"db file scattered read" too high and Query going for full table scan-Why ?
Hi,
I had a big table of around 200mb and had a index on it.
In my query I am using the where clause which has to use the
index. I am neither using any not null condition
nor using any function on the index fields.
Still my query is not using the index.
It is going for full table scan.
Also the statspack report is showing the
"db file scattered read" too high.
Can any body help and suggest me why this is happenning.
Also tell me the possible solution for it.
Thanks
Arun Tayal
"db file scattered read" are physical reads/multi block reads. This wait occurs when the session reading data blocks from disk and writing into the memory.
Take the execution plan of the query and see what is wrong and why the index is not being used.
However, FTS are not always bad. By the way, what is your db_block_size and db_file_multiblock_read_count values?
If those values are set to high, Optimizer always favour FTS thinking that reading multiblock is always faster than single reads (index scans).
Dont see oracle not using index, just find out why oracle is not using index. Use the INDEX hint to force optimizer to use index. Take the execution with/witout index and compare the cardinality,cost and of course, logical reads.
Jaffar
Message was edited by:
The Human Fly
Similar Messages
-
Query is doing full table scan
Hi All,
The below query is doing full table scan. So many threads from application trigger this query and doing full table scan. Can you please tell me how to improve the performance of this query?
Env is 11.2.0.3 RAC (4 node). Unique index on VZ_ID, LOGGED_IN. The table row count is 2,501,103.
Query is :-
select ccagentsta0_.LOGGED_IN as LOGGED1_404_, ccagentsta0_.VZ_ID as VZ2_404_, ccagentsta0_.ACTIVE as ACTIVE404_, ccagentsta0_.AGENT_STATE as AGENT4_404_,
ccagentsta0_.APPLICATION_CODE as APPLICAT5_404_, ccagentsta0_.CREATED_ON as CREATED6_404_, ccagentsta0_.CURRENT_ORDER as CURRENT7_404_,
ccagentsta0_.CURRENT_TASK as CURRENT8_404_, ccagentsta0_.HELM_ID as HELM9_404_, ccagentsta0_.LAST_UPDATED as LAST10_404_, ccagentsta0_.LOCATION as LOCATION404_,
ccagentsta0_.LOGGED_OUT as LOGGED12_404_, ccagentsta0_.SUPERVISOR_VZID as SUPERVISOR13_404_, ccagentsta0_.VENDOR_NAME as VENDOR14_404_
from AGENT_STATE ccagentsta0_ where ccagentsta0_.VZ_ID='v790531' and ccagentsta0_.ACTIVE='Y';
Table Scan AGENT_STATE 2.366666667
Table Scan AGENT_STATE 0.3666666667
Table Scan AGENT_STATE 1.633333333
Table Scan AGENT_STATE 0.75
Table Scan AGENT_STATE 1.866666667
Table Scan AGENT_STATE 2.533333333
Table Scan AGENT_STATE 0.5333333333
Table Scan AGENT_STATE 1.95
Table Scan AGENT_STATE 0.8
Table Scan AGENT_STATE 0.2833333333
Table Scan AGENT_STATE 1.983333333
Table Scan AGENT_STATE 2.5
Table Scan AGENT_STATE 1.866666667
Table Scan AGENT_STATE 1.883333333
Table Scan AGENT_STATE 0.9
Table Scan AGENT_STATE 2.366666667
But the explain plan shows the query is taking the index
Explain plan output:-
PLAN_TABLE_OUTPUT
Plan hash value: 1946142815
| Id | Operation | Name | Rows | Bytes | Cost (%C
PU)| Time |
PLAN_TABLE_OUTPUT
| 0 | SELECT STATEMENT | | 1 | 106 | 244
(0)| 00:00:03 |
|* 1 | TABLE ACCESS BY INDEX ROWID| AGENT_STATE | 1 | 106 | 244
(0)| 00:00:03 |
|* 2 | INDEX RANGE SCAN | AGENT_STATE_IDX | 229 | | 4
(0)| 00:00:01 |
PLAN_TABLE_OUTPUT
Predicate Information (identified by operation id):
1 - filter("CCAGENTSTA0_"."ACTIVE"='Y')
2 - access("CCAGENTSTA0_"."VZ_ID"='v790531')
The values (VZ_ID) i have given are dummy values picked from the table. I dont get the actual values since the query is coming with bind variables. Please let me know your suggestion on this.
Thanks,
ManiHi,
But I am not getting what is the issue..its a simple select query and index is there on one of the leading columns (VZ_ID --- PK). Explain plan says its using its using Index and it only select fraction of rows from the table. Then why it is doing FTS. For Optimizer, why its like a query doing FTS.
The rule-based optimizer would have picked the plan with the index. The cost-based optimizer, however, is picking the plan with the lowest cost. Apparently, the lowest cost plan is the one with the full table scan. And the optimizer isn't necessarily wrong about this.
Reading data from a table via index probes is only efficient when selecting a relatively small percentage of rows. For larger percentages, a full table scan is generally better.
