| General |
Note: Compression is available for heap tables, partitioned tables (named
partitions may be excluded, LOBs, multi-column indexes, and SecureFile tablespaces.
Advanced compression only kicks in if it believes the saving will exceed
20%. This is done to avoid wasting CPU. Be sure to test compressibility
before implementing this technology. |
| Related Data Dictionary Objects |
| ind$ |
dba_indexes |
all_indexes |
user_indexes |
| tab$ |
dba_tables |
all_tables |
user_tables |
|
| |
| Compressed Table |
COMPRESSED |
The table_compression clause is valid only for heap-organized tables. Use
this clause to instruct the database whether to compress data segments to reduce disk use. This clause is especially
useful in environments such as data warehouses, where the amount of insert and update operations is small. The COMPRESS
keyword enables table compression. The NOCOMPRESS keyword disables table compression. NOCOMPRESS is the default.
When you enable table compression, Oracle Database attempts to compress data during direct-path INSERT operations when
it is productive to do so. The original import utility (imp) does not support direct-path INSERT, and therefore cannot
import data in a compressed format. You can specify table compression for the following portions of a heap-organized table.
- For an entire table, in the physical_properties clause of relational_table or object_table
- For a range partition, in the table_partition_description of the range_partitioning clause
- For a list partition, in the table_partition_description of the list_partitioning clause
- For the storage table of a nested table, in the nested_table_col_properties clause
Table compression saves disk space and reduces memory use in the buffer cache, and is completely
transparent to applications. Compression ratios as high as 3.5 : 1 can be achieved. Table compression can also speed up query
execution during reads. There is, however, a cost in CPU overhead for DML.
|
Compressed Tablespace |
CREATE TABLESPACE ...
DEFAULT COMPRESS; |
conn / as sysdba
CREATE TABLESPACE compressed
DATAFILE 'compressed.dbf' SIZE 5M
DEFAULT COMPRESS;
SELECT tablespace_name, def_tab_compression
FROM dba_tablespaces;
ALTER USER uwclass
QUOTA unlimited ON compressed;
conn uwclass/uwclass
CREATE TABLE regtab
TABLESPACE uwdata AS
SELECT * FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
CREATE TABLE comptab
TABLESPACE compressed AS
SELECT * from all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
SELECT table_name, tablespace_name, compression
FROM user_tables
WHERE table_name IN ('REGTAB', 'COMPTAB');
exec dbms_stats.gather_table_stats(USER, 'REGTAB');
exec dbms_stats.gather_table_stats(USER, 'COMPTAB');
SELECT table_name, blocks
FROM user_tables
WHERE table_name IN ('REGTAB', 'COMPTAB');
conn / as sysdba
DROP TABLESPACE compressed INCLUDING CONTENTS AND DATAFILES; |
Compressed Table
New in 11g |
CREATE TABLE ...
COMPRESS; |
conn uwclass/uwclass
CREATE TABLE regtab
TABLESPACE uwdata AS
SELECT * FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
CREATE TABLE comptab
TABLESPACE uwdata COMPRESS AS
