PG中添加和查询注释comment

COMMENT命令简介

PG中可以使用comment命令为表、列、索引、视图等对象添加注释。

命令

COMMENT ON
{
  ACCESS METHOD object_name |
  AGGREGATE aggregate_name ( aggregate_signature ) |
  CAST (source_type AS target_type) |
  COLLATION object_name |
  COLUMN relation_name.column_name |
  CONSTRAINT constraint_name ON table_name |
  CONSTRAINT constraint_name ON DOMAIN domain_name |
  CONVERSION object_name |
  DATABASE object_name |
  DOMAIN object_name |
  EXTENSION object_name |
  EVENT TRIGGER object_name |
  FOREIGN DATA WRAPPER object_name |
  FOREIGN TABLE object_name |
  FUNCTION function_name [ ( [ [ argmode ] [ argname ] argtype [, ...] ] ) ] |
  INDEX object_name |
  LARGE OBJECT large_object_oid |
  MATERIALIZED VIEW object_name |
  OPERATOR operator_name (left_type, right_type) |
  OPERATOR CLASS object_name USING index_method |
  OPERATOR FAMILY object_name USING index_method |
  POLICY policy_name ON table_name |
  [ PROCEDURAL ] LANGUAGE object_name |
  PROCEDURE procedure_name [ ( [ [ argmode ] [ argname ] argtype [, ...] ] ) ] |
  PUBLICATION object_name |
  ROLE object_name |
  ROUTINE routine_name [ ( [ [ argmode ] [ argname ] argtype [, ...] ] ) ] |
  RULE rule_name ON table_name |
  SCHEMA object_name |
  SEQUENCE object_name |
  SERVER object_name |
  STATISTICS object_name |
  SUBSCRIPTION object_name |
  TABLE object_name |
  TABLESPACE object_name |
  TEXT SEARCH CONFIGURATION object_name |
  TEXT SEARCH DICTIONARY object_name |
  TEXT SEARCH PARSER object_name |
  TEXT SEARCH TEMPLATE object_name |
  TRANSFORM FOR type_name LANGUAGE lang_name |
  TRIGGER trigger_name ON table_name |
  TYPE object_name |
  VIEW object_name
} IS 'text'

其中 aggregate_signature 是：

* |
[ argmode ] [ argname ] argtype [ , ... ] |
[ [ argmode ] [ argname ] argtype [ , ... ] ] ORDER BY [ argmode ] [ argname ] argtype [ , ... ]

描述

COMMENT存储关于一个数据库对象的注释。

对每一个对象只保存一个注释字符串，因此为了修改一段注释，对同一个对象 发出一个新的COMMENT命令。要移除一段注释，可在文本字符串的位置上写上NULL。当对象被删除时，其注释 也会被自动删除。

对大部分类型的对象，只有对象的拥有者可以设置注释。角色没有拥有者，因此 COMMENT ON ROLE的规则是你必须作为一个超级用户来对一个 超级用户角色设置注释，或者具有CREATEROLE特权来对非超级用 户角色设置注释。同样的，访问方法也没有拥有者，你必须作为一个超级用户来 对一个访问方法设置注释。当然，一个超级用户可以对任何东西设置注释。

使用psql的\d 命令家族可以查看注释。其他检索注释的用户接口可以构建在 psql使用的内建函数之上，即 obj_description、col_description 以及shobj_description （见表 9.73）。

参数

• object_name relation_name.column_name aggregate_name constraint_name function_name operator_name policy_name procedure_name routine_name rule_name trigger_name

  要被注释的对象的名称。表、聚集、排序方式、转换、域、外部表、函数、 索引、操作符、操作符类、操作符族、存储过程、例程、序列、统计信息、文本搜索对象、类型和视图 的名称可以被模式限定。在注释一列时， *relation_name*必须 引用一个表、视图、组合类型或者外部表。

• table_name domain_name

  当在一个约束、触发器、规则或者策略上创建一段注释时，这些参数指定在其上定义 该对象的表或域的名称。

• source_type

  造型的源数据类型的名称。

• target_type

  造型的目标数据类型的名称。

• argmode

  一个函数，存储过程或者聚集函数的参数的模式：IN、 OUT、INOUT或者VARIADIC。 如果被省略，默认值是IN。注意 COMMENT并不真正关心 OUT参数，因为决定函数的身份只需要输入参数。因此 列出IN、INOUT和VARIADIC 参数就足够了。