Consider a simple example: a query that selects from a table with biographies for all people on the planet. Suppose you are interested in all people from a certain country.
select * from all_people where country='Vatican'
would only return only 800 rows (as Vatican is an extremely small country with population of just 800 people). For this case, obviously, using an index would be very efficient.
Now if we run this query:
select * from all_people where contry = 'India',
we'd be getting over a billion of rows. For this case, a full table scan would be several thousand times faster.
Now consider the third case:
select * from all_people where country = :b1
What plan should the optimizer choose? The value of :b1 bind variable is generally not known during the parse time, it will be passed by the user when the query is already parsed, during run-time.
In this case, one of two scenarios takes place: either the optimizer relies on some built-in default selectivities (basically, it takes a wild guess), or the optimizer postpones taking the final decision until the
first time the query is run, 'peeks' the value of the bind, and optimizes the query for this case.
In means, that if the first time the query is parsed, it was called with :b1 = 'India', a plan with a full table scan will be generated and cached for subsequent usage. And until the cursor is aged out of library cache
or invalidated for some reason, this will be the plan for this query.
If the first time it was called with :b1='Vatican', then an index-based plan will be picked.
Either way, bind peeking only gives good results if the subsequent usage of the query is the same kind as the first usage. I.e. in the first case it will be efficient, if the query would always be run for countries with big popultions.
And in the second case, if it's always run for countries with small populations.
This mechanism is called 'bind peeking' and it's one of the most common causes of performance problems. In 11g, there are more sophisticated mechanisms, such a cardinality feedback, but they don't always work as expected.
This mechanism is the most likely explanation for your issue. However, without proper diagnostic information we cannot be 100% sure.
Best regards,
Nikolay -
Update Query is Performing Full table Scan of 1 Millions Records
Hello Everyboby I have one update query ,
UPDATE tablea SET
task_status = 12
WHERE tablea.link_id >0
AND tablea.task_status <> 0
AND tablea.event_class='eventexception'
AND Exists(SELECT 1 from tablea ltask where ltask.task_id=tablea.link_id
AND ltask.task_status = 0)
When I do explain plan it shows following result...
Execution Plan
0 UPDATE STATEMENT Optimizer=CHOOSE
1 0 UPDATE OF 'tablea'
2 1 FILTER
3 2 TABLE ACCESS (FULL) OF 'tablea'
4 2 TABLE ACCESS (BY INDEX ROWID) OF 'tablea'
5 4 INDEX (UNIQUE SCAN) OF 'PK_tablea' (UNIQUE)
NOW tablea may have more than 10 MILLION Records....This would take hell of time even if it has to
update 2 records....please suggest me some optimal solutions.....
Regards
MaheshI see your point but my question or logic say i have index on all columns used in where clauses so i find no reason for oracle to do full table scan,,,,
UPDATE tablea SET
task_status = 12
WHERE tablea.link_id >0
AND tablea.task_status <> 0
AND tablea.event_class='eventexception'
AND Exists(SELECT 1 from tablea ltask where ltask.task_id=tablea.link_id
AND ltask.task_status = 0)
I am clearly statis where task_status <> 0 and event_class= something and tablea.link_id >0
so ideal case FOR optimizer should be
Step 1)Select all the rowid having this condition...
Step 2)
For each row in rowid get all the row where task_status=0
and where taskid=linkid of rowid selected above...
Step 3)While looping for each rowid if it find any condition try for rowid obtained from ltask in task 2 update that record....
I want to have this kind of plan,,,,,does anyone know how to make oracle obtained this kind of plan......
It is try FULL TABLE SCAN is harmfull alteast not better than index scan..... -
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 -
Query to identify full table scans in progress
Does anybody have a query that would help me identify:
1) Full table scans in progress.
2) Long running queries in progress.
Thanks,
Thomas
nullDoes anybody have a query that would help me identify:
1) Full table scans in progress.Not sure.
2) Long running queries in progressDon’t have a query readily available, but you can write one based on the following –
Try querying the view V$SESSION_LONGOPS. You will need to join this to V$SQL using SQL_ADDRESS to identify all the SQL’s running for more than ‘x’ minutes.
Current System Time - V$SESSION_LONGOPS.Start_Time should give you the duration.
Shailender Mehta -
Slow query due to large table and full table scan
Hi,
We have a large Oracle database, v 10g. Two of the tables in the database have over one million rows.
We have a few queries which take a lot of time to execute. Not always though, it that seems when load is high the queries tend
to take much longer. Average time may be 1 or 2 seconds, but maxtime can be up to 2 minutes.
We have now used Oracle Grid to help us examine the queries. We have found that some of the queries require two or three full table scans.
Two of the full table scans are of the two large tables mentioned above.
This is an example query:
SELECT table1.column, table2.column, table3.column
FROM table1
JOIN table2 on table1.table2Id = table2.id
LEFT JOIN table3 on table2.table3id = table3.id
WHERE table1.id IN(
SELECT id
FROM (
(SELECT a.*, rownum rnum FROM(
SELECT table1.id
FROM table1,
table2,
table3
WHERE
table1.table2id = table2.id
AND
table2.table3id IS NULL OR table2.table3id = :table3IdParameter
) a
WHERE rownum <= :end))
WHERE rnum >= :start
Table1 and table2 are the large tables in this example. This query starts two full table scans on those tables.
