COMMENT — 定义或修改对象的注释
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 }
其中 aggregate_signature 是：
* |
[ argmode ] [ argname ] argtype [ , ... ] |
[ [ argmode ] [ argname ] argtype [ , ... ] ] ORDER BY [ argmode ] [ argname ] argtype [ , ... ]
COMMENT用于存储有关数据库对象的注释。
每个对象只存储一个注释字符串，因此要修改注释，请对同一对象再次发出 COMMENT命令。要移除注释，请在文本字符串的位置写入 NULL。当对象被删除时，其注释也会自动删除。
将在要添加注释的对象上获取一个SHARE UPDATE EXCLUSIVE锁。
对于大多数对象类型，只有对象的拥有者才能设置注释。角色没有拥有者，因此 COMMENT ON ROLE的规则是：若要为超级用户角色添加注释，你必须是 超级用户；否则，你必须拥有CREATEROLE权限，并且在目标角色上已获授 ADMIN OPTION。同样，访问方法也没有拥有者；要为访问方法添加注释， 你必须是超级用户。当然，超级用户可以为任何对象添加注释。
可以使用psql的\d 系列命令查看这些注释。其他用于检索注释的用户界面，也可以建立在 psql使用的同一组内置函数之上，即 obj_description、col_description 和shobj_description （见Table 9.80）。
object_namerelation_name.column_nameaggregate_nameconstraint_namefunction_nameoperator_namepolicy_nameprocedure_nameroutine_namerule_nametrigger_name #要添加注释的对象名称。位于模式中的对象（表、函数等）的名称可以带模式限定。 为列添加注释时，relation_name必须 引用一个表、视图、复合类型或外部表。
table_namedomain_name #当为约束、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、过程或聚合函数参数的模式：IN、 OUT、INOUT或VARIADIC。 如果省略，默认值是IN。注意COMMENT 实际上并不关心OUT参数，因为确定函数标识只需要输入参数。 因此，列出IN、INOUT和VARIADIC 参数就足够了。
argname #函数、过程或聚合函数参数的名称。注意COMMENT 实际上并不关心参数名称，因为确定函数标识只需要参数数据类型。
argtype #函数、过程或聚合函数参数的数据类型。
large_object_oid #大对象的 OID。
left_typeright_type #操作符参数的数据类型（可以是模式限定的）。对于前缀操作符缺失的参数， 请写成NONE。
PROCEDURAL #这是一个噪声词。
type_name #该转换所对应的数据类型名称。
lang_name #该转换所用语言的名称。
string_literal #新的注释内容，以字符串字面量形式写成。
NULL #写入NULL以移除该注释。
目前没有用于查看注释的安全机制：任何连接到某个数据库的用户都可以看到 该数据库中所有对象的注释。对于数据库、角色和表空间这类共享对象， 注释是全局存储的，因此连接到集簇中任一数据库的任何用户都可以看到 共享对象的全部注释。因此，不要在注释中放入与安全密切相关的信息。
为表mytable附加一条注释：
COMMENT ON TABLE mytable IS 'This is my table.';
再将其移除：
COMMENT ON TABLE mytable IS NULL;
更多示例：
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';
SQL 标准中没有COMMENT命令。
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