• argname

  一个函数，存储过程或者聚集函数参数的名称。注意 COMMENT并不真正关心参数名称， 因为决定函数的身份只需要参数数据类型。

• argtype

  一个函数，存储过程或者聚集函数参数的数据类型。

• large_object_oid

  大对象的 OID。

• left_type right_type

  操作符的参数的数据类型（可以是模式限定的）。对一个前缀后后缀操作符 的缺失参数可以写NONE。

• PROCEDURAL

  这是一个噪声词。

• type_name

  该转换的数据类型的名称。

• lang_name

  该转换的语言的名称。

• text

  写成一个字符串的新注释。如果要删除注释，写成NULL。

注解

当前对查看注释没有安全机制：任何连接到一个数据库的用户能够看到 该数据库中所有对象的注释。对于数据库、角色、表空间这类共享对象， 注释被全局存储，因此连接到集簇中任何数据库的任何用户可以看到共 享对象的所有注释。因此，不要在注释中放置有安全性风险的信息。

示例

为表mytable附加一段注释：

COMMENT ON TABLE mytable IS 'This is my table.';

移除它：

COMMENT ON TABLE mytable IS NULL;

更多的一些例子：

COMMENT ON ACCESS METHOD gin IS 'GIN index access method';
COMMENT ON AGGREGATE my_aggregate (double precision) IS 'Computes sample variance';
COMMENT ON CAST (text AS int4) IS 'Allow casts from text to int4';
COMMENT ON COLLATION "fr_CA" IS 'Canadian French';
COMMENT ON COLUMN my_table.my_column IS 'Employee ID number';
COMMENT ON CONVERSION my_conv IS 'Conversion to UTF8';
COMMENT ON CONSTRAINT bar_col_cons ON bar IS 'Constrains column col';
COMMENT ON CONSTRAINT dom_col_constr ON DOMAIN dom IS 'Constrains col of domain';
COMMENT ON DATABASE my_database IS 'Development Database';
COMMENT ON DOMAIN my_domain IS 'Email Address Domain';
COMMENT ON EVENT TRIGGER abort_ddl IS 'Aborts all DDL commands';
COMMENT ON EXTENSION hstore IS 'implements the hstore data type';
COMMENT ON FOREIGN DATA WRAPPER mywrapper IS 'my foreign data wrapper';
COMMENT ON FOREIGN TABLE my_foreign_table IS 'Employee Information in other database';
COMMENT ON FUNCTION my_function (timestamp) IS 'Returns Roman Numeral';
COMMENT ON INDEX my_index IS 'Enforces uniqueness on employee ID';
COMMENT ON LANGUAGE plpython IS 'Python support for stored procedures';
COMMENT ON LARGE OBJECT 346344 IS 'Planning document';
COMMENT ON MATERIALIZED VIEW my_matview IS 'Summary of order history';
COMMENT ON OPERATOR ^ (text, text) IS 'Performs intersection of two texts';
COMMENT ON OPERATOR - (NONE, integer) IS 'Unary minus';
COMMENT ON OPERATOR CLASS int4ops USING btree IS '4 byte integer operators for btrees';
COMMENT ON OPERATOR FAMILY integer_ops USING btree IS 'all integer operators for btrees';
COMMENT ON POLICY my_policy ON mytable IS 'Filter rows by users';
COMMENT ON PROCEDURE my_proc (integer, integer) IS 'Runs a report';
COMMENT ON PUBLICATION alltables IS 'Publishes all operations on all tables';
COMMENT ON ROLE my_role IS 'Administration group for finance tables';
COMMENT ON ROUTINE my_routine (integer, integer) IS 'Runs a routine (which is a function or procedure)';
COMMENT ON RULE my_rule ON my_table IS 'Logs updates of employee records';
COMMENT ON SCHEMA my_schema IS 'Departmental data';
COMMENT ON SEQUENCE my_sequence IS 'Used to generate primary keys';
COMMENT ON SERVER myserver IS 'my foreign server';
COMMENT ON STATISTICS my_statistics IS 'Improves planner row estimations';
COMMENT ON SUBSCRIPTION alltables IS 'Subscription for all operations on all tables';
COMMENT ON TABLE my_schema.my_table IS 'Employee Information';
COMMENT ON TABLESPACE my_tablespace IS 'Tablespace for indexes';
COMMENT ON TEXT SEARCH CONFIGURATION my_config IS 'Special word filtering';
COMMENT ON TEXT SEARCH DICTIONARY swedish IS 'Snowball stemmer for Swedish language';
COMMENT ON TEXT SEARCH PARSER my_parser IS 'Splits text into words';
COMMENT ON TEXT SEARCH TEMPLATE snowball IS 'Snowball stemmer';
COMMENT ON TRANSFORM FOR hstore LANGUAGE plpythonu IS 'Transform between hstore and Python dict';
COMMENT ON TRIGGER my_trigger ON my_table IS 'Used for RI';
COMMENT ON TYPE complex IS 'Complex number data type';
COMMENT ON VIEW my_view IS 'View of departmental costs';