Can we avoid this? We have, what we think are, the correct indexes.
/best regards, Håkan>
Hi Håkan - welcome to the forum.
We have a large Oracle database, v 10g. Two of the tables in the database have over one million rows.
We have a few queries which take a lot of time to execute. Not always though, it that seems when load is high the queries tend
to take much longer. Average time may be 1 or 2 seconds, but maxtime can be up to 2 minutes.
We have now used Oracle Grid to help us examine the queries. We have found that some of the queries require two or three full table scans.
Two of the full table scans are of the two large tables mentioned above.
This is an example query:Firstly, please read the forum FAQ - top right of page.
Please format your SQL using tags [code /code].
In order to help us to help you.
Please post table structures - relevant (i.e. joined, FK, PK fields only) in the form - note use of code tags - we can just run table create script.
CREATE TABLE table1
Field1 Type1,
Field2 Type2,
FieldN TypeN
);Then give us some table data - not 100's of records - just enough in the form
INSERT INTO Table1 VALUES(Field1, Field2.... FieldN);
..Please post EXPLAIN PLAN - again with tags.
HTH,
Paul...
/best regards, Håkan -
User I/O and db file parallel read is high in AWR report
Hi,
We have one performance issue during a job execution.
From the awr report we have identified one query with a table having millions of records causing problems and then we had also fine tuned that query by changing it's code and by using the optmizer hints. It is being executed in plsql batches. After fine tuning, On the first batch execution(first 5000 records) the query is taking only 5 mins, but on the consecutive batches it is consuming more time( more than 30 mins).
From the awr report I got the statistics as
Release : 11.2.0.2.0
Instance Efficiency Percentages (Target 100%)
Buffer Nowait %: 100.00 Redo NoWait %: 100.00
Buffer Hit %: 85.44 In-memory Sort %: 99.98
Library Hit %: 99.76 Soft Parse %: 99.15
Execute to Parse %: 88.91 Latch Hit %: 100.00
Parse CPU to Parse Elapsd %: 87.32 % Non-Parse CPU: 98.65
The buffer hit % is good. On each batch execution it is taking different set of records.
Top 5 Timed Foreground Events
Event Waits Time(s) Avg wait (ms) % DB time Wait Class
db file parallel read 120,485 42,540 353 89.60 User I/O
DB CPU 3,794 7.99
db file sequential read 145,074 606 4 1.28 User I/O
db file scattered read 70,030 556 8 1.17 User I/O
direct path write temp 12,423 21 2 0.04 User I/O
So the I/O is our main concern since that query contains one table with millions of records.
Host CPU (CPUs: 24 Cores: 24 Sockets: 4)
Load Average Begin Load Average End %User %System %WIO %Idle
1.40 1.45 0.6 0.3 3.7 99.0
Load is also normal.
From the Time model statistics , sql execute elapsed time is 98.27% of db time and only 7.99% is that of DB CPU.
Memory Statistics
Begin End
Host Mem (MB): 64,318.0 64,318.0
SGA use (MB): 30,720.0 30,720.0
PGA use (MB): 488.2 497.1
% Host Mem used for SGA+PGA: 48.52 48.54
Both the size of sga_max_size and sga_target are 32,212,254,720(32gb) bytes and that of
pga_aggregate target is 629,145,600(600mb)
from this it is evident that the memory is still available(so increase in memory size is not an option).
The sql statistics for that query shows like that
Elapsed Time (s) Executions Elapsed Time per Exec (s) %Total %CPU %IO SQL Id SQL Text
44,950.03 55 817.27 94.67 6.99 94.72 79dgmrxh4kv74 SELECT /*+ index(cdr_data cdr_...
I can't understand whether the problem is in the database side or with the query?
If the problem is with the query, then how it has been executed in 5 mins for the first batch ?
(all the batches are having 5000 records each).
And how can we reduce the db file parallel read ?
Your valuable advice will be greatly appreciated.
Thanks in advance
Manoj Kumar N"db file parallel read" is likely to be associated with something like index prefetching.
See:
http://www.freelists.org/post/oracle-l/RE-Calculating-LIOs,11
http://aprakash.wordpress.com/2012/05/29/index-range-scan-and-db-file-scattered-read-as-session-wait-event/
http://jonathanlewis.wordpress.com/2006/12/15/index-operations/
Tune the SQL.
Review the execution plan.
Check whether the statistics are accurate.
Review whether the index hint (and others that we can't see) is appropriate. -
Serial table scan with direct path read compared to db file scattered read
Hi,
The environment
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit
8K block size
db_file_multiblock_read_count is 128
show sga
Total System Global Area 1.6702E+10 bytes
Fixed Size 2219952 bytes
Variable Size 7918846032 bytes
Database Buffers 8724152320 bytes
Redo Buffers 57090048 bytes
16GB of SGA with 8GB of db buffer cache.
