PostgreSQL修改表列属性

PostgreSQL修改表列属性的SQL语法与Oracle不同：
（1）Oracle改列属性：
delete from cfg_public_special_service where length(mail_class)>1;
commit;
alter table cfg_public_special_service modify mail_class varchar2(1);
（2）PostgreSQL改列属性：
delete from cfg_public_special_service where length(mail_class)>1;
alter table cfg_public_special_service alter column mail_class type character varying(1);
（3）cpost=# help
You are using psql, the command-line interface to PostgreSQL.
Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help with psql commands
       \g or terminate with semicolon to execute query
       \q to quit
cpost=# \h alter table
Command:     ALTER TABLE
Description: change the definition of a table
Syntax:ALTER TABLE [ ONLY ] name [ * ] action [, ... ]
ALTER TABLE [ ONLY ] name [ * ] RENAME [ COLUMN ] column TO new_column
ALTER TABLE name RENAME TO new_name
ALTER TABLE name SET SCHEMA new_schema
where action is one of:
    ADD [ COLUMN ] column data_type [ COLLATE collation ] [ column_constraint [ ... ] ]
    DROP [ COLUMN ] [ IF EXISTS ] column [ RESTRICT | CASCADE ]
    ALTER [ COLUMN ] column [ SET DATA ] TYPE data_type [ COLLATE collation ] [ USING expression ]
    ALTER [ COLUMN ] column SET DEFAULT expression
    ALTER [ COLUMN ] column DROP DEFAULT
    ALTER [ COLUMN ] column { SET | DROP } NOT NULL
    ALTER [ COLUMN ] column SET STATISTICS integer
    ALTER [ COLUMN ] column SET ( attribute_option = value [, ... ] )
    ALTER [ COLUMN ] column RESET ( attribute_option [, ... ] )
    ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
    ADD table_constraint [ NOT VALID ]
    ADD table_constraint_using_index
    VALIDATE CONSTRAINT constraint_name
    DROP CONSTRAINT [ IF EXISTS ]  constraint_name [ RESTRICT | CASCADE ]
    DISABLE TRIGGER [ trigger_name | ALL | USER ]
    ENABLE TRIGGER [ trigger_name | ALL | USER ]
    ENABLE REPLICA TRIGGER trigger_name
    ENABLE ALWAYS TRIGGER trigger_name
    DISABLE RULE rewrite_rule_name
    ENABLE RULE rewrite_rule_name
    ENABLE REPLICA RULE rewrite_rule_name
    ENABLE ALWAYS RULE rewrite_rule_name
    CLUSTER ON index_name
    SET WITHOUT CLUSTER
    SET WITH OIDS
    SET WITHOUT OIDS
    SET ( storage_parameter = value [, ... ] )
    RESET ( storage_parameter [, ... ] )
    INHERIT parent_table
    NO INHERIT parent_table
    OF type_name
    NOT OF
    OWNER TO new_owner
    SET TABLESPACE new_tablespace
and table_constraint_using_index is:
    [ CONSTRAINT constraint_name ]
    { UNIQUE | PRIMARY KEY } USING INDEX index_name
    [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]