SELECT * FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
SELECT table_name, tablespace_name, compression
FROM user_tables
WHERE table_name IN ('REGTAB', 'COMPTAB');
exec dbms_stats.gather_table_stats(USER, 'REGTAB');
exec dbms_stats.gather_table_stats(USER, 'COMPTAB');
SELECT table_name, blocks
FROM user_tables
WHERE table_name IN ('REGTAB', 'COMPTAB'); |
Another example |
CREATE TABLE t1 (
testcol VARCHAR2(50))
TABLESPACE uwdata;
CREATE TABLE t2 (
testcol VARCHAR2(50))
TABLESPACE uwdata COMPRESS;
DECLARE
x t1.testcol%TYPE;
BEGIN
FOR i IN 1 .. 100000 LOOP
SELECT dbms_crypto.randombytes(25)
INTO x
FROM dual;
INSERT INTO t1 VALUES (x);
INSERT INTO t2 VALUES (x);
END LOOP;
COMMIT;
END;
/
-- some sample rows:
3605CAA721159CAC4E462B841419CCB7390F1AE3484FF14963
05B7AE0B6BB076EEAF3E8E7DBA1BE9D5C8F97737AA1FDF21A5
40756BCEBF00CCB80ACA5F4F6BF3AFE6BC19D19EA74F10212B
234812A15930421A208BCF19C943762B5FA11D0C0C7E811F5E
4177AFC94C248D6B6765B8CE45FE3E49E2E5456BA6BA48C147
exec dbms_stats.gather_table_stats(USER, 'T1');
exec dbms_stats.gather_table_stats(USER, 'T2');
SELECT table_name, blocks
FROM user_tables
WHERE table_name IN ('T1', 'T2'); |
Another example |
conn sh/sh
CREATE TABLE sh_regular AS
SELECT * FROM sales;
CREATE TABLE sh_compressed COMPRESS AS
SELECT * FROM sales;
exec dbms_stats.gather_table_stats(USER, 'SALES');
-- partitioned
exec dbms_stats.gather_table_stats(USER, 'SH_COMPRESSED');
exec dbms_stats.gather_table_stats(USER, 'SH_REGULAR');
SELECT table_name, num_rows, blocks, compression
FROM user_tables
WHERE table_name LIKE 'S%'; |
Compressed Direct Load
New in 11g |
CREATE TABLE ...
COMPRESS FOR DIRECT_LOAD OPERATIONS; |
CREATE TABLE regtab
TABLESPACE uwdata AS
SELECT /*+ APPEND */ *
FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
CREATE TABLE comptab
TABLESPACE uwdata COMPRESS FOR DIRECT_LOAD OPERATIONS AS
SELECT /*+ APPEND */ *
FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
SELECT table_name, tablespace_name, compression
FROM user_tables
WHERE table_name IN ('REGTAB', 'COMPTAB');
exec dbms_stats.gather_table_stats(USER, 'REGTAB');
exec dbms_stats.gather_table_stats(USER, 'COMPTAB');
SELECT table_name, blocks
FROM user_tables
WHERE table_name IN ('REGTAB', 'COMPTAB'); |
| |
| Compressed Indexes |
Create Compressed Index |
CREATE INDEX <index_name>
ON (<first_column>, <second_column>)
PCTFREE <integer>
TABLESPACE <tablespace_name>
COMPRESS <integer *>;
* Must be at least 1 less than the number of indexed columns |
-- For a copy of postal_code visit the Setup Page: [Click Here]
CREATE INDEX ix_pcodes
ON postal_code (state_abbrev, city_name, zip_code)
PCTFREE 0
TABLESPACE uwdata;
ANALYZE INDEX ix_pcodes VALIDATE STRUCTURE;
exec dbms_stats.gather_index_stats(USER, 'IX_PCODES');
SELECT opt_cmpr_count, opt_cmpr_pctsave
FROM index_stats;
SELECT sum(bytes)
FROM user_segments
WHERE segment_name = 'IX_PCODES';
DROP INDEX ix_pcodes;
CREATE INDEX ix_pcodes
ON postal_code (state_abbrev, city_name, zip_code)
PCTFREE 0
TABLESPACE uwdata
COMPRESS 1;
ANALYZE INDEX ix_pcodes VALIDATE STRUCTURE;
exec dbms_stats.gather_index_stats(USER, 'IX_PCODES');
SELECT opt_cmpr_count, opt_cmpr_pctsave