-- database is built on Solid State Disks
-- SQL trace and wait events
DBMS_MONITOR.SESSION_TRACE_ENABLE ( waits=>true )
-- The underlying table is called tdash. It has 1.7 Million rows based on data in all_objects. NO index
TABLE_NAME Rows Table Size/MB Used/MB Free/MB
TDASH 1,729,204 15,242 15,186 56
TABLE_NAME Allocated blocks Empty blocks Average space/KB Free list blocks
TDASH 1,943,823 7,153 805 0
Objectives
To show that when serial scans are performed on database built on Solid State Disks (SSD) compared to Magnetic disks (HDD), the performance gain is far less compared to random reads with index scans on SSD compared to HDD
Approach
We want to read the first 100 rows of tdash table randomly into buffer, taking account of wait events and wait times generated. The idea is that on SSD the wait times will be better compared to HDD but not that much given the serial nature of table scans.
The code used
ALTER SESSION SET TRACEFILE_IDENTIFIER = 'test_with_tdash_ssdtester_noindex';
DECLARE
type array is table of tdash%ROWTYPE index by binary_integer;
l_data array;
l_rec tdash%rowtype;
BEGIN
SELECT
a.*
,RPAD('*',4000,'*') AS PADDING1
,RPAD('*',4000,'*') AS PADDING2
BULK COLLECT INTO
l_data
FROM ALL_OBJECTS a;
DBMS_MONITOR.SESSION_TRACE_ENABLE ( waits=>true );
FOR rs IN 1 .. 100
LOOP
BEGIN
SELECT * INTO l_rec FROM tdash WHERE object_id = l_data(rs).object_id;
EXCEPTION
WHEN NO_DATA_FOUND THEN NULL;
END;
END LOOP;
END;
/Server is rebooted prior to any tests
Whern run as default, the optimizer (although some attribute this to the execution engine) chooses direct path read into PGA in preference to db file scattered read.
With this choice it takes 6,520 seconds to complete the query. The results are shown below
SQL ID: 78kxqdhk1ubvq
Plan Hash: 1148949653
SELECT *
FROM
TDASH WHERE OBJECT_ID = :B1
call count cpu elapsed disk query current rows
Parse 1 0.01 0.00 2 47 0 0
Execute 100 0.00 0.00 1 51 0 0
Fetch 100 10.88 6519.89 194142802 194831012 0 100
total 201 10.90 6519.90 194142805 194831110 0 100
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 96 (SSDTESTER) (recursive depth: 1)
Rows Row Source Operation
1 TABLE ACCESS FULL TDASH (cr=1948310 pr=1941430 pw=0 time=0 us cost=526908 size=8091 card=1)
Rows Execution Plan
0 SELECT STATEMENT MODE: ALL_ROWS
1 TABLE ACCESS MODE: ANALYZED (FULL) OF 'TDASH' (TABLE)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
Disk file operations I/O 3 0.00 0.00
db file sequential read 2 0.00 0.00
direct path read 1517504 0.05 6199.93
asynch descriptor resize 196 0.00 0.00
DECLARE
type array is table of tdash%ROWTYPE index by binary_integer;
l_data array;
l_rec tdash%rowtype;
BEGIN
SELECT
a.*
,RPAD('*',4000,'*') AS PADDING1
,RPAD('*',4000,'*') AS PADDING2
BULK COLLECT INTO
l_data
FROM ALL_OBJECTS a;
DBMS_MONITOR.SESSION_TRACE_ENABLE ( waits=>true );
FOR rs IN 1 .. 100
LOOP
BEGIN
SELECT * INTO l_rec FROM tdash WHERE object_id = l_data(rs).object_id;
EXCEPTION
WHEN NO_DATA_FOUND THEN NULL;
END;
END LOOP;
END;
call count cpu elapsed disk query current rows
Parse 0 0.00 0.00 0 0 0 0
Execute 1 3.84 4.03 320 48666 0 1
Fetch 0 0.00 0.00 0 0 0 0
total 1 3.84 4.03 320 48666 0 1
Misses in library cache during parse: 0
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 96 (SSDTESTER)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 1 0.00 0.00
SQL*Net message from client 1 0.00 0.00
SQL ID: 9babjv8yq8ru3
Plan Hash: 0
BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 1
Fetch 0 0.00 0.00 0 0 0 0
total 2 0.00 0.00 0 0 0 1
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: 96 (SSDTESTER)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 1 0.00 0.00
SQL*Net message from client 1 0.00 0.00
OVERALL TOTALS FOR ALL NON-RECURSIVE STATEMENTS
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 0 0 0 0
Execute 2 3.84 4.03 320 48666 0 2
Fetch 0 0.00 0.00 0 0 0 0
total 3 3.84 4.03 320 48666 0 2
Misses in library cache during parse: 0
Misses in library cache during execute: 1
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 2 0.00 0.00
SQL*Net message from client 2 0.00 0.00
log file sync 1 0.00 0.00
OVERALL TOTALS FOR ALL RECURSIVE STATEMENTS
call count cpu elapsed disk query current rows
Parse 9 0.01 0.00 2 47 0 0
Execute 129 0.01 0.00 1 52 2 1
Fetch 140 10.88 6519.89 194142805 194831110 0 130
total 278 10.91 6519.91 194142808 194831209 2 131
Misses in library cache during parse: 9
Misses in library cache during execute: 8
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
db file sequential read 5 0.00 0.00
Disk file operations I/O 3 0.00 0.00
direct path read 1517504 0.05 6199.93
asynch descriptor resize 196 0.00 0.00
102 user SQL statements in session.
