Oracle vs PostgreSQL，研發注意事項（4）- PageSize對資料儲存的影響
本文簡單介紹了PageSize對資料儲存的影響。Oracle有BlockSize的概念，但對資料儲存沒有太多的影響，看起來對於使用者來說是透明的，但在PG中，BlockSize對於不同的Column Storage選項會有不同的影響。
一、Oracle
Oracle資料庫，Block Size設定為8K
TEST-cndb@pc15.infogov>show parameter db_block_size
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_size integer 8192
-- 為方便分析，建立一個只有128K的表空間
TEST-cndb@pc15.infogov>create tablespace tbs_tmp datafile '/data/oradata/testtbs/tbs_tmp01.dbf' size 128K autoextend off;
Tablespace created.
-- 建立一張表，儲存在tbs_tmp表空間，一行的大小 > 8K
TEST-cndb@pc15.infogov>drop table t2 purge;
Table dropped.
TEST-cndb@pc15.infogov>
TEST-cndb@pc15.infogov>create table t2(id int,c1 char(2000),c2 char(2000),c3 char(2000),c4 char(2000),c5 char(2000)) tablespace tbs_tmp;
Table created.
TEST-cndb@pc15.infogov>insert into t2 values(1,rpad('1',2000,'1'),rpad('2',2000,'2'),rpad('3',2000,'3'),rpad('4',2000,'4'),rpad('5',2000,'5'));
1 row created.
TEST-cndb@pc15.infogov>commit;
Commit complete.
-- 執行checkpoint，flush資料到磁碟
TEST-cndb@pc15.infogov>alter system checkpoint;
System altered.
在一行資料大於Block Size時，Oracle使用行連結的方式實現跨塊儲存。
TEST-cndb@pc15.infogov>drop table CHAINED_ROWS purge;
Table dropped.
TEST-cndb@pc15.infogov>create table CHAINED_ROWS (
2 owner_name varchar2(30),
3 table_name varchar2(30),
4 cluster_name varchar2(30),
5 partition_name varchar2(30),
6 subpartition_name varchar2(30),
7 head_rowid rowid,
8 analyze_timestamp date
9 );
Table created.
TEST-cndb@pc15.infogov>analyze table t2 list chained rows;
Table analyzed.
TEST-cndb@pc15.infogov>select table_name,head_rowid from CHAINED_ROWS;
TABLE_NAME HEAD_ROWID
------------------------------ ------------------
T2 AAAZroAB9AAAAAPAAA
二、PG
PG的預設Block Size為8K，可以在編譯安裝時修改，不作任何調整，建立一張預期行佔用空間可能 > 8K的資料表，插入測試資料：
testdb=# drop table if exists t2;
n_filepath('t2');DROP TABLE
testdb=# create table t2(id int,c1 char(4000),c2 char(4000),c3 char(4000));
CREATE TABLE
testdb=#
testdb=# \d+ t2
Table "public.t2"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+-----------------+-----------+----------+---------+----------+--------------+-------------
id | integer | | | | plain | |
c1 | character(4000) | | | | extended | |
c2 | character(4000) | | | | extended | |
c3 | character(4000) | | | | extended | |
testdb=#
testdb=# insert into t2 values(1,'11','12','13');
INSERT 0 1
testdb=# insert into t2 values(2,rpad('2',4000,'2'),rpad('3',4000,'3'),rpad('4',4000,'4'));
INSERT 0 1
testdb=#
testdb=# checkpoint;
CHECKPOINT
testdb=# select pg_relation_filepath('t2');
pg_relation_filepath
----------------------
base/16477/26700
(1 row)
[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/26700
00000000 01 00 00 00 d0 d8 26 db 00 00 00 00 20 00 68 1e |......&..... .h.|
00000010 00 20 04 20 00 00 00 00 30 9f 9e 01 68 9e 88 01 |. . ....0...h...|
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001e60 00 00 00 00 00 00 00 00 07 9c 18 00 00 00 00 00 |................|
00001e70 00 00 00 00 00 00 00 00 02 00 04 00 02 08 18 00 |................|
00001e80 02 00 00 00 e2 00 00 00 a0 0f 00 00 fe 32 0f 01 |.............2..|
00001e90 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff |................|
00001ea0 0f 01 ff ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff |................|
00001eb0 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 9f e2 00 00 00 |................|
00001ec0 a0 0f 00 00 fe 33 0f 01 ff 0f 01 ff 0f 01 ff 0f |.....3..........|
00001ed0 01 ff 0f 01 ff 0f 01 ff 0f 01 ff ff 0f 01 ff 0f |................|
00001ee0 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 |................|
00001ef0 ff 0f 01 9f e2 00 00 00 a0 0f 00 00 fe 34 0f 01 |.............4..|
00001f00 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff |................|
00001f10 0f 01 ff ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff |................|
00001f20 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 9f 00 00 00 00 |................|
00001f30 06 9c 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001f40 01 00 04 00 02 08 18 00 01 00 00 00 ee 00 00 00 |................|
00001f50 a0 0f 00 00 f8 31 31 20 0f 01 ff 0f 01 ff 0f 01 |.....11 ........|
