1. Database Tuning - User and Rollback Data Spaces, Recovery, Backup
ISYS366 – Week 5-6
Database Tuning - User and Rollback Data Spaces, Recovery, Backup
9/12/2018
ISYS366 - Week05

2. Database Deliverables of a System Development Process
ERD
Relational model
Disk space requirements
Tuning goals – response time, etc.
Security requirements
Data requirements
Data entry and retrieval
Backup and recovery
Execution plans – EXPLAIN PLAN
Acceptance testing
9/12/2018
ISYS366 - Week05

3. Disk Space Requirements – Sizing tables
CREATE TABLE employee (…)
TABLESPACE user_work
PCTFREE 20
PCTUSED 60
STORAGE(INITIAL 10M NEXT 5K PCTINCREASE 0);
9/12/2018
ISYS366 - Week05

4. Disk Space Requirements – Sizing tables
Blocks
Block header = 90 bytes
2K block size: = 1958 bytes
4K block size: = 4006 bytes
9/12/2018
ISYS366 - Week05

5. Disk Space Requirements – Sizing tables
Blocks
Block header = 90 bytes
2K block size: = 1958 bytes
4K block size: = 4006 bytes
pctfree
the amount of space reserved in the block for updates that will expand the row
the purpose is to minimize disk I/O by reducing row chaining and row migration
row chaining occurs when you have to go to another block to get the rest of the row
row migration occurs when you have to write a row to another data block (can be cause by too low pctfree)
9/12/2018
ISYS366 - Week05

6. Disk Space Requirements – Sizing tables
pctfree
used for both tables and indexes
Default is 10%
The more volatile you data, the higher pctfree
For DW and OLAP, pctfree can be 0
1958*(10/100) = 196 bytes (round up)
9/12/2018
ISYS366 - Week05

7. Disk Space Requirements – Sizing tables
pctused
Determine when used blocks can be readded to the list of blocks into which rows can be added
Default is 40, which is too low for frequent deletes
For frequent deletes, try: 95 - pctfree
9/12/2018
ISYS366 - Week05

8. Disk Space Requirements – Sizing tables
Free list space
Each table has associated with it lists of data blocks that have been allocated for that table
These lists contain free spaces for inserting rows called "free lists“
Blocks in the free list is less than PCTUSED full
2048 – 90 – 196 = 1762 bytes for data
9/12/2018
ISYS366 - Week05

9. Disk Space Requirements – Sizing tables
Space used per row
Bytes per row
When data is available
Average bytes per row
SELECT AVG(NVL(VSIZE(col1),0)) +
AVG(NVL(VSIZE(col2),0)) +
AVG(NVL(VSIZE(col3),0)) +
AVG(NVL(VSIZE(col4),0))
FROM table;
9/12/2018
ISYS366 - Week05

10. Disk Space Requirements – Sizing tables
Space used per row
When data is available
Maximum bytes per row
SELECT column_name, data_type, data_length
FROM table
WHERE table_name = 'table_name';
3 bytes for row header
1 byte for each column
1 byte for each long column
10 bytes for each ROWID (Oracle 8)
Rows per block
Free space/bytes per row
9/12/2018
ISYS366 - Week05

11. Disk Space Requirements – Sizing tables
Determining if pctfree is correct
ANALYZE TABLE table COMPUTE STATISTICS;
RETRIEVE DATA
SELECT Num_rows, Blocks, Num_rows/Blocks
FROM User_Tables
WHERE Table_name = 'TABLE_NAME';
Perform an update
Run ANALYZE again
If the number of rows is fewer, then that means they have been moved to a new data block. Thus, the pctfree is NOT high enough.
If, however, the Avg_space (also generated by ANALYZE) is large, then the pctfree is TOO high
9/12/2018
ISYS366 - Week05

12. Determining actual storage
SELECT COUNT(*) FROM mytable;
SELECT COUNT (DISTINCT (SUBSTR (ROWID, 1, 8)) || (SUBSTR (ROWID, 15,4))) FROM mytable;
#rows per block = #rows/#blocks OR
ANALYZE TABLE table COMPUTE STATISTICS;
RETRIEVE DATA
SELECT Num_rows, Blocks, Num_rows/Blocks FROM User_Tables WHERE Table_name = 'TABLE_NAME';
9/12/2018
ISYS366 - Week05

13. Disk Space Requirements – Sizing indexes
Blocks
Block header = 161 bytes
2K block size: = 1887 bytes
4K block size: = 3935 bytes
pctfree
Same as for tables
9/12/2018
ISYS366 - Week05

14. Disk Space Requirements – Sizing indexes
Space used per index row
Average bytes per indexed column
8 bytes for index row header
1 byte for each column
1 byte for each long column
1 byte if index is UNIQUE
6-10 bytes for each ROWID (Oracle 8)
Blocking Factor (rows per block)
Free space/bytes per row
9/12/2018
ISYS366 - Week05

15. Disk Space Requirements – Sizing indexes
N.B. deleted index space is rarely reused, so indexes may grow even if tables don't!
9/12/2018
ISYS366 - Week05

16. Sizing Tablespaces 9/12/2018 ISYS366 - Week05
SQL> select * From user_tablespaces;
TABLESPACE_NAME INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS MAX_EXTENTS PCT_INCREASE
MIN_EXTLEN STATUS CONTENTS LOGGING EXTENT_MAN ALLOCATIO
SYSTEM
0 ONLINE PERMANENT LOGGING DICTIONARY USER
RBS
USERS
TEMP
0 ONLINE TEMPORARY LOGGING DICTIONARY USER
INDX
OEM_REPOSITORY E
ONLINE PERMANENT LOGGING DICTIONARY USER
DRSYS
WEBBOARD
8 rows selected.
9/12/2018
ISYS366 - Week05