29 internal SQL statements in session.
131 SQL statements in session.
1 statement EXPLAINed in this session.
Trace file: mydb_ora_16394_test_with_tdash_ssdtester_noindex.trc
Trace file compatibility: 11.1.0.7
Sort options: default
1 session in tracefile.
102 user SQL statements in trace file.
29 internal SQL statements in trace file.
131 SQL statements in trace file.
11 unique SQL statements in trace file.
1 SQL statements EXPLAINed using schema:
ssdtester.plan_table
Schema was specified.
Table was created.
Table was dropped.
1531657 lines in trace file.
6520 elapsed seconds in trace file.I then force the query not to use direct path read by invoking
ALTER SESSION SET EVENTS '10949 trace name context forever, level 1' -- No Direct path read ;In this case the optimizer uses db file scattered read predominantly and the query takes 4,299 seconds to finish which is around 34% faster than using direct path read (default).
The report is shown below
SQL ID: 78kxqdhk1ubvq
Plan Hash: 1148949653
SELECT *
FROM
TDASH WHERE OBJECT_ID = :B1
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 2 47 0 0
Execute 100 0.00 0.00 2 51 0 0
Fetch 100 143.44 4298.87 110348670 194490912 0 100
total 201 143.45 4298.88 110348674 194491010 0 100
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 96 (SSDTESTER) (recursive depth: 1)
Rows Row Source Operation
1 TABLE ACCESS FULL TDASH (cr=1944909 pr=1941430 pw=0 time=0 us cost=526908 size=8091 card=1)
Rows Execution Plan
0 SELECT STATEMENT MODE: ALL_ROWS
1 TABLE ACCESS MODE: ANALYZED (FULL) OF 'TDASH' (TABLE)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
Disk file operations I/O 3 0.00 0.00
db file sequential read 129759 0.01 17.50
db file scattered read 1218651 0.05 3770.02
latch: object queue header operation 2 0.00 0.00
DECLARE
type array is table of tdash%ROWTYPE index by binary_integer;
l_data array;
l_rec tdash%rowtype;
BEGIN
SELECT
a.*
,RPAD('*',4000,'*') AS PADDING1
,RPAD('*',4000,'*') AS PADDING2
BULK COLLECT INTO
l_data
FROM ALL_OBJECTS a;
DBMS_MONITOR.SESSION_TRACE_ENABLE ( waits=>true );
FOR rs IN 1 .. 100
LOOP
BEGIN
SELECT * INTO l_rec FROM tdash WHERE object_id = l_data(rs).object_id;
EXCEPTION
WHEN NO_DATA_FOUND THEN NULL;
END;
END LOOP;
END;
call count cpu elapsed disk query current rows
Parse 0 0.00 0.00 0 0 0 0
Execute 1 3.92 4.07 319 48625 0 1
Fetch 0 0.00 0.00 0 0 0 0
total 1 3.92 4.07 319 48625 0 1
Misses in library cache during parse: 0
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 96 (SSDTESTER)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 1 0.00 0.00
SQL*Net message from client 1 0.00 0.00
SQL ID: 9babjv8yq8ru3
Plan Hash: 0
BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 1
Fetch 0 0.00 0.00 0 0 0 0
total 2 0.00 0.00 0 0 0 1
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: 96 (SSDTESTER)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 1 0.00 0.00
SQL*Net message from client 1 0.00 0.00
OVERALL TOTALS FOR ALL NON-RECURSIVE STATEMENTS
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 0 0 0 0
Execute 2 3.92 4.07 319 48625 0 2
Fetch 0 0.00 0.00 0 0 0 0
total 3 3.92 4.07 319 48625 0 2
Misses in library cache during parse: 0
Misses in library cache during execute: 1
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 2 0.00 0.00
SQL*Net message from client 2 0.00 0.00
log file sync 1 0.00 0.00
OVERALL TOTALS FOR ALL RECURSIVE STATEMENTS
call count cpu elapsed disk query current rows
Parse 9 0.01 0.00 2 47 0 0
Execute 129 0.00 0.00 2 52 2 1
Fetch 140 143.44 4298.87 110348674 194491010 0 130
total 278 143.46 4298.88 110348678 194491109 2 131
Misses in library cache during parse: 9
Misses in library cache during execute: 8
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
db file sequential read 129763 0.01 17.50
Disk file operations I/O 3 0.00 0.00
db file scattered read 1218651 0.05 3770.02
latch: object queue header operation 2 0.00 0.00
102 user SQL statements in session.
29 internal SQL statements in session.
131 SQL statements in session.
1 statement EXPLAINed in this session.
Trace file: mydb_ora_26796_test_with_tdash_ssdtester_noindex_NDPR.trc
Trace file compatibility: 11.1.0.7
Sort options: default
1 session in tracefile.
102 user SQL statements in trace file.
29 internal SQL statements in trace file.
131 SQL statements in trace file.
11 unique SQL statements in trace file.
