Why does not a query go by index but FULL TABLE SCAN

I have two tables;
table 1 has 1400 rwos and more than 30 columns , one of them is named as 'site_code' , a index was created on this column;
table 2 has more 150 rows than 20 column , this primary key is 'site_code' also.
the two tables were analysed by dbms_stats.gather_table()...
when I run the explain for the 2 sqls below:
select * from table1 where site_code='XXXXXXXXXX';
select * from table1 where site_code='XXXXXXXXXX';
certainly the oracle explain report show that 'Index scan'
but the problem raised that
I try to explain the sql
select *
from table1,table2
where 1.'site_code'=2.'site_code'
the explain report that :
select .....
FULL Table1 Scan
FULL Table2 Scan
why......

Nikolay Ivankin wrote:
BluShadow wrote:
Nikolay Ivankin wrote:
Try to use hint, but I really doubt it will be faster.No, using hints should only be advised when investigating an issue, not recommended for production code, as it assumes that, as a developer, you know better than the Oracle Optimizer how the data is distributed in the data files, and how the data is going to grow and change over time, and how best to access that data for performance etc.Yes, you are absolutly right. But aren't we performing such an investigation, are we? ;-)The way you wrote it, made it sound that a hint would be the solution, not just something for investigation.
select * from .. always performs full scan, so limit your query.No, select * will not always perform a full scan, that's just selecting all the columns.
A select without a where clause or that has a where clause that has low selectivity will result in full table scans.But this is what I have ment.But not what you said.

Similar Messages

Why does not my query use an index?

