背景
PostgreSQL是一个强类型数据库，因此你输入的变量、常量是什么类型，是强绑定的，例如
在调用操作符时，需要通过操作符边上的数据类型，选择对应的操作符。
在调用函数时，需要根据输入的类型，选择对应的函数。
如果类型不匹配，就会报操作符不存在，或者函数不存在的错误。
postgres=# select '1' + '1';
ERROR: operator is not unique: unknown + unknown
LINE 1: select '1' + '1';
^
HINT: Could not choose a best candidate operator. You might need to add explicit type casts.
那么使用起来是不是很不方便呢？
PostgreSQL开放了类型转换的接口，同时也内置了很多的自动类型转换。来简化操作。
查看目前已有的类型转换：
postgres=# \dC+
List of casts
Source type | Target type | Function | Implicit"char" | character | bpchar | in assignment |
"char" | character varying | text | in assignment |
"char" | integer | int4 | no |
"char" | text | text | yes |
abstime | date | date | in assignment |
abstime | integer | (binary coercible) | no |
abstime | time without time zone | time | in assignment |
................................
timestamp without time zone | timestamp with time zone | timestamptz | yes |
timestamp without time zone | timestamp without time zone | timestamp | yes |
xml | character | (binary coercible) | in assignment |
xml | character varying | (binary coercible) | in assignment |
xml | text | (binary coercible) | in assignment |
(246 rows)
如果你发现有些类型转换没有内置，怎么办呢？我们可以自定义转换。
当然你也可以使用这种语法，对类型进行强制转换：
CAST(x AS typename) or x::typename
如何自定义类型转换(CAST)
自定义CAST的语法如下：
CREATE CAST (source_type AS target_type) WITH FUNCTION function_name [ (argument_type [, ...]) ] [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_type AS target_type) WITHOUT FUNCTION [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_type AS target_type) WITH INOUT [ AS ASSIGNMENT | AS IMPLICIT ]
解释：
1、WITH FUNCTION，表示转换需要用到什么函数。
2、WITHOUT FUNCTION，表示被转换的两个类型，在数据库的存储中一致，即物理存储一致。例如text和varchar的物理存储一致。不需要转换函数。
Two types can be binary coercible, which means that the conversion can be performed “for free” without invoking any function. This requires that corresponding values use the same internal representation. For instance, the types text and varchar are binary coercible both ways. Binary coercibility is not necessarily a symmetric relationship. For example, the cast from xml to text can be performed for free in the present implementation, but the reverse direction requires a function that performs at least a syntax check. (Two types that are binary coercible both ways are also referred to as binary compatible.)
3、WITH INOUT，表示使用内置的IO函数进行转换。每一种类型，都有INPUT 和OUTPUT函数。使用这种方法，好处是不需要重新写转换函数。
除非有特殊需求，我们建议直接使用IO函数来进行转换。
List of functions
Schema | Name | Result data type | Argument data types | Type
------------+-----------------+------------------+---------------------+--------
pg_catalog | textin | text | cstring | normal
pg_catalog | textout | cstring | text | normal
pg_catalog | date_in | date | cstring | normal
pg_catalog | date_out | cstring | date | normal
You can define a cast as an I/O conversion cast by using the WITH INOUT syntax.
An I/O conversion cast is performed by invoking the output function of the source data type,
and passing the resulting string to the input function of the target data type.
In many common cases, this feature avoids the need to write a separate cast function for conversion.
An I/O conversion cast acts the same as a regular function-based cast; only the implementation is different.
4、AS ASSIGNMENT，表示在赋值时，自动对类型进行转换。例如字段类型为TEXT，输入的类型为INT，那么可以创建一个 cast(int as text) as ASSIGNMENT。
If the cast is marked AS ASSIGNMENT then it can be invoked implicitly when assigning a value to a column of the target data type. For example, supposing that foo.f1 is a column of type text, then: INSERT INTO foo (f1) VALUES (42); will be allowed if the cast from type integer to type text is marked AS ASSIGNMENT, otherwise not. (We generally use the term assignment cast to describe this kind of cast.)