00001f60 ff 0f 01 ff 0f 01 ff ff 0f 01 ff 0f 01 ff 0f 01 |................|
00001f70 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff |................|
00001f80 03 0f 01 ff 0f 01 9d 00 ee 00 00 00 a0 0f 00 00 |................|
00001f90 f8 31 32 20 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff |.12 ............|
00001fa0 0f 01 ff ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff |................|
00001fb0 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 03 0f 01 ff |................|
00001fc0 0f 01 9d 00 ee 00 00 00 a0 0f 00 00 f8 31 33 20 |.............13 |
00001fd0 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff ff |................|
00001fe0 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f 01 ff 0f |................|
00001ff0 01 ff 0f 01 ff 0f 01 ff 03 0f 01 ff 0f 01 9d 00 |................|
00002000
可以看到，雖然行資料>8K，但在PG中，這些資料都儲存在一個block中（顯然使用了壓縮），00000000~00002000為一個block 8K。
實際上，在PG中，PG使用稱為TOAST (The Oversized-Attribute Storage Technique)的技術：
If any of the columns of a table are TOAST-able, the table will have an associated TOAST table, whose OID is stored in the table's pg_class.reltoastrelid entry. On-disk TOASTed values are kept in the TOAST table, as described in more detail below.
對於資料表的列，有四種儲存選項：
PLAIN prevents either compression or out-of-line storage; furthermore it disables use of single-byte headers for varlena types. This is the only possible strategy for columns of non-TOAST-able data types.
EXTENDED allows both compression and out-of-line storage. This is the default for most TOAST-able data types. Compression will be attempted first, then out-of-line storage if the row is still too big.
EXTERNAL allows out-of-line storage but not compression. Use of EXTERNAL will make substring operations on wide text and bytea columns faster (at the penalty of increased storage space) because these operations are optimized to fetch only the required parts of the out-of-line value when it is not compressed.
MAIN allows compression but not out-of-line storage. (Actually, out-of-line storage will still be performed for such columns, but only as a last resort when there is no other way to make the row small enough to fit on a page.)
預設選項為EXTENDED。
我們不妨嘗試PLAIN和EXTERNAL這兩種選項：
PLAIN
-- PLAIN
testdb=# drop table if exists t3;
11','12','13');
insert into t3 values(2,rpad('2',4000,'2'),rpad('3',4000,'3'),rpad('4',4000,'4'));DROP TABLE
testdb=# create table t3(id int,c1 char(4000),c2 char(4000),c3 char(4000));
CREATE TABLE
testdb=#
testdb=# alter table t3 alter c1 set storage plain;
ALTER TABLE
testdb=# alter table t3 alter c2 set storage plain;
ALTER TABLE
testdb=# alter table t3 alter c3 set storage plain;
ALTER TABLE
testdb=#
testdb=# \d+ t3
Table "public.t3"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+-----------------+-----------+----------+---------+---------+--------------+-------------
id | integer | | | | plain | |
c1 | character(4000) | | | | plain | |
c2 | character(4000) | | | | plain | |
c3 | character(4000) | | | | plain | |
testdb=#
testdb=# insert into t3 values(1,'11','12','13');
2018-07-28 11:32:07.561 CST [1576] ERROR: row is too big: size 12040, maximum size 8160
2018-07-28 11:32:07.561 CST [1576] STATEMENT: insert into t3 values(1,'11','12','13');
ERROR: row is too big: size 12040, maximum size 8160
testdb=# insert into t3 values(2,rpad('2',4000,'2'),rpad('3',4000,'3'),rpad('4',4000,'4'));
2018-07-28 11:32:08.687 CST [1576] ERROR: row is too big: size 12040, maximum size 8160
2018-07-28 11:32:08.687 CST [1576] STATEMENT: insert into t3 values(2,rpad('2',4000,'2'),rpad('3',4000,'3'),rpad('4',4000,'4'));
ERROR: row is too big: size 12040, maximum size 8160
-- 如果使用PLAIN選項，資料行限制為8160（32Bytes用於頭部資訊、ItemId和Special space等）
EXTERNAL
testdb=# -- EXTERNAL選項，RowSize > 8K
testdb=# drop table if exists t4;
TERNAL;
insert into t4 values(1,'11','12','13');
insert into t4 values(2,rpad('2',4000,'2'),rpad('3',4000,'3'),rpad('4',4000,'4'));DROP TABLE
testdb=# create table t4(id int,c1 char(4000),c2 char(4000),c3 char(4000));
CREATE TABLE
testdb=#
testdb=# alter table t4 alter c1 set storage EXTERNAL;
ALTER TABLE