1 SQL statements EXPLAINed using schema:
ssdtester.plan_table
Schema was specified.
Table was created.
Table was dropped.
1357958 lines in trace file.
4299 elapsed seconds in trace file.I note that there are 1,517,504 waits with direct path read with total time of nearly 6,200 seconds. In comparison with no direct path read, there are 1,218,651 db file scattered read waits with total wait time of 3,770 seconds. My understanding is that direct path read can use single or multi-block read into the PGA. However db file scattered reads do multi-block read into multiple discontinuous SGA buffers. So it is possible given the higher number of direct path waits that the optimizer cannot do multi-block reads (contigious buffers within PGA) and hence has to revert to single blocks reads which results in more calls and more waits?.
Appreciate any advise and apologies for being long winded.
Thanks,
MichHi Charles,
I am doing your tests for t1 table using my server.
Just to clarify my environment is:
I did the whole of this test on my server. My server has I7-980 HEX core processor with 24GB of RAM and 1 TB of HDD SATA II for test/scratch backup and archive. The operating system is RHES 5.2 64-bit installed on a 120GB OCZ Vertex 3 Series SATA III 2.5-inch Solid State Drive.
Oracle version installed was 11g Enterprise Edition Release 11.2.0.1.0 -64bit. The binaries were created on HDD. Oracle itself was configured with 16GB of SGA, of which 7.5GB was allocated to Variable Size and 8GB to Database Buffers.
For Oracle tablespaces including SYS, SYSTEM, SYSAUX, TEMPORARY, UNDO and redo logs, I used file systems on 240GB OCZ Vertex 3 Series SATA III 2.5-inch Solid State Drive. With 4K Random Read at 53,500 IOPS and 4K Random Write at 56,000 IOPS (manufacturer’s figures), this drive is probably one of the fastest commodity SSDs using NAND flash memory with Multi-Level Cell (MLC). Now my T1 table created as per your script and has the following rows and blocks (8k block size)
SELECT
NUM_ROWS,
BLOCKS
FROM
USER_TABLES
WHERE
TABLE_NAME='T1';
NUM_ROWS BLOCKS
12000000 178952which is pretty identical to yours.
Then I run the query as brelow
set timing on
ALTER SESSION SET TRACEFILE_IDENTIFIER = 'test_bed_T1';
ALTER SESSION SET EVENTS '10046 TRACE NAME CONTEXT FOREVER, LEVEL 8';
SELECT
COUNT(*)
FROM
T1
WHERE
RN=1;
which gives
COUNT(*)
60000
Elapsed: 00:00:05.29
tkprof output shows
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.02 5.28 178292 178299 0 1
total 4 0.02 5.28 178292 178299 0 1
Compared to yours:
Fetch 2 0.60 4.10 178493 178498 0 1
It appears to me that my CPU utilisation is by order of magnitude better but my elapsed time is worse!
Now the way I see it elapsed time = CPU time + wait time. Further down I have
Rows Row Source Operation
1 SORT AGGREGATE (cr=178299 pr=178292 pw=0 time=0 us)
60000 TABLE ACCESS FULL T1 (cr=178299 pr=178292 pw=0 time=42216 us cost=48697 size=240000 card=60000)
Rows Execution Plan
0 SELECT STATEMENT MODE: ALL_ROWS
1 SORT (AGGREGATE)
60000 TABLE ACCESS MODE: ANALYZED (FULL) OF 'T1' (TABLE)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 3 0.00 0.00
SQL*Net message from client 3 0.00 0.00
Disk file operations I/O 3 0.00 0.00
direct path read 1405 0.00 4.68
Your direct path reads are
direct path read 1404 0.01 3.40Which indicates to me you have faster disks compared to mine, whereas it sounds like my CPU is faster than yours.
With db file scattered read I get
Elapsed: 00:00:06.95
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 1.22 6.93 178293 178315 0 1
total 4 1.22 6.94 178293 178315 0 1
Rows Row Source Operation
1 SORT AGGREGATE (cr=178315 pr=178293 pw=0 time=0 us)
60000 TABLE ACCESS FULL T1 (cr=178315 pr=178293 pw=0 time=41832 us cost=48697 size=240000 card=60000)
Rows Execution Plan
0 SELECT STATEMENT MODE: ALL_ROWS
1 SORT (AGGREGATE)
60000 TABLE ACCESS MODE: ANALYZED (FULL) OF 'T1' (TABLE)
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 2 0.00 0.00
Disk file operations I/O 3 0.00 0.00
db file sequential read 1 0.00 0.00
db file scattered read 1414 0.00 5.36
SQL*Net message from client 2 0.00 0.00
compared to your
db file scattered read 1415 0.00 4.16On the face of it with this test mine shows 21% improvement with direct path read compared to db scattered file read. So now I can go back to re-visit my original test results:
First default with direct path read
call count cpu elapsed disk query current rows
Parse 1 0.01 0.00 2 47 0 0
Execute 100 0.00 0.00 1 51 0 0
Fetch 100 10.88 6519.89 194142802 194831012 0 100
total 201 10.90 6519.90 194142805 194831110 0 100
CPU ~ 11 sec, elapsed ~ 6520 sec
wait stats
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
direct path read 1517504 0.05 6199.93
roughly 0.004 sec for each I/ONow with db scattered file read I get
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 2 47 0 0
Execute 100 0.00 0.00 2 51 0 0
Fetch 100 143.44 4298.87 110348670 194490912 0 100
total 201 143.45 4298.88 110348674 194491010 0 100
CPU ~ 143 sec, elapsed ~ 4299 sec
and waits:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
db file sequential read 129759 0.01 17.50
db file scattered read 1218651 0.05 3770.02
roughly 17.5/129759 = .00013 sec for single block I/O and 3770.02/1218651 = .0030 for multi-block I/ONow my theory is that the improvements comes from the large buffer cache (8320MB) inducing it to do some read aheads (async pre-fetch). Read aheads are like quasi logical I/Os and they will be cheaper compared to physical I/O. When there is large buffer cache and read aheads can be done then using buffer cache is a better choice than PGA?