I have a table with some processed rows (state: 9) and some unprocessed rows (states: 0,1,2,3,4).
This table has over 120000 rows, but this number will grow.
Most of the rows are processed and most of them also contain a group id. Number of groups is relatively small (let's assume 20).
I would like to obtain the oldest some_date for every group. This values has to be outer joined to a on-line report (contains one row for each group).
Here is my set-up:
Tested on: 10.2.0.4 (Solaris), 10.2.0.1 (WinXp)
drop table t purge;
create table t(
id number not null primary key,
grp_id number,
state number,
some_date date,
pad char(200)
insert into t(id, grp_id, state, some_date, pad)
select level,
     trunc(dbms_random.value(0,20)),
        9,
        sysdate+dbms_random.value(1,100),
        'x'
from dual
connect by level <= 120000;
insert into t(id, grp_id, state, some_date, pad)
select level + 120000,
     trunc(dbms_random.value(0,20)),
        trunc(dbms_random.value(0,5)),
        sysdate+dbms_random.value(1,100),
        'x'
from dual
connect by level <= 2000;
commit;
exec dbms_stats.gather_table_stats(user, 'T', estimate_percent=>100, method_opt=>'FOR ALL COLUMNS SIZE 1');
Tom Kyte's printtab
==============================
TABLE_NAME                    : T
NUM_ROWS                      : 122000
BLOCKS                        : 3834I know, this could be easily solved by fast refresh on commit materialized view like this:
select
grp_id,
min(some_date),
from
t
where
state in (0,1,2,3,4)
grpup by
grp_id;+ I have to create log on (grp_id, some_date, state)
Number of rows with active state will be always relatively small. Let's assume 1000-2000.
So my another idea was to create a selective index. An index which would contain only data for rows with an active state.
Something like this:
create index fidx_active on t (
case state
    when 0 then grp_id
    when 1 then grp_id
    when 2 then grp_id
    when 3 then grp_id
    when 4 then grp_id
end,
case state
    when 0 then some_date
    when 1 then some_date
    when 2 then some_date
    when 3 then some_date
    when 4 then some_date
end) compress 1; so a tuple (group_id, some_date) is projected to tuple (null, null) when the state is not an active state and therefore it is not indexed.
We can save even more space by compressing 1st expression.
analyze index idx_grp_state_date validate structure;
select * from index_stats
@pr
Tom Kyte's printtab
==============================
HEIGHT                        : 2
BLOCKS                        : 16
NAME                          : FIDX_ACTIV
LF_ROWS                       : 2000 <-- we're indexing only active rows
LF_BLKS                       : 6 <-- small index: 1 root block with 6 leaf blocks
BR_ROWS                       : 5
BR_BLKS                       : 1
DISTINCT_KEYS                 : 2000
PCT_USED                      : 69
PRE_ROWS                      : 25
PRE_ROWS_LEN                  : 224
OPT_CMPR_COUNT                : 1
OPT_CMPR_PCTSAVE              : 0Note: @pr is a Tom Kyte's print table script adopted by Tanel Poder (I'm using Tanel's library) .
Then I created a query to be outer joined to the report (report contains a row for every group).
I want to achieve a full scan of the index.
select
case state -- 1st expression
    when 0 then grp_id
    when 1 then grp_id
    when 2 then grp_id
    when 3 then grp_id
    when 4 then grp_id
end grp_id,
min(case state --second expression
        when 0 then some_date
        when 1 then some_date
        when 2 then some_date
        when 3 then some_date
        when 4 then some_date
      end) as mintime
from t
where
case state --1st expression: at least one index column has to be not null
    when 0 then grp_id
    when 1 then grp_id
    when 2 then grp_id
    when 3 then grp_id
    when 4 then grp_id
end is not null
group by
case state --1st expression
    when 0 then grp_id
    when 1 then grp_id
    when 2 then grp_id
    when 3 then grp_id
    when 4 then grp_id
end;-------------
Doc's snippet:
13.5.3.6 Full Scans
A full scan is available if a predicate references one of the columns in the index. The predicate does not need to be an index driver. A full scan is also available when there is no predicate, if both the following conditions are met:
All of the columns in the table referenced in the query are included in the index.
At least one of the index columns is not null.
A full scan can be used to eliminate a sort operation, because the data is ordered by the index key. It reads the blocks singly.
13.5.3.7 Fast Full Index Scans
Fast full index scans are an alternative to a full table scan when the index contains all the columns that are needed for the query, and at least one column in the index key has the NOT NULL constraint. A fast full scan accesses the data in the index itself, without accessing the table. It cannot be used to eliminate a sort operation, because the data is not ordered by the index key. It reads the entire index using multiblock reads, unlike a full index scan, and can be parallelized.
You can specify fast full index scans with the initialization parameter OPTIMIZER_FEATURES_ENABLE or the INDEX_FFS hint. Fast full index scans cannot be performed against bitmap indexes.
A fast full scan is faster than a normal full index scan in that it can use multiblock I/O and can be parallelized just like a table scan.
So the question is: Why does oracle do a full table scan?
Everything needed is in the index and one expression is not null, but index (fast) full scan is not even considered by CBO (I did a 10053 trace)
| Id | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   1 | HASH GROUP BY     |      |      1 |     85 |     20 |00:00:00.11 |    3841 |
|* 2 |   TABLE ACCESS FULL| T    |      1 |   6100 |   2000 |00:00:00.10 |    3841 |
Predicate Information (identified by operation id):
   2 - filter(CASE "STATE" WHEN 0 THEN "GRP_ID" WHEN 1 THEN "GRP_ID" WHEN 2
              THEN "GRP_ID" WHEN 3 THEN "GRP_ID" WHEN 4 THEN "GRP_ID" END IS NOT NULL)Let's try some minimalistic examples. Firstly with no FBI.
create index idx_grp_id on t(grp_id);
select grp_id,
       min(grp_id) min
from t
where grp_id is not null
group by grp_id;
| Id | Operation             | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads |
|   1 | HASH GROUP BY        |            |      1 |     20 |     20 |00:00:01.00 |     244 |    237 |
|* 2 |   INDEX FAST FULL SCAN| IDX_GRP_ID |      1 |    122K|    122K|00:00:00.54 |     244 |    237 |
Predicate Information (identified by operation id):
   2 - filter("GRP_ID" IS NOT NULL)This kind of output I was expected to see with FBI. Index FFS was used although grp_id has no NOT NULL constraint.
Let's try a simple FBI.
create index fidx_grp_id on t(trunc(grp_id));
select trunc(grp_id),
       min(trunc(grp_id)) min
from t
where trunc(grp_id) is not null
group by trunc(grp_id);
| Id | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   1 | HASH GROUP BY     |      |      1 |     20 |     20 |00:00:00.94 |    3841 |
|* 2 |   TABLE ACCESS FULL| T    |      1 |   6100 |    122K|00:00:00.49 |    3841 |
Predicate Information (identified by operation id):
   2 - filter(TRUNC("GRP_ID") IS NOT NULL)Again, index (fast) full scan not even considered by CBO.
I tried:
alter table t modify grp_id not null;
alter table t add constraint trunc_not_null check (trunc(grp_id) is not null);I even tried to set table hidden column (SYS_NC00008$) to NOT NULL
It has no effect, FTS is still used..
Let's try another query:
select distinct trunc(grp_id)
from t
where trunc(grp_id) is not null
| Id | Operation             | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   1 | HASH UNIQUE          |             |      1 |     20 |     20 |00:00:00.85 |     244 |
|* 2 |   INDEX FAST FULL SCAN| FIDX_GRP_ID |      1 |    122K|    122K|00:00:00.49 |     244 |
Predicate Information (identified by operation id):
   2 - filter("T"."SYS_NC00008$" IS NOT NULL)Here the index FFS is used..
Let's try one more query, very similar to the above query:
select trunc(grp_id)
from t
where trunc(grp_id) is not null
group by trunc(grp_id)
| Id | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   1 | HASH GROUP BY     |      |      1 |     20 |     20 |00:00:00.86 |    3841 |
|* 2 |   TABLE ACCESS FULL| T    |      1 |    122K|    122K|00:00:00.49 |    3841 |
Predicate Information (identified by operation id):
   2 - filter(TRUNC("GRP_ID") IS NOT NULL)And again no index full scan..
So my next question is:
What are the restrictions which prevent index (fast) fullscan to be used in these scenarios?
Thank you very much for your answers.
Edited by: user1175494 on 16.11.2010 15:23
Edited by: user1175494 on 16.11.2010 15:25

