bitmap index mechanism.
Supposing their is a table T1 with column c1 having records
Y
N
N
Y
Y
N
and i query the table like
select * from t1 where c1 = 'Y'
How bit map index functions?
thanks,
VinodhI think you should check the google's new feature to explore -> [How bitmap Index Works|http://www.google.co.in/search?hl=en&rlz=1G1GGLQ_ENIN333&q=how+bitmap+index+works&meta=&aq=0&oq=how+bitma]
Regards.
Satyaki De. -
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