FROM index_stats;
SELECT sum(bytes)
FROM user_segments
WHERE segment_name = 'IX_PCODES';
DROP INDEX ix_pcodes;
CREATE INDEX ix_pcodes
ON postal_code (state_abbrev, city_name, zip_code)
PCTFREE 0
TABLESPACE uwdata
COMPRESS 2;
ANALYZE INDEX ix_pcodes VALIDATE STRUCTURE;
exec dbms_stats.gather_index_stats(USER, 'IX_PCODES');
SELECT opt_cmpr_count, opt_cmpr_pctsave
FROM index_stats;
SELECT sum(bytes)
FROM user_segments
WHERE segment_name = 'IX_PCODES';
-- so pay attention to opt_comp_count |
| Uncompressing an index |
ALTER INDEX <index_name> NOCOMPRESS; |
| ALTER INDEX cix_pcodes NOCOMPRESS; |
| |
| Test Cases |
|
Test 1: INSERT Performance |
conn uwclass/uwclass
CREATE TABLE regtab
TABLESPACE uwdata AS
SELECT * FROM all_objects
WHERE 1=2;
CREATE TABLE comptab
TABLESPACE uwdata COMPRESS AS
SELECT * FROM all_objects
WHERE 1=2;
SELECT table_name, tablespace_name, compression
FROM user_tables
WHERE table_name IN ('REGTAB', 'COMPTAB');
CREATE OR REPLACE PROCEDURE regtest1 IS
BEGIN
FOR i IN 1..10 LOOP
INSERT INTO regtab
SELECT * FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
END LOOP;
COMMIT;
END regtest1;
/
CREATE OR REPLACE PROCEDURE comptest1 IS
BEGIN
FOR i IN 1..10 LOOP
INSERT INTO comptab
SELECT * FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
END LOOP;
COMMIT;
END comptest1;
/
|
Test 2: UPDATE Performance |
SELECT MIN(object_id),
MAX(object_id)
FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'WZZZZZ';
CREATE OR REPLACE PROCEDURE regtest2 IS
j PLS_INTEGER;
BEGIN
FOR i IN 1..1000 LOOP
j := TRUNC(dbms_random.value(2, 80984));
UPDATE regtab
SET namespace = i
WHERE object_id = j;
END LOOP;
COMMIT;
END regtest2;
/
CREATE OR REPLACE PROCEDURE comptest2 IS
j PLS_INTEGER;
BEGIN
FOR i IN 1..1000 LOOP
j := TRUNC(dbms_random.value(2, 80984));
UPDATE comptab
SET namespace = i
WHERE object_id = j;
END LOOP;
COMMIT;
END comptest2;
/
|
Test 3: DELETE Performance |
CREATE OR REPLACE PROCEDURE regtest3 IS
CURSOR s_cur IS
SELECT DISTINCT namespace
FROM regtab;
TYPE NumTab IS TABLE OF regtab.namespace%TYPE INDEX BY PLS_INTEGER;
s_array NumTab;
BEGIN
OPEN s_cur;
FETCH s_cur BULK COLLECT INTO s_array LIMIT 500;
FOR i IN 1..s_array.LAST LOOP
DELETE FROM regtab
WHERE namespace = s_array(i);
END LOOP;
CLOSE s_cur;
COMMIT;
END regtest3;
/
CREATE OR REPLACE PROCEDURE comptest3 IS
CURSOR s_cur IS
SELECT DISTINCT namespace
FROM comptab;
TYPE NumTab IS TABLE OF comptab.namespace%TYPE INDEX BY PLS_INTEGER;
s_array NumTab;
BEGIN
OPEN s_cur;
FETCH s_cur BULK COLLECT INTO s_array LIMIT 500;
FOR i IN 1..s_array.COUNT LOOP
DELETE FROM comptab
WHERE namespace = s_array(i);
END LOOP;
CLOSE s_cur;
COMMIT;
END comptest3;
/
|
Testing |
set timing on
exec regtest1
exec comptest1
Insert Results
| Proc |
Run 1 |
Run 2 |
Run 3 |
| Reg |
9.21 |
9.15 |
9.31 |
| Comp |
9.21 |
8.92 |
9.07 |
exec regtest2
exec comptest2
Update Results
| Proc |
Run 1 |
Run 2 |
Run 3 |
| Reg |
21.78 |
23.21 |
23.09 |
| Comp |
22.56 |
22.78 |
23.20 |
exec regtest3
exec comptest3
Delete Results
| Proc |
Run 1 |
Run 2 |
Run 3 |
| Reg |
7.35 |
8.48 |
11.81 |
| Comp |
8.95 |
7.68 |
9.18 |
TRUNCATE TABLE regtab;
TRUNCATE TABLE comptab; |
|