Regards,
Mich -
求教:oracle10g如果设置查询语句可以通过直接路径读,而不走db file scattered read
如题，在Oracle10g版本中，是不是有什么提示，可以让某个查询语句在读大表时，是走直接路径访问，而不走db file scattered read.
我只知道 ，好象只要加上并行，就可以走直接路径访问了.
是不是还有其它的方法？
多问一下，如果是版本是11g有什么方法？
谢谢！session level改_SMALL_TABLE_THRESHOLD，这个参数很危险；10和11应该都能用。
还有一个参数alter session set "_serial_direct_read"=true;但是我测试的结果它不怎么靠谱 -
Error message Translational currency is too high when posting GR for a PO
Hi,
I am trying to post a GR for a PO,with document currency say in GBP and local currency in EUR,when i tried to post a GR the system gives the message with-"Translational currency is too high" and also the value updated for previous GRs(for a qty 10) is vary high like 37,056,361,610.68 GBP,just i want to know how this value is picked by the system,i know exhange rate table maintained is plays important role,but the value i am getting is too high which is not at all related with rates we maintained in the table TCURR.
I want to know why system is giving such a high value.
further PO quantity is 17,000 and net price is 1 GBP.
we have posted very sucessfull GR earlier,but now this high value and error in GR we are facing.
with regards,Hi,
Good afternoon and greetings,
Wishing you a Happy New Year 2007
Please go through the following OSS Note
Note 191927 - Posting logic: GR for foreign currency PO
Please reward points if found useful
Thanking you
With kindest regards
Ramesh Padmanabhan -
Too stupid and/or lazy for anything but Arch?
Many times during the last 15 years, I made an effort to use GNU/Linux, mostly so-called "beginner-friendly" distributions, as my main operating system but never successfully. There were always some issues that I was too stupid or too lazy to understand and fix. So I kept using Windows and later on Mac OS X.
While still using Mac OS X, I inexplicably stumbled upon Arch some months ago and gave it a try. I followed the awesome Beginner's Guide to setup Arch on my notebook. For the first time there were no issues I could not resolve. Overall, it was beginner-friendly, transparent and fun. I have been using Arch as my sole operating system since then. Best computing experience ever.
Recently, I am becoming more and more interested in other GNU/Linux distributions and *BSDs. I tried to find a (desktop) system that:
1) followed KISS,
2) (easily) allowed a minimal installation,
3) was documented for the stupid and/or lazy.
I looked at Debian but its init system scared me. I looked at Gentoo but its compiling scared me. I looked at Slackware but its documentation scared me.
I am wondering: Am I unfoundedly scared by either of these? Have I overlooked a system? Or am I just too stupid and/or lazy for anything but Arch?it sounds to me like Arch is just the best fit for you
...and i can share my own perspective on the matter...
I've used many many distro's; gentoo (and a few derivatives), Fedora (and a few remixes), Ubuntu (and many of it's derivatives), puppy (and...same goes..), slackware, FreeBSD, PClinuxOS, debian (various versions), etc, etc....
i haven't found any distribution that i like better than Arch....and i do not 'distro-hop' anymore at all. there is absolutely no point for me to do so...
So I don't think you're too stupid or lazy for anything but Arch, it's quite the opposite... Arch is simple/easy to use, but does require the user to be somewhat competent, right? - i say this because things can break (although not too often), and the end-user has to be able to do things a little more hands on than some of the so-called 'user-friendly distro's'. we also all tend to be aware of where all config files are, how to edit them, where they are located, etc...the average archer knows the underlying system fairly well, i tend to think.
Arch also tends to offer some of the advantages of both meta/binary distributions; ie: ubuntu, fedora, etc....while also offering the advantages of source-based distributions; ie: gentoo, yoper, etc. because it covers both types, you also do away with some of the disadvantages of both types; ie: compiling from source is more often than not as simple as using tools like 'yum' or 'apt-get' (which i suppose could be taken as laziness, but i tend to think it is just a better design), while in say Fedora or Ubuntu, there is more to do, to accomplish the exact same end-result... another example, we also don't have to sit around and wait for the whole system to compile on system upgrades (unless you choose too, using pacbuilder), this to me is a significant advantage over source-based distro's too....