testdb=# alter table t4 alter c2 set storage EXTERNAL;
ALTER TABLE
testdb=# alter table t4 alter c3 set storage EXTERNAL;
ALTER TABLE
testdb=#
testdb=# insert into t4 values(1,'11','12','13');
INSERT 0 1
testdb=# insert into t4 values(2,rpad('2',4000,'2'),rpad('3',4000,'3'),rpad('4',4000,'4'));
INSERT 0 1
testdb=#
testdb=# checkpoint;
CHECKPOINT
testdb=#
testdb=# select pg_relation_filepath('t4');
pg_relation_filepath
----------------------
base/16477/26712
(1 row)
[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/26712
00000000 01 00 00 00 30 57 2b db 00 00 00 00 20 00 50 1f |....0W+..... .P.|
00000010 00 20 04 20 00 00 00 00 a8 9f a4 00 50 9f a4 00 |. . ........P...|
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001f50 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001f60 02 00 04 00 06 08 18 00 02 00 00 00 01 12 a4 0f |................|
00001f70 00 00 a0 0f 00 00 61 68 00 00 5b 68 00 00 01 12 |......ah..[h....|
00001f80 a4 0f 00 00 a0 0f 00 00 62 68 00 00 5b 68 00 00 |........bh..[h..|
00001f90 01 12 a4 0f 00 00 a0 0f 00 00 63 68 00 00 5b 68 |..........ch..[h|
00001fa0 00 00 00 00 00 00 00 00 14 9c 18 00 00 00 00 00 |................|
00001fb0 00 00 00 00 00 00 00 00 01 00 04 00 06 08 18 00 |................|
00001fc0 01 00 00 00 01 12 a4 0f 00 00 a0 0f 00 00 5e 68 |..............^h|
00001fd0 00 00 5b 68 00 00 01 12 a4 0f 00 00 a0 0f 00 00 |..[h............|
00001fe0 5f 68 00 00 5b 68 00 00 01 12 a4 0f 00 00 a0 0f |_h..[h..........|
00001ff0 00 00 60 68 00 00 5b 68 00 00 00 00 00 00 00 00 |..`h..[h........|
00002000
-- 在資料表的block沒有儲存使用者資料，實際的儲存資料檔案使用以下SQL獲得：
testdb=# select reltoastrelid from pg_class where relname = 't4';
reltoastrelid
---------------
26715
(1 row)
[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/26715
00000000 01 00 00 00 08 08 2b db 00 00 00 00 28 00 00 08 |......+.....(...|
00000010 00 20 04 20 00 00 00 00 10 98 e0 0f 20 90 e0 0f |. . ........ ...|
00000020 f0 8f 58 00 00 88 e0 0f 00 00 00 00 00 00 00 00 |..X.............|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000800 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000810 04 00 03 00 02 08 18 00 5f 68 00 00 00 00 00 00 |........_h......|
00000820 40 1f 00 00 31 32 20 20 20 20 20 20 20 20 20 20 |@...12 |
00000830 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
*
00000ff0 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001000 03 00 03 00 02 08 18 00 5e 68 00 00 02 00 00 00 |........^h......|
00001010 30 00 00 00 20 20 20 20 20 20 20 20 00 00 00 00 |0... ....|
00001020 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001030 02 00 03 00 02 08 18 00 5e 68 00 00 01 00 00 00 |........^h......|
00001040 40 1f 00 00 20 20 20 20 20 20 20 20 20 20 20 20 |@... |
00001050 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
*
00001810 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001820 01 00 03 00 02 08 18 00 5e 68 00 00 00 00 00 00 |........^h......|
00001830 40 1f 00 00 31 31 20 20 20 20 20 20 20 20 20 20 |@...11 |
00001840 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
*
00002000 01 00 00 00 30 22 2b db 00 00 00 00 2c 00 d0 07 |....0"+.....,...|
00002010 00 20 04 20 00 00 00 00 10 98 e0 0f e0 97 58 00 |. . ..........X.|
00002020 f0 8f e0 0f 00 88 e0 0f d0 87 58 00 00 00 00 00 |..........X.....|
00002030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000027d0 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 01 00 |................|
000027e0 05 00 03 00 02 08 18 00 60 68 00 00 02 00 00 00 |........`h......|
000027f0 30 00 00 00 20 20 20 20 20 20 20 20 00 00 00 00 |0... ....|
00002800 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 01 00 |................|
00002810 04 00 03 00 02 08 18 00 60 68 00 00 01 00 00 00 |........`h......|
00002820 40 1f 00 00 20 20 20 20 20 20 20 20 20 20 20 20 |@... |
00002830 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
*
00002ff0 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 01 00 |................|
00003000 03 00 03 00 02 08 18 00 60 68 00 00 00 00 00 00 |........`h......|
00003010 40 1f 00 00 31 33 20 20 20 20 20 20 20 20 20 20 |@...13 |
