Retrieving Comments from a PostgreSQL DB

Question

I'm running a project on a Postgres database and need to retrieve the comments on columns within the DB to be used as table headings and such. I have seen that there are a couple of built in functions (pg_description and col_description) but i haven't been able to find examples on how to use them and playing around with them has proved pretty futile.

So I was wondering if any has been able to do this before and if so, how?

Answer 1

Enhance for @Nick and @mat suggestions: use
SELECT obj_description('schemaName.tableName'::regclass, 'pg_class'); when you have string name (not oid).

To avoid to remember 'pg_class' parameter, and to avoid ugly concatenations at the function calls, as (tname||'.'||schema)::regclass, an useful overload for obj_description:

  CREATE FUNCTION obj_description(
      p_rname text, p_schema text DEFAULT NULL, 
      p_catalname text DEFAULT 'pg_class'
  ) RETURNS text AS $f$
     SELECT obj_description((CASE 
        WHEN strpos($1, '.')>0 OR $2 IS NULL OR $2='' THEN $1
        ELSE $2||'.'||$1
     END)::regclass, $3);
  $f$ LANGUAGE SQL IMMUTABLE;
 -- USAGE: obj_description('mytable') 
 --        SELECT obj_description('s.t'); 
 -- PS: obj_description('s.t', 'otherschema') is a syntax error, 
 --     but not generates exception: returns the same as ('s.t') 

Now is easy to use, because the table name (rname parameter) is a varchar and can be expressed with a separated field for schema name, as in the main tables and queries.

See also "Getting list of table comments in PostgreSQL" or the new pg9.3 Guide

Answer 2

This works for me using the PostBooks 3.2.2 DB:

select cols.column_name,
(select pg_catalog.obj_description(oid) from pg_catalog.pg_class c where c.relname=cols.table_name) as table_comment
,(select pg_catalog.col_description(oid,cols.ordinal_position::int) from pg_catalog.pg_class c where c.relname=cols.table_name) as column_comment
from information_schema.columns cols
where cols.table_catalog='postbooks' and cols.table_name='apapply'

Regards, Sylnsr

Answer 3

I asked a similar question about Postgresql comments last month. If you dig through that, you'll come across some Perl code over on my blog that automates the process of extracting a comment.

To pull out the column names of a table, you can use something like the following:

select
     a.attname  as "colname"
    ,a.attrelid as "tableoid"
    ,a.attnum   as "columnoid"
from
    pg_catalog.pg_attribute a
    inner join pg_catalog.pg_class c on a.attrelid = c.oid
where
        c.relname = 'mytable' -- better to use a placeholder
    and a.attnum > 0
    and a.attisdropped is false
    and pg_catalog.pg_table_is_visible(c.oid)
order by a.attnum

You can then use the tableoid,columnoid tuple to extract the comment of each column (see my question).

Answer 4

This answer is a little late, but it popped up on a google search I did to research this problem. We only needed Table descriptions, but the method would be the same for columns. The column descriptions are in the pg_description table also, referenced by objoid.

Add this view:

CREATE OR REPLACE VIEW our_tables AS 
 SELECT c.oid, n.nspname AS schemaname, c.relname AS tablename, d.description,
   pg_get_userbyid(c.relowner) AS tableowner, t.spcname AS "tablespace", 
   c.relhasindex AS hasindexes, c.relhasrules AS hasrules, c.reltriggers > 0 AS hastriggers
   FROM pg_class c
   LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
   LEFT JOIN pg_tablespace t ON t.oid = c.reltablespace
   LEFT JOIN pg_description d ON c.oid = d.objoid
  WHERE c.relkind = 'r'::"char";

ALTER TABLE our_tables OWNER TO postgres;