本节以数值型相互转换以及数值型和字符型的转换为例大体介绍了Oracle和PostgreSQL类型转换上的部分异同，可据此思路推广到其他类型。

下面以数值类型为例子说明,包括运算结果的转换和强制类型转换.
运算结果
以除运算为例说明.
PostgreSQL的除运算

testdb=# select 1/4; ?column? ---------- 0(1 row)

Oracle的除运算

TEST-orcl@server4>select 1/4 from dual; 1/4---------- .25

两个整型值1和4参与除法运算,结果PostgreSQL为整型的0,Oracle为浮点型的0.25,两者的行为不一致.
为何PostgreSQL执行整型运算返回的结果是整型?当然,这是PG的机制(整型/整型=整型)使然,在PG中,运算的结果类型可查询pg_operator获得:

testdb=# \xExpanded display is on.testdb=# select * from pg_operator where oprname = '/' and oprleft=21 and oprright = 21;-[ RECORD 1 ]+--------oprname | / -->运算符oprnamespace | 11oprowner | 10oprkind | boprcanmerge | foprcanhash | foprleft | 21 -->int2(占用2个字节的整型,通过select * from pg_type where oid=21查询可得)oprright | 21 -->同上oprresult | 21 -->整型/整型,结果也是整型oprcom | 0oprnegate | 0oprcode | int2divoprrest | -oprjoin | -

在PostgreSQL中,要想获得0.25的结果,需要进行转换:

testdb=# select 1/4::float; ?column? ---------- 0.25(1 row)二、强制类型转换

以字符型->整型为例说明.
PostgreSQL

testdb=# drop table if exists t_cast ;DROP TABLEtestdb=# create table t_cast (c_int int,c_s varchar(20));CREATE TABLEtestdb=# insert into t_cast values(1,'1');INSERT 0 1testdb=# insert into t_cast values(2,'2');INSERT 0 1testdb=# select * from t_cast where c_int = 1; c_int | c_s -------+----- 1 | 1(1 row)testdb=# select * from t_cast where c_s = 1;ERROR: operator does not exist: character varying = integer -->可变长字符型转换为整型LINE 1: select * from t_cast where c_s = 1; ^HINT: No operator matches the given name and argument types. You might need to add explicit type casts.

Oracle

TEST-orcl@server4>drop table t_cast;Table dropped.TEST-orcl@server4>create table t_cast (c_int int,c_s varchar2(20)) tablespace users;Table created.TEST-orcl@server4>insert into t_cast values(1,'1');1 row created.TEST-orcl@server4>insert into t_cast values(2,'2');1 row created.TEST-orcl@server4>select * from t_cast where c_int = 1; C_INT C_S---------- -------------------- 1 1TEST-orcl@server4>select * from t_cast where c_s = 1; C_INT C_S---------- -------------------- 1 1

PG,整型不能转换为字符型,而Oracle可以.
PG可以通过显式类型转换或者自定义类型转换的机制实现字符型->整型的转换:

-- 显式转换testdb=# select * from t_cast where c_s = 1::varchar; c_int | c_s -------+----- 1 | 1(1 row)-- 自定义类型转换testdb=# create cast(varchar as integer) with inout as implicit;CREATE CASTtestdb=# select * from t_cast where c_s = 1; c_int | c_s -------+----- 1 | 1(1 row)

通过数据字典表pg_cast可查询PG支持的类型转换.

testdb=# select oid,a.* from pg_cast a where castsource=1043 and casttarget = 23; oid | castsource | casttarget | castfunc | castcontext | castmethod -------+------------+------------+----------+-------------+------------ 16774 | 1043 | 23 | 0 | i | i --> 这是新加的记录三、参考资料

CREATE CAST
PostgreSQL 自定义自动类型转换(CAST)