5、AS IMPLICIT，表示在表达式中，或者在赋值操作中，都对类型进行自动转换。（包含了AS ASSIGNMENT，它只对赋值进行转换）
If the cast is marked AS IMPLICIT then it can be invoked implicitly in any context, whether assignment or internally in an expression. (We generally use the term implicit cast to describe this kind of cast.) For example, consider this query: SELECT 2 + 4.0; The parser initially marks the constants as being of type integer and numeric respectively. There is no integer + numeric operator in the system catalogs, but there is a numeric + numeric operator. The query will therefore succeed if a cast from integer to numeric is available and is marked AS IMPLICIT — which in fact it is. The parser will apply the implicit cast and resolve the query as if it had been written SELECT CAST ( 2 AS numeric ) + 4.0;
6、注意，AS IMPLICIT需要谨慎使用，为什么呢？因为操作符会涉及到多个算子，如果有多个转换，目前数据库并不知道应该选择哪个？
Now, the catalogs also provide a cast from numeric to integer. If that cast were marked AS IMPLICIT — (which it is not — ) then the parser would be faced with choosing between the above interpretation and the alternative of casting the numeric constant to integer and applying the integer + integer operator. Lacking any knowledge of which choice to prefer, it would give up and declare the query ambiguous. The fact that only one of the two casts is implicit is the way in which we teach the parser to prefer resolution of a mixed numeric-and-integer expression as numeric; there is no built-in knowledge about that.
因此，建议谨慎使用AS IMPLICIT。建议使用AS IMPLICIT的CAST应该是非失真转换转换，例如从INT转换为TEXT，或者int转换为numeric。
而失真转换，不建议使用as implicit，例如numeric转换为int。
It is wise to be conservative about marking casts as implicit. An overabundance of implicit casting paths can cause PostgreSQL to choose surprising interpretations of commands, or to be unable to resolve commands at all because there are multiple possible interpretations. A good rule of thumb is to make a cast implicitly invokable only for information-preserving transformations between types in the same general type category. For example, the cast from int2 to int4 can reasonably be implicit, but the cast from float8 to int4 should probably be assignment-only. Cross-type-category casts, such as text to int4, are best made explicit-only.
注意事项 + 例子
不能嵌套转换。例子
1、将text转换为date
错误方法
create or replace function text_to_date(text) returns date as $$ select cast($1 as date); $$ language sql strict; create cast (text as date) with function text_to_date(text) as implicit;
嵌套转换后出现死循环
postgres=# select text '2017-01-01' + 1; ERROR: stack depth limit exceeded HINT: Increase the configuration parameter "max_stack_depth" (currently 2048kB), after ensuring the platform's stack depth limit is adequate. CONTEXT: SQL function "text_to_date" during startup SQL function "text_to_date" statement 1 SQL function "text_to_date" statement 1 SQL function "text_to_date" statement 1 ......