I feel as an Archer i reap the benefits of all distros, in other ways too. for example, if i want some patch from Ubuntu, or say some software that is found in a PPA - usually i can find it in AUR. i need not bother with adding software sources, or any of that stuff, all i have to do is use 'yaourt -S *pkgname*'... the whole thing is automated, it grabs the source, builds it, installs it and i am done. easy peasy
then, there is the part, where using the kernel (with the patches i want), is much easier than say Fedora or Ubuntu, etc. I used to have to download and patch everything myself for the most part. but same goes as the example above - now all that it requires it 'yaourt -S *pkgname*'... which is much better, you want to use your applications, not spend your evening patching and compiling everything manually, when you don't have too - right? I am a tinkerer, but i still try to take the most sensible approach, ya know...?
then, as someone else pointed out. We have some of the best documentation available in ArchWiki - this is why i switched to Arch in the first place. i had been using ArchWiki with other distributions for a long time. it got to the point, that i started to realize that maybe Arch as a better fit for me...and the wiki showed me some of the other advantages of using Arch over other distros...
anyway, that's my 2 cents on the matter
cheerz -
I desperately need to uninstall adobe reader from my Mac book air. I sent adobe reader to trash and afterwards look for internet plugs in library but this folder is empty in my library. Can anybody help me?
I looked for plugs in in my library but the folder internet plugs in is empty
-
Help - i recently made put together a high quality movie for a relative, it has taken me months to complete and it goes for a total of 9 hours and 43 minutes ,however, it won't let me export the video at all! please help - its taken ages to make it!
9 hours??!
Twice the length of a cinema epic?
How are you expecting to distribute it?
iDVD encoding settings:
http://docs.info.apple.com/article.html?path=iDVD/7.0/en/11417.html
Short version:
Best Performance is for videos of up to 60 minutes
Best Quality is for videos of up to 120 minutes
Professional Quality is also for up to 120 minutes but even higher quality (and takes much longer)
That was for single-layer DVDs. Double these numbers for dual-layer DVDs.
Professional Quality: The Professional Quality option uses advanced technology to encode your video, resulting in the best quality of video possible on your burned DVD. You can select this option regardless of your project’s duration (up to 2 hours of video for a single-layer disc and 4 hours for a double-layer disc). Because Professional Quality encoding is time-consuming (requiring about twice as much time to encode a project as the High Quality option, for example) choose it only if you are not concerned abo
In both cases the maximum length includes titles, transitions and effects etc. Allow about 15 minutes for these.
You can use the amount of video in your project as a rough determination of which method to choose. If your project has an hour or less of video (for a single-layer disc), choose Best Performance. If it has between 1 and 2 hours of video (for a single-layer disc), choose High Quality. If you want the best possible encoding quality for projects that are up to 2 hours (for a single-layer disc), choose Professional Quality. This option takes about twice as long as the High Quality option, so select it only if time is not an issue for you.
Use the Capacity meter in the Project Info window (choose Project > Project Info) to determine how many minutes of video your project contains.
NOTE: With the Best Performance setting, you can turn background encoding off by choosing Advanced > “Encode in Background.” The checkmark is removed to show it’s no longer selected. Turning off background encoding can help performance if your system seems sluggish.
And whilst checking these settings in iDVD Preferences, make sure that the settings for NTSC/PAL and DV/DV Widescreen are also what you want.
http://support.apple.com/kb/HT1502?viewlocale=en_US -
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. -
Regd: full table scan in sql query
hi friends,
i have one query below.
SELECT wm_concat (d.cp_hrchy_desc) LOCATION
FROM am_tm_chnl_prtnr_hrchy d, am_tl_user_hrchy e
WHERE d.pk_cp_hrchy_id = e.fk_cp_hrchy_id
AND e.fk_ent_mst_customers = 13754
its given me 'Full Table Scan' on table am_tm_chnl_prtnr_hrchy.its just simple query.
want to remove Full Scan.
see below plan of above query.
Plan
SELECT STATEMENT ALL_ROWSCost: 10 Bytes: 18 Cardinality: 1
4 SORT AGGREGATE Bytes: 18 Cardinality: 1
3 NESTED LOOPS Cost: 10 Bytes: 1,314 Cardinality: 73
1 TABLE ACCESS FULL TABLE BTSTARGET.AM_TM_CHNL_PRTNR_HRCHY
Cost: 9 Bytes: 50,831 Cardinality: 4,621
2 INDEX UNIQUE SCAN INDEX (UNIQUE) BTSTARGET.PK_USER_HRCHY_01 Cost: 0 Bytes: 7 Cardinality: 1Full table scan is not evil. Oracle selects full table scan only when its optimal.
So now tell us how many rows are you selecting from am_tm_chnl_prtnr_hrchy table. Give the percentage of data fetched from this table.
If its high say 90% then a FTS is a good option. Also do you have any index defined on the table am_tm_chnl_prtnr_hrchy. If yes give us the details.
See the below thread they will give you a nice idea of how to post a performance related question.
{thread:id=501834} and {thread:id=863295}.
Maybe you are looking for
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