00003020 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
*
000037e0 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 01 00 |................|
000037f0 02 00 03 00 02 08 18 00 5f 68 00 00 02 00 00 00 |........_h......|
00003800 30 00 00 00 20 20 20 20 20 20 20 20 00 00 00 00 |0... ....|
00003810 14 9c 18 00 00 00 00 00 00 00 00 00 00 00 01 00 |................|
00003820 01 00 03 00 02 08 18 00 5f 68 00 00 01 00 00 00 |........_h......|
00003830 40 1f 00 00 20 20 20 20 20 20 20 20 20 20 20 20 |@... |
00003840 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
*
00004000 01 00 00 00 50 3c 2b db 00 00 00 00 28 00 00 08 |....P<+.....(...|
00004010 00 20 04 20 00 00 00 00 10 98 e0 0f 20 90 e0 0f |. . ........ ...|
00004020 f0 8f 58 00 00 88 e0 0f 00 00 00 00 00 00 00 00 |..X.............|
00004030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00004800 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 02 00 |................|
00004810 04 00 03 00 02 08 18 00 62 68 00 00 00 00 00 00 |........bh......|
00004820 40 1f 00 00 33 33 33 33 33 33 33 33 33 33 33 33 |@...333333333333|
00004830 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33 |3333333333333333|
*
00004ff0 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 02 00 |................|
00005000 03 00 03 00 02 08 18 00 61 68 00 00 02 00 00 00 |........ah......|
00005010 30 00 00 00 32 32 32 32 32 32 32 32 00 00 00 00 |0...22222222....|
00005020 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 02 00 |................|
00005030 02 00 03 00 02 08 18 00 61 68 00 00 01 00 00 00 |........ah......|
00005040 40 1f 00 00 32 32 32 32 32 32 32 32 32 32 32 32 |@...222222222222|
00005050 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 |2222222222222222|
*
00005810 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 02 00 |................|
00005820 01 00 03 00 02 08 18 00 61 68 00 00 00 00 00 00 |........ah......|
00005830 40 1f 00 00 32 32 32 32 32 32 32 32 32 32 32 32 |@...222222222222|
00005840 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 |2222222222222222|
*
00006000 01 00 00 00 78 56 2b db 00 00 00 00 2c 00 d0 07 |....xV+.....,...|
00006010 00 20 04 20 00 00 00 00 10 98 e0 0f e0 97 58 00 |. . ..........X.|
00006020 f0 8f e0 0f 00 88 e0 0f d0 87 58 00 00 00 00 00 |..........X.....|
00006030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000067d0 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 03 00 |................|
000067e0 05 00 03 00 02 08 18 00 63 68 00 00 02 00 00 00 |........ch......|
000067f0 30 00 00 00 34 34 34 34 34 34 34 34 00 00 00 00 |0...44444444....|
00006800 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 03 00 |................|
00006810 04 00 03 00 02 08 18 00 63 68 00 00 01 00 00 00 |........ch......|
00006820 40 1f 00 00 34 34 34 34 34 34 34 34 34 34 34 34 |@...444444444444|
00006830 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 |4444444444444444|
*
00006ff0 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 03 00 |................|
00007000 03 00 03 00 02 08 18 00 63 68 00 00 00 00 00 00 |........ch......|
00007010 40 1f 00 00 34 34 34 34 34 34 34 34 34 34 34 34 |@...444444444444|
00007020 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 |4444444444444444|
*
000077e0 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 03 00 |................|
000077f0 02 00 03 00 02 08 18 00 62 68 00 00 02 00 00 00 |........bh......|
00007800 30 00 00 00 33 33 33 33 33 33 33 33 00 00 00 00 |0...33333333....|
00007810 15 9c 18 00 00 00 00 00 00 00 00 00 00 00 03 00 |................|
00007820 01 00 03 00 02 08 18 00 62 68 00 00 01 00 00 00 |........bh......|
00007830 40 1f 00 00 33 33 33 33 33 33 33 33 33 33 33 33 |@...333333333333|
00007840 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33 |3333333333333333|
*
00008000
[xdb@localhost utf8db]$
三、小結
知識要點：
1、實際行大小大於PageSize：
A.Oracle使用行連結方式儲存
B.PG預設使用EXTENDED方式，首先壓縮，如果仍不足以儲存資料，那麼使用out-of-line方式儲存
2、PG列資料儲存有4種選項，分別是PLAIN/EXTENDED/EXTERNAL/MAIN
四、參考文件
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/6906/viewspace-2374916/，如需轉載，請註明出處，否則將追究法律責任。
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