正确方法
create or replace function text_to_date(text) returns date as $$ select to_date($1,'yyyy-mm-dd'); $$ language sql strict; create cast (text as date) with function text_to_date(text) as implicit; postgres=# select text '2017-01-01' + 1; "htmlcode">postgres=# create cast (text as date) with inout as implicit; CREATE CAST postgres=# select text '2017-01-01' + 1; "htmlcode">postgres=# create table cas_test(id int, c1 boolean); CREATE TABLE postgres=# \set VERBOSITY verbose postgres=# insert into cas_test values (1, int '1'); ERROR: 42804: column "c1" is of type boolean but expression is of type integer LINE 1: insert into cas_test values (1, int '1'); ^ HINT: You will need to rewrite or cast the expression. LOCATION: transformAssignedExpr, parse_target.c:591又或者数据库定义的是时间，用户输入的是字符串：
postgres=# create table tbl123(id int, crt_time timestamp); CREATE TABLE postgres=# insert into tbl123 values (1, text '2017-01-01 10:00:00'); ERROR: column "crt_time" is of type timestamp without time zone but expression is of type text LINE 1: insert into tbl123 values (1, text '2017-01-01 10:00:00'); ^ HINT: You will need to rewrite or cast the expression.从错误提示来看，数据库已经很清晰的告诉你为什么了。那么怎么让数据库自动转换呢？
PostgreSQL有一个语法，支持数据类型的转换（赋值、参数、表达式 等位置的自动转换）。
postgres=# \h create cast Command: CREATE CAST Description: define a new cast Syntax: CREATE CAST (source_type AS target_type) WITH FUNCTION function_name [ (argument_type [, ...]) ] [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_type AS target_type) WITHOUT FUNCTION [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_type AS target_type) WITH INOUT [ AS ASSIGNMENT | AS IMPLICIT ]数据库内置了很多转换法则：
postgres=# \dC List of casts Source type | Target type | Function | Implicit"_blank" href="https://github.com/digoal/blog/blob/master/201710/20171013_03.md">PostgreSQL 自定义自动类型转换(CAST)》我们看到整型转布尔是有内置的转换规则的，那么为什么没有自动转呢？
postgres=# \dC List of casts Source type | Target type | Function | Implicit"htmlcode">postgres=# select cast ((int '1') as boolean); bool ------ t (1 row)pg_cast里面的context转换为可读的内容(e表示no, a表示assignment, 否则表示implicit)
如果让数据库赋值时自动将字符串转换为时间，自动将整型转换为布尔
1、如果数据库已经内置了转换规则，那么可以通过更新系统表的方式，修改自动转换规则。
例如，将这个INT转BOOLEAN的规则，修改为assignment的规则。
postgres=# update pg_cast set castcontext='a' where castsource ='integer'::regtype and casttarget='boolean'::regtype; UPDATE 1修改后，我们再查看这个转换规则，就变成这样了
\dC List of casts Source type | Target type | Function | Implicit"htmlcode">postgres=# create table cas_test(id int, c1 boolean); CREATE TABLE postgres=# insert into cas_test values (1, int '1'); INSERT 0 12、如果系统中没有两种类型转换的CAST规则，那么我们需要自定义一个。
例如
postgres=# create cast (text as timestamp) with inout as ASSIGNMENT; CREATE CAST List of casts Source type | Target type | Function | Implicit"htmlcode">postgres=# insert into tbl123 values (1, text '2017-01-01 10:00:00'); INSERT 0 1 postgres=# select * from tbl123; id | crt_time ----+--------------------- 1 | 2017-01-01 10:00:00 (1 row)删掉这个转换，就会报错。
postgres=# drop cast (text as timestamp); DROP CAST postgres=# insert into tbl123 values (1, text '2017-01-01 10:00:00'); ERROR: column "crt_time" is of type timestamp without time zone but expression is of type text LINE 1: insert into tbl123 values (1, text '2017-01-01 10:00:00'); ^ HINT: You will need to rewrite or cast the expression.3、如果没有内置的转换函数，我们可能需要自定义转换函数来支持这种转换。
例子
自定义一个函数，用于输入TEXT，返回TIMESTAMPTZ
postgres=# create or replace function cast_text_to_timestamp(text) returns timestamptz as $$ select to_timestamp($1, 'yyyy-mm-dd hh24:mi:ss'); $$ language sql strict ; CREATE FUNCTION建立规则
postgres=# create cast (text as timestamptz) with function cast_text_to_timestamp as ASSIGNMENT; CREATE CAST postgres=# \dC List of casts Source type | Target type | Function | Implicit"htmlcode">postgres=# create table tbl1234(id int, crt_time timestamptz); CREATE TABLE postgres=# insert into tbl1234 values (1, text '2017-01-01 10:10:10'); INSERT 0 1当然，这些类型实际上内部都有内部的存储格式，大多数时候，如果存储格式通用，就可以直接使用INOUT来转换，不需要写转换函数。
仅仅当两种类型在数据库的内部存储格式不一样的时候，需要显示的写函数来转换。
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。如有错误或未考虑完全的地方，望不吝赐教。
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