COMMENT — 定义或更改一个对象的注释
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 { string_literal | NULL }
where aggregate_signature is:
* |
[ argmode ] [ argname ] argtype [ , ... ] |
[ [ argmode ] [ argname ] argtype [ , ... ] ] ORDER BY [ argmode ] [ argname ] argtype [ , ... ]
COMMENT 存储、替换或移除一个数据库对象上的注释。
每个对象只存储一个注释字符串。对同一个对象再次发出 COMMENT 命令会替换现有的注释。指定 NULL 或空字符串（''）会移除注释。对象被删除时，其注释会被自动删除。
会在被注释的对象上获取一个 SHARE UPDATE EXCLUSIVE 锁。
对于大多数类型的对象，只有对象的所有者才能设置注释。角色没有所有者，因此 COMMENT ON ROLE 的规则是：要注释一个超级用户角色，你必须自己是超级用户，或者拥有 CREATEROLE 特权并且已被授予目标角色上的 ADMIN OPTION。同样，访问方法也没有所有者；要注释一个访问方法，你必须自己是超级用户。当然，超级用户可以注释任何东西。
可以使用 psql 的 \d 命令族来查看注释。也可以基于 psql 所使用的相同内建函数构建其他检索注释的用户界面，这些函数是 obj_description、col_description 和 shobj_description（参见 Table 9.82）。
object_namerelation_name.column_nameaggregate_nameconstraint_namefunction_nameoperator_namepolicy_nameprocedure_nameroutine_namerule_nametrigger_name 要注释的对象的名称。位于模式中的对象（表、函数等）的名称可以用模式限定。在注释一个列时，relation_name 必须引用表、视图、复合类型或外部表。
table_namedomain_name在为约束、触发器、规则或策略创建注释时，这些参数指定定义该对象的表或域的名称。
source_type类型转换的源数据类型的名称。
target_type类型转换的目标数据类型的名称。
argmode 函数、过程或聚合参数的模式：IN、OUT、INOUT 或 VARIADIC。如果省略，默认为 IN。注意 COMMENT 实际上并不关注 OUT 参数，因为确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 函数、过程或聚合参数的名称。注意 COMMENT 实际上并不关注参数名称，因为确定函数的身份只需要参数的数据类型。
argtype函数、过程或聚合参数的数据类型。
large_object_oid大对象的 OID。
left_typeright_type 操作符各参数的数据类型（可以用模式限定）。对于前缀操作符缺失的参数，写 NONE。
PROCEDURAL这是一个噪声词。
type_name转换的数据类型的名称。
lang_name转换的语言的名称。
string_literal 新注释的内容，以字符串字面值的形式书写。空字符串（''）会移除注释。
NULL 写 NULL 来移除注释。
目前没有用于查看注释的安全机制：任何连接到数据库的用户都能看到该数据库中所有对象的全部注释。对于数据库、角色和表空间这类共享对象，注释被全局存储，因此任何连接到集群中任意数据库的用户都能看到共享对象的全部注释。因此，不要把安全关键信息放在注释中。
为表 mytable 附加一个注释：
COMMENT ON TABLE mytable IS 'This is my table.';
再次将其移除：
COMMENT ON TABLE mytable IS NULL;
再举一些例子：
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 plpython3u 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'; COMMENT ON VIEW my_view IS NULL;
SQL 标准中没有 COMMENT 命令。