首先，Toast是一个名字缩写，全写是The OverSized Attribute Storage Technique，即超尺寸字段存储技术，顾名思义，是说超长字段在Postgres的一个存储方式。Postgres采用的存储默认是每个页面存储固定8Kb大小的数据，并且元组不允许跨页面存储，所以并不能直接存储大字段数据。Toast就是为此应运而生，它会将大字段值压缩或者分散为多个物理行来存储。对于用户来说完全不用关注这一技术实现，完全是透明的。
二、TOAST的存储方式
Postgres的部分类型数据支持toast，不是全部类型是因为有些字段类型是不会产生大字段数据的，完全没必要用到Toast技术(比如date,time,boolean等)。支持Toast的数据类型应当时变长的(variable-length)，变长字段最多可选择32bit的列头(header),Toast占用两个变长的bit位来作为FLAG，故Toast的逻辑尺寸限制是(2^30-1)~1GB，当两个bit都是0是，这个数据类型的值就是非Toast的(untoasted)。当表中字段任何一个有Toast，那这个表都会有这一个相关联的Toast表，OID被存储在pg_class.reltoastrelid里面。超出的的数值将会被分割成chunks，并最多toast_max_chunk_size 个byte(缺省是2Kb)，当存储的行数据超过toast_tuple_threshold值(通常是2kB)，就会触发toast存储，这时toast将会压缩或者移动字段值直到超出部分比toast_tuple_targer值小(这个值通常也是2KB)。
相比较普通表(MAIN TABLE),TOAST有额外的三个字段
chunk_id :标识TOAST表的OID字段
chunk_seq :chunk的序列号，与chunk_id的组合唯一索引可以加速访问
chunk_data :存储TOAST表的实际数据--plain避免压缩或行外存储
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允许压缩和行外存储(默认toast存储)
EXTENDED模式同时支持压缩和外部存储。对于大多数可TOAST的数据类型来说，这是默认的模式。它首先会尝试进行压缩，如果行数据仍然过大，才会采用外部存储的方式。
--external允许行外但不允许压缩
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允许压缩但不允许行外存储
MAIN允许压缩，但不支持离线存储。（实际上，对于此类列，离线存储仍会执行，但仅在没有其他方法能使行足够小以适应页面时才作为最后的手段。）源码见： postgresql-9.2.3/src/backend/utils/adt/pg_lzcompress.c
TOAST可以通过SQL来更改存储方式，示例：
ALTER TABLE table_name ALTER COLUMN column_name SET STORAGE {PLAIN | EXTENDED | MAIN | EXTERNAL};
postgres=# d+ t_kenyon
Table "public.t_kenyon"
Column | Type | Modifiers | Storage | Stats target | Description
--------+-------------------+-----------+----------+--------------+-------------
dd | character varying | | extended | |
Has OIDs: no
postgres=# alter table t_kenyon alter column dd set storage main;
ALTER TABLE
postgres=# d+ t_kenyon
Table "public.t_kenyon"
Column | Type | Modifiers | Storage | Stats target | Description
--------+-------------------+-----------+---------+--------------+-------------
dd | character varying | | main | |
Has OIDs: no计算一个表的大小时要注意统计Toast的大小，因为对超长字段存储时，在基础表上可能只存了20%，另外的数据都存到了Toast里面去了，计算大小时要结合起来看，索引也是一样，对于表里有main或者extended类型的会创建Toast表，两者的关联是通过pg_class里的OID去关联的。下面举例子：
1.TOAST表关联查询
Example a:
[postgres@localhost ~]$ psql
psql (9.2.3)
Type "help" for help.
