create table s1 as select * from SH.SALES;
create table s2 as select * from SH.SALES;
set timing on;
select * from s1 where prod_id=1;
select * from s2 where prod_id=1;
SELECT A.OWNER, A.TABLE_OWNER, A.TABLE_NAME, A.INDEX_NAME, A.INDEX_TYPE,
B.COLUMN_POSITION, B.COLUMN_NAME, C.TABLESPACE_NAME,
FROM DBA_INDEXES A, DBA_IND_COLUMNS B, DBA_TABLES C
WHERE A.OWNER = UPPER ('hr')
AND A.OWNER = B.INDEX_OWNER
AND A.OWNER = C.OWNER
AND A.TABLE_NAME LIKE UPPER ('DEPARTMENTS')
AND A.TABLE_NAME = B.TABLE_NAME
AND A.TABLE_NAME = C.TABLE_NAME
AND A.INDEX_NAME = B.INDEX_NAME
ORDER BY A.OWNER, A.TABLE_OWNER, A.TABLE_NAME, A.INDEX_NAME, B.COLUMN_POSITION
SELECT OWNER, TABLE_NAME
WHERE OWNER NOT IN ('SYS','SYSTEM','OUTLN','DBSNMP') AND OWNER = UPPER ('scott')
SELECT OWNER, TABLE_NAME
WHERE OWNER NOT IN ('SYS','SYSTEM','OUTLN','DBSNMP')
SELECT OWNER, TABLE_NAME, COUNT (*) "count"
WHERE OWNER NOT IN ('SYS','SYSTEM','OUTLN','DBSNMP') AND OWNER = UPPER ('hr')
GROUP BY OWNER, TABLE_NAME
HAVING COUNT (*) > ('4')
create table table1 as select * from SH.SALES;
create table table2 as select * from SH.SALES;
SELECT count(*) FROM table1 where prod_id=30;
set timing on;
update table1 set cust_id=1 where prod_id=30;
update table2 set cust_id=1 where prod_id=30;
A full table scan occurs when every block is read from a table. Full table scans are often a preferred performance option in batch-style applications, such as decision support. We have seen some excellent run-time improvements in decision support systems that use the parallel query option, which relies on full table scans to operate. However, full table scans at an OLTP site during prime online usage times can create havoc with response times. Full table scans, even on small tables, can degrade response times particularly when the small table drives the query, and this table is not always the most efficient access path.
The following query reports how many full table scans are taking place:
SELECT name, value
WHERE name LIKE '%table %'
ORDER BY name;
The values relating to the full table scans are:
table scans (long tables) - a scan of a table that has more than five database blocks
table scans (short tables) - a count of full table scans with five or fewer blocks
If the number of long table scans is significant, there is a strong possibility that SQL statements in your application need tuning or indexes need to be added.
To get an appreciation of how many rows and blocks are being accessed on average for the long full table scans, use this calculation (the sample data comes from an OLTP application):
Average Long Table Scan Blocks
= (table scan blocks gotten - (short table scans * 5))
/ long table scans
= (3,540,450 - (160,618 * 5)) / 661
= (3,540,450 - (803,090)) / 661
= 4,141 blocks read per full table scan
In our example, 4141 average disk reads performed on an OLTP application 661 times in the space of a few short hours is not a healthy situation.
If you can identify the users who are experiencing the full table scans, you can find out what they were running to cause these scans. Below is a script that allows you to do this:
REM FILE NAME: fullscan.sql
REM LOCATION: Database Tuning\File I/O Reports
REM FUNCTION: Identifies users of full table scans
REM TESTED ON: 126.96.36.199, 188.8.131.52, 8.1.5, 8.1.7, 9.0.1, 184.108.40.206
REM PLATFORM: non-specific
REM REQUIRES: v$session, v$sesstat, v$statname
REM This view is used by the fscanavg.sql script
REM This is a part of the Knowledge Xpert for Oracle Administration REM library.
REM Copyright (C) 2001 Quest Software
REM All rights reserved.
REM************ Knowledge Xpert for Oracle Administration *************
DROP VIEW full_table_scans;
CREATE VIEW full_table_scans
|| ') ' "User Process",
SUM (DECODE (NAME, 'table scans (short tables)', VALUE)) "Short Scans",
SUM (DECODE (NAME, 'table scans (long tables)', VALUE)) "Long Scans",
SUM (DECODE (NAME, 'table scan rows gotten', VALUE)) "Rows Retrieved"
FROM v$session ss, v$sesstat se, v$statname sn
WHERE se.statistic# = sn.statistic#
AND ( NAME LIKE '%table scans (short tables)%'
OR NAME LIKE '%table scans (long tables)%'
OR NAME LIKE '%table scan rows gotten%'
AND se.sid = ss.sid
AND ss.username IS NOT NULL
GROUP BY ss.username
|| ') ';