兼容性

SQL 标准中没有COMMENT命令。

查询注释

表 9.73中展示的函数抽取注释，注释是由COMMENT命令在以前存储的。如果对指定参数找不到注释，则返回空值。

表 9.73. 注释信息函数

我的示例

注意：::regclass是oid的别名。

regclass是一种数据类型，它实际上是oid的别名。相当于是oid和对象名之间的双向符号，既可以把oid翻译成表名，也可以把表名翻译成oid。通过使用::regclass, 我们就不需要通过查询pg_class表来获取对象与oid的对应关系了。

drop table t_hash;
CREATE TABLE t_hash AS 
SELECT id, md5(id::text) md5_text
FROM generate_series(1, 500) AS id;

SELECT oid FROM pg_class WHERE relname = 't_hash';
select oid,oid::regclass tbname,relname,reltuples from pg_class where oid='t_hash'::regclass;

comment on table t_hash is 'table t_hash';
comment on column t_hash.id is 'The ID';
comment on column t_hash.md5_text is 'The text';

select
    table_catalog,
    table_schema,
    table_name,
    column_name, data_type,
    obj_description(table_name::regclass) tb_comment,
    col_description(table_name::regclass, ordinal_position::int) col_comment
from  information_schema.columns
where table_name = 't_hash';

过程：

postgres=# drop table t_hash;
DROP TABLE
postgres=# CREATE TABLE t_hash AS 
postgres-# SELECT id, md5(id::text) md5_text
postgres-# FROM generate_series(1, 500) AS id;
SELECT 500
postgres=# 
postgres=# SELECT oid FROM pg_class WHERE relname = 't_hash';
  oid  
-------
 17863
(1 row)

postgres=# 
postgres=# select oid,oid::regclass tbname,relname,reltuples from pg_class where oid='t_hash'::regclass;
  oid  | tbname | relname | reltuples 
-------+--------+---------+-----------
 17863 | t_hash | t_hash  |         0
(1 row)

postgres=# \d+ t_hash
                                    Table "public.t_hash"
  Column  |  Type   | Collation | Nullable | Default | Storage  | Stats target | Description 
----------+---------+-----------+----------+---------+----------+--------------+-------------
 id       | integer |           |          |         | plain    |              | 
 md5_text | text    |           |          |         | extended |              | 
Access method: heap

postgres=# 
postgres=# comment on table t_hash is 'table t_hash';
COMMENT
postgres=# comment on column t_hash.id is 'The ID';
COMMENT
postgres=# comment on column t_hash.md5_text is 'The text';
COMMENT
postgres=# \d+ t_hash
                                    Table "public.t_hash"
  Column  |  Type   | Collation | Nullable | Default | Storage  | Stats target | Description 
----------+---------+-----------+----------+---------+----------+--------------+-------------
 id       | integer |           |          |         | plain    |              | The ID
 md5_text | text    |           |          |         | extended |              | The text
Access method: heap
postgres=# 
postgres=# select
postgres-#     table_catalog,
postgres-#     table_schema,
postgres-#     table_name,
postgres-#     column_name, data_type,
postgres-#     obj_description(table_name::regclass,) tb_comment,
postgres-#     col_description(table_name::regclass, ordinal_position::int) col_comment
postgres-# from  information_schema.columns
postgres-# where table_name = 't_hash';
 table_catalog | table_schema | table_name | column_name | data_type |  tb_comment  | col_comment 
---------------+--------------+------------+-------------+-----------+--------------+-------------
 postgres      | public       | t_hash     | id          | integer   | table t_hash | The ID
 postgres      | public       | t_hash     | md5_text    | text      | table t_hash | The text
(2 rows)

参考

http://postgres.cn/docs/14/sql-comment.html