I'll start off with the caveat that i'm no Johnathan Lewis so hopefully someone will be able to come along and give you a more coherent explanation than i'm going to attempt here.
It looks like the application of the MIN function against the case statement is confusing the optimizer and disallowing the usage of your FBI. I tested this against my 11.2.0.1 instance and your query chooses the fast full scan without being nudged in the right direction.
That being said, i was able to get this to use a fast full scan on my 10 instance, but i had to jiggle the wires a bit. I modified your original query slightly, just to make it easier to do my fiddling.
original (in the sense that it still takes the full table scan) query
with data as
select
    case state -- 1st expression
      when 0 then grp_id
      when 1 then grp_id
      when 2 then grp_id
      when 3 then grp_id
      when 4 then grp_id
    end as grp_id,
    case state --second expression
          when 0 then some_date
          when 1 then some_date
          when 2 then some_date
          when 3 then some_date
          when 4 then some_date
    end as mintime
from t
where
    case state --1st expression: at least one index column has to be not null
      when 0 then grp_id
      when 1 then grp_id
      when 2 then grp_id
      when 3 then grp_id
      when 4 then grp_id
    end is not null
and
    case state --second expression
          when 0 then some_date
          when 1 then some_date
          when 2 then some_date
          when 3 then some_date
          when 4 then some_date
    end is not null
select--+ GATHER_PLAN_STATISTICS
grp_id,
min(mintime)
from data
group by grp_id;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL, NULL, 'allstats +peeked_binds'));
| Id | Operation        | Name | Starts | E-Rows | A-Rows |      A-Time   | Buffers |
|   1 | HASH GROUP BY        |       |      2 |      33 |       40 |00:00:00.07 |    7646 |
|* 2 |   TABLE ACCESS FULL| T       |      2 |      33 |     4000 |00:00:00.08 |    7646 |
Predicate Information (identified by operation id):
   2 - filter((CASE "STATE" WHEN 0 THEN "GRP_ID" WHEN 1 THEN "GRP_ID" WHEN 2
           THEN "GRP_ID" WHEN 3 THEN "GRP_ID" WHEN 4 THEN "GRP_ID" END IS NOT NULL AND
           CASE "STATE" WHEN 0 THEN "SOME_DATE" WHEN 1 THEN "SOME_DATE" WHEN 2 THEN
           "SOME_DATE" WHEN 3 THEN "SOME_DATE" WHEN 4 THEN "SOME_DATE" END IS NOT
           NULL))
modified version where we prevent the MIN function from being applied too early, by using ROWNUM
with data as
select
    case state -- 1st expression
      when 0 then grp_id
      when 1 then grp_id
      when 2 then grp_id
      when 3 then grp_id
      when 4 then grp_id
    end as grp_id,
    case state --second expression
          when 0 then some_date
          when 1 then some_date
          when 2 then some_date
          when 3 then some_date
          when 4 then some_date
    end as mintime
from t
where
    case state --1st expression: at least one index column has to be not null
      when 0 then grp_id
      when 1 then grp_id
      when 2 then grp_id
      when 3 then grp_id
      when 4 then grp_id
    end is not null
and
    case state --second expression
          when 0 then some_date
          when 1 then some_date
          when 2 then some_date
          when 3 then some_date
          when 4 then some_date
    end is not null
and rownum > 0
select--+ GATHER_PLAN_STATISTICS
grp_id,
min(mintime)
from data
group by grp_id;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL, NULL, 'allstats +peeked_binds'));
| Id | Operation           | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   1 | HASH GROUP BY           |            |      2 |     20 |     40 |00:00:00.01 |      18 |
|   2 |   VIEW                |            |      2 |     33 |   4000 |00:00:00.07 |      18 |
|   3 |    COUNT           |            |      2 |      |   4000 |00:00:00.05 |      18 |
|* 4 |     FILTER           |            |      2 |      |   4000 |00:00:00.03 |      18 |
|* 5 |      INDEX FAST FULL SCAN| FIDX_ACTIVE |      2 |     33 |   4000 |00:00:00.01 |      18 |
Predicate Information (identified by operation id):
   4 - filter(ROWNUM>0)
   5 - filter(("T"."SYS_NC00006$" IS NOT NULL AND "T"."SYS_NC00007$" IS NOT NULL))