postgres=# create table t_kenyon(id int);
CREATE TABLE
postgres=# select relname,reltoastrelid from pg_class where relname = 't_kenyon';
relname | reltoastrelid
----------+---------------
t_kenyon | 0
(1 row)
postgres=# d+ t_kenyon
Table "public.t_kenyon"
Column | Type | Modifiers | Storage | Stats target | Description
--------+---------+-----------+---------+--------------+-------------
id | integer | | plain | |
Has OIDs: noExample b:
postgres=# select relname,reltoastrelid from pg_class where relname = 't_kenyon';
relname | reltoastrelid
----------+---------------
t_kenyon | 16411
(1 row)
postgres=# select relname from pg_class where oid = 16411;
relname
----------------
pg_toast_16408
(1 row)postgres=# drop table t_kenyon;
DROP TABLE
postgres=# create table t_kenyon(id int,vname varchar(48),remark text);
CREATE TABLE
postgres=# d+ t_kenyon
Table "public.t_kenyon"
Column | Type | Modifiers | Storage | Stats target | Description
--------+-----------------------+-----------+----------+--------------+-------------
id | integer | | plain | |
vname | character varying(48) | | extended | |
remark | text | | extended | |
Has OIDs: no
postgres=# select oid,relname,reltoastrelid from pg_class where relname = 't_kenyon';
oid | relname | reltoastrelid
--------+----------+---------------
121174 | t_kenyon | 121177
(1 row)
postgres=# insert into t_kenyon select generate_series(1,2000),repeat('kenyon here'||'^_^',2),repeat('^_^ Kenyon is not God',500);
INSERT 0 2000
postgres=# insert into t_kenyon select generate_series(1,2),repeat('kenyon here'||'^_^',2),repeat('^_^ Kenyon is not God,Remark here!!',2000);
INSERT 0 2
postgres=# select pg_column_size(id),pg_column_size(vname),pg_column_size(remark) from t_kenyon limit 2;
pg_column_size | pg_column_size | pg_column_size
----------------+----------------+----------------
4 | 29 | 851
4 | 29 | 851
(2 rows)
--查看基础表和Toast的大小
postgres=# select pg_relation_size(121174);
pg_relation_size
------------------
8192
(1 row)
postgres=# select pg_relation_size(121177);
pg_relation_size
------------------
0
(1 row)
--文本数据量增多，这时可以看到toast表字段大小在2kb左右时有大小了
postgres=# insert into t_kenyon select generate_series(3,4),repeat('kenyon here'||'^_^',2),repeat('^_^ Kenyon is not God,Remark here!!',4000);
INSERT 0 2
postgres=# select pg_relation_size(121174);
pg_relation_size
------------------
8192
(1 row)
postgres=# select pg_relation_size(121177);
pg_relation_size
------------------
0
(1 row)
postgres=# insert into t_kenyon select generate_series(5,6),repeat('kenyon here'||'^_^',2),repeat('^_^ Kenyon is not God,Remark here!!',5500);
INSERT 0 2
postgres=# select pg_column_size(id),pg_column_size(vname),pg_column_size(remark) from t_kenyon;
pg_column_size | pg_column_size | pg_column_size
----------------+----------------+----------------
4 | 29 | 851
4 | 29 | 851
4 | 29 | 1651
4 | 29 | 1651
4 | 29 | 2247
4 | 29 | 2247
(6 rows)
postgres=# insert into t_kenyon select generate_series(1,2),repeat('kenyon here'||'^_^',2),repeat('^_^ Kenyon is not God,Remark here!!',10000);
INSERT 0 2
postgres=# select pg_relation_size(121174);
pg_relation_size
------------------
8192
(1 row)
postgres=# select pg_relation_size(121177);
pg_relation_size
------------------
16384
(1 row)
postgres=# insert into t_kenyon select generate_series(7,8),repeat('kenyon here'||'^_^',2),repeat('^_^ Kenyon is not God,Remark here!!',20000);
INSERT 0 2
postgres=# select id,pg_column_size(id),pg_column_size(vname),pg_column_size(remark) from t_kenyon;
id | pg_column_size | pg_column_size | pg_column_size
----+----------------+----------------+----------------
1 | 4 | 29 | 851
2 | 4 | 29 | 851
3 | 4 | 29 | 1651
4 | 4 | 29 | 1651
5 | 4 | 29 | 2247
6 | 4 | 29 | 2247
7 | 4 | 29 | 8056
8 | 4 | 29 | 8056
(8 rows)
postgres=# select pg_relation_size(121174);
pg_relation_size
------------------
8192
(1 row)
postgres=# select pg_relation_size(121177);
pg_relation_size
------------------
24576
(1 row)四、TOAST的优缺点
1.可以存储超长超大字段，避免之前不能直接存储的限制
2.物理上与普通表是分离的，检索查询时不检索到该字段会极大地加快速度
3.更新普通表时，该表的Toast数据没有被更新时，不用去更新Toast表
Toast的劣势：
1.对大字段的索引创建是一个问题，有可能会失败，其实通常也不建议在大字段上创建，全文检索倒是一个解决方案。
2.大字段的更新会有点慢，其它DB也存在，通病
五、其他
Toast的源码:
postgresql-9.2.3/src/backend/access/heap/tuptoaster.c
postgresql-9.2.3/src/backend/catalog/toasting.c
