How to retrieve the comment of a PostgreSQL database? - postgresql

Question

I recently discovered you can attach a comment to all sort of objects in PostgreSQL. In particular, I'm interested on playing with the comment of a database. For example, to set the comment of a database:
COMMENT ON DATABASE mydatabase IS 'DB Comment';
However, what is the opposite statement, to get the comment of mydatabase?
From the psql command line, I can see the comment along with other information as a result of the \l+ command; which I could use with the aid of awk in order to achieve my goal. But I'd rather use an SQL statement, if possible.

Answer 1

First off, your query for table comments can be simplified using a cast to the appropriate object identifier type:
SELECT description
FROM pg_description
WHERE objoid = 'myschema.mytbl'::regclass;
The schema part is optional. If you omit it, your current search_path decides visibility of any table named mytbl.
Better yet, there are dedicated functions in PostgreSQL to simplify and canonize these queries. The manual:
obj_description(object_oid, catalog_name) ... get comment for a
database object
shobj_description(object_oid, catalog_name) ... get comment for a shared database object
Description for table:
SELECT obj_description('myschema.mytbl'::regclass, 'pg_class');
Description for database:
SELECT pg_catalog.shobj_description(d.oid, 'pg_database') AS "Description"
FROM pg_catalog.pg_database d
WHERE datname = 'mydb';
How do you find out about that?
Well, reading the excellent manual is enlightening. :)
But there is a more direct route in this case: most psql meta commands are implemented with plain SQL. Start a session with psql -E, to see the magic behind the curtains. The manual:
-E
--echo-hidden
Echo the actual queries generated by \d and other backslash commands. You can use this to study psql's internal operations. This
is equivalent to setting the variable ECHO_HIDDEN to on.

Answer 2

To get the comment on the database, use the following query:
select description from pg_shdescription
join pg_database on objoid = pg_database.oid
where datname = '<database name>'
This query will get you table comment for the given table name:
select description from pg_description
join pg_class on pg_description.objoid = pg_class.oid
where relname = '<your table name>'
If you use the same table name in different schemas, you need to modify it a bit:
select description from pg_description
join pg_class on pg_description.objoid = pg_class.oid
join pg_namespace on pg_class.relnamespace = pg_namespace.oid
where relname = '<table name>' and nspname='<schema name>'

Answer 3

For tables, try
\dd TABLENAME
This shows the comment I added to a table

Answer 4

This query will get only table comment for all tables
SELECT RelName,Description
FROM pg_Description
JOIN pg_Class
ON pg_Description.ObjOID = pg_Class.OID
WHERE ObjSubID = 0

Answer 5

This query will return the comment of a table
SELECT obj_description('public.myTable'::regclass)
FROM pg_class
WHERE relkind = 'r' limit 1

Answer 6

To get the comments on all the databases (not on their objects like tables etc.) :
SELECT datname, shobj_description( oid, 'pg_database' ) AS comment
FROM pg_database
ORDER BY datname
An example showing databases, sizes and descriptions from a shell script:
psql -U postgres -c "SELECT datname,
format('%8s MB.', pg_database_size(datname)/1000000) AS size,
shobj_description( oid, 'pg_database' ) as comment
FROM pg_database ORDER BY datname"
Sample output:
datname | size | comment
----------------------+--------------+-----------------------------------------------------
last_wikidb | 18 MB. | Wiki backup from yesterday
postgres | 7 MB. | default administrative connection database
previous_wikidb | 18 MB. | Wiki backup from the day before yesterday
some_db | 82 MB. |
template0 | 7 MB. | unmodifiable empty database
template1 | 7 MB. | default template for new databases