Why does query do a full table scan?

I have a simple select query that filters on the last 10 or 11 days of data from a table. In the first case it executes in 1 second. In the second case it is taking 15+ minutes and still not done.
I can tell that the second query (11 days) is doing a full table scan.
- Why is this happening? ... I guess some sort of threshold calculation???
- Is there a way to prevent this? ... or encourage Oracle to play nice.
I find it confusing from a front end/query perspective to get vastly different performance.
Jason
Oracle 10g
Quest Toad 10.6
CREATE TABLE delme10 AS
SELECT *
FROM ed_visits
WHERE first_contact_dt >= TRUNC(SYSDATE-10,'D');
Plan hash value: 915912709
| Id | Operation                    | Name              | Rows | Bytes | Cost (%CPU)| Time     |
|   0 | CREATE TABLE STATEMENT       |                   | 4799 | 5534K| 4951   (1)| 00:01:00 |
|   1 | LOAD AS SELECT              | DELME10           |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| ED_VISITS         | 4799 | 5534K| 4796   (1)| 00:00:58 |
|* 3 |    INDEX RANGE SCAN          | NDX_ED_VISITS_020 | 4799 |       |    15   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - access("FIRST_CONTACT_DT">=TRUNC(SYSDATE@!-10,'fmd'))
CREATE TABLE delme11 AS
SELECT *
FROM ed_visits
WHERE first_contact_dt >= TRUNC(SYSDATE-11,'D');
Plan hash value: 1113251513
| Id | Operation              | Name      | Rows | Bytes | Cost (%CPU)| Time     |    TQ |IN-OUT| PQ Distrib |
|   0 | CREATE TABLE STATEMENT |           | 25157 |    28M| 14580   (1)| 00:02:55 |        |      |            |
|   1 | LOAD AS SELECT        | DELME11   |       |       |            |          |        |      |            |
|   2 |   PX COORDINATOR       |           |       |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM) | :TQ10000 | 25157 |    28M| 14530   (1)| 00:02:55 | Q1,00 | P->S | QC (RAND) |
|   4 |     PX BLOCK ITERATOR |           | 25157 |    28M| 14530   (1)| 00:02:55 | Q1,00 | PCWC |            |
|* 5 |      TABLE ACCESS FULL | ED_VISITS | 25157 |    28M| 14530   (1)| 00:02:55 | Q1,00 | PCWP |            |
Predicate Information (identified by operation id):
   5 - filter("FIRST_CONTACT_DT">=TRUNC(SYSDATE@!-11,'fmd'))

Hi Jason,
I think you're right with kind of "threshold". You can verify CBO costing with event 10053 enabled. There are many possible ways to change this behaviour. Most straightforward would be probably INDEX hint, but you can also change some index-cost related parameters, check histograms, decrease degree of paralellism on table, create stored outline, etc.
Lukasz

PreparedStatement.setTimestamp does not allo use of DATE index..

(...although they' re not suposed be related)
Hi;
I use Oracle 10g with Java JDBC driver ojdbc14.jar and I have a PreparedStatement on which among other "bind" JDBC parameters I have a java.util.Date. The ORM that we use (Hibernate 2.0.3) passes this Date instance as a java.sql.Timestamp binding it with setTimestamp on the PreparedStatement.
Interestingly enaugh, this way of binding the value does not make the database use the date column index, although the explain available from TOAD / Session browse, for instance, tells me the planner first demands an index scan there (it is possible i think that explain only operates on prepared SQL alone, disregarding value initial types and values). Doing a preparedStatement.setDate(value) instead, passing a java.sql.Date, uses the index and probably fully respects the Explain Plan that I have seen.
I assure you I have done many tests, with exact same query, parameter types and values, session origination and environment and changing one thing at a time. The only thing that being changed enables the query to make use of the DATE column index and return under a second, compared to tens of seconds of full table scan with expensive function calls, is to replace the preparedStatement.setTimestamp with setDate.
This can only be a JDBC bug, as far as I can tell. Do you know of this problem? What may be the cause?
As a note, since I need to continue to use that version of Hibernate on that spot, I momentarily worked around this apparent bug by modifying the SQL WHERE clause in order to provide the DATE value as to_date function output from a String bind, so doing date_col = to_date('YYYY-MM-DD', :value) instead of date_col = :value. Works fain, but just a work around.
Looking forward for your answer and suggestions,
Nicu Marasoiu

A few remarks:
- Toad explain plans can be frought by implicit data type conversions.
Generally speaking everything is treated as a varchar2.
- You would need to show the plan from v$sql_plan.
- It would be a JDBC bug if you can show 'correct' behavior from sql*plus, making sure you actually do use bind variables of the correct type (so VARIABLE foo date prior to running the SQL, the SELECT referencing :foo)
- In the past I have observed strange CBO behavior because the date wasn't a complete date (ie time was missing). This can be rectified by function based indexes.
- You probably can enable event 10053 in your JDBC session, or event 10046 level 12
Sybrand Bakker
Senior Oracle DBA

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Thank you so much for your patience and expertise. You spent a lot of time
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luck to you. Joyce
From: Phillip Jones <[email protected]>
Reply-To: <[email protected]>
Date: Fri, 23 Apr 2010 16:22:02 -0600
To: Joyce Hinds <[email protected]>
Subject: Re: Why does Acrobat Standard 6.0 recognize
scanner but Pro 9 will not?
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Hi,
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[https://www.sdn.sap.com/irj/sdn/go/portal/prtroot/docs/library/uuid/9f4a452b-0301-0010-8ca6-ef25a095834a]
[http://help.sap.com/saphelp_nw70/helpdata/en/41/b987eb1443534ba78a793f4beed9d5/frameset.htm]
[http://help.sap.com/saphelp_nw70/helpdata/en/d9/31363dc992752de10000000a114084/frameset.htm]
[http://help.sap.com/saphelp_nw04/helpdata/en/2e/caceae8dd08e48a63c2da60c8ffa5e/frameset.htm]
[https://www.sdn.sap.com/irj/sdn/go/portal/prtroot/docs/library/uuid/e0501cb9-d51c-2a10-239c-c141a22c45a6]
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https://www.apple.com/legal/internet-services/icloud/ww/
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eng.robo wrote:
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of necessary information stored therein.
Thanks for any help....

I was having this issue also. I found this thread and disabling Exchange Notes did the trick. Obviously, this is the solution only if you were using Exchange to begin with.
If you want to continue to use Exchange notes or perhaps try this to see if it works in general, I found a fix on this site. All works well now.
1. Delete your exchange account from the Mail, Contacts & Calendars pane in System Preferences.
2. Go to Finder, click "Go" in the menu bar, click "Go to Folder" and type in "~/Library" (without the quotes) in the box. This will take you to your user Library folder.
3. Use the search box in the finder and type in "com.apple.notes" and click "Library" in the search bar that appears. This should now find several folders and files related to the Notes app. Rename or delete everything except the help files (with the lifesaver icons on them).
4. Restart your Mac. Put your exchange account back into System Preferences, you can go ahead and activate the Notes sync service.
5. Open Notes and it should be working now!

I m coming from linux platform ... now need my g to work with vim ... how to install g which supports C 0x ... why does not it has repository with apple ... its so easy rather than going to UI ...

i m coming from linux platform ... now need my g to work with vim ... how to install g which supports C 0x ... why does not it has repository with apple ... its so easy rather than going to UI ...

tom i have been using ubuntu for c++ project
i use lambda and atomic (c++0x) but now as I am working on MAC not able to use them
installed xcode(4.3) and changed compiler settings to LLVM3.1 but still both features are not available
to compile wanted to have g++ which supports c++0x features ...
new to MAC (2-3 days) so was in rush to solution
let me know if you have some suggestion ...
else will try with eclipse cdt ....

Why does not a query go by index but FULL TABLE SCAN

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