## Name

COMMENT — define or change the comment of an object

## Synopsis

    COMMENT ON
    {
      TABLE object_name |
      COLUMN table_name.column_name |
      AGGREGATE agg_name (agg_type) |
      CAST (sourcetype AS targettype) |
      CONSTRAINT constraint_name ON table_name |
      CONVERSION object_name |
      DATABASE object_name |
      DOMAIN object_name |
      FUNCTION func_name ( [ [ argmode ] [ argname ] argtype [, ...] ] ) |
      INDEX object_name |
      LARGE OBJECT large_object_oid |
      OPERATOR op (leftoperand_type, rightoperand_type) |
      OPERATOR CLASS object_name USING index_method |
      [ PROCEDURAL ] LANGUAGE object_name |
      RULE rule_name ON table_name |
      SCHEMA object_name |
      SEQUENCE object_name |
      TRIGGER trigger_name ON table_name |
      TYPE object_name |
      VIEW object_name
    } IS 'text'

## Description

`COMMENT` stores a comment about a database object.

To modify a comment, issue a new `COMMENT` command for the same object. Only one comment string is stored for each object. To remove a comment, write `NULL` in place of the text string. Comments are automatically dropped when the object is dropped.

Comments can be easily retrieved with the psql commands `\dd`, `\d+`, and `\l+`. Other user interfaces to retrieve comments can be built atop the same built-in functions that psql uses, namely `obj_description` and `col_description` (see Table 9.43, “Comment Information Functions”).

## Parameters

`object_name`
`table_name.column_name`
`agg_name`
`constraint_name`
`func_name`
`op`
`rule_name`
`trigger_name`

  The name of the object to be commented. Names of tables, aggregates, domains, functions, indexes, operators, operator classes, sequences, types, and views may be schema-qualified.

`agg_type`

  The argument data type of the aggregate function, or `*` if the function accepts any data type.

`sourcetype`

  The name of the source data type of the cast.

`targettype`

  The name of the target data type of the cast.

`argmode`

  The mode of a function argument: either `IN`, `OUT`, or `INOUT`. If omitted, the default is `IN`. Note that `COMMENT ON FUNCTION` does not actually pay any attention to `OUT` arguments, since only the input arguments are needed to determine the function's identity. So it is sufficient to list the `IN` and `INOUT` arguments.

`argname`

  The name of a function argument. Note that `COMMENT ON FUNCTION` does not actually pay any attention to argument names, since only the argument data types are needed to determine the function's identity.

`argtype`

  The data type(s) of the function's arguments (optionally schema-qualified), if any.

`large_object_oid`

  The OID of the large object.

`PROCEDURAL`

  This is a noise word.

`text`

  The new comment, written as a string literal; or `NULL` to drop the comment.

## Notes

A comment for a database can only be created in that database, and will only be visible in that database, not in other databases.

There is presently no security mechanism for comments: any user connected to a database can see all the comments for objects in that database (although only superusers can change comments for objects that they don't own). Therefore, don't put security-critical information in comments.

## Examples

Attach a comment to the table `mytable`:

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

Remove it again:

    COMMENT ON TABLE mytable IS NULL;

Some more examples:

    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 COLUMN my_table.my_column IS 'Employee ID number';
    COMMENT ON CONVERSION my_conv IS 'Conversion to UTF8';
    COMMENT ON DATABASE my_database IS 'Development Database';
    COMMENT ON DOMAIN my_domain IS 'Email Address Domain';
    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 OPERATOR ^ (text, text) IS 'Performs intersection of two texts';
    COMMENT ON OPERATOR - (NONE, text) IS 'This is a prefix operator on text';
    COMMENT ON OPERATOR CLASS int4ops USING btree IS '4 byte integer operators for btrees';
    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 TABLE my_schema.my_table IS 'Employee Information';
    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';

## Compatibility

There is no `COMMENT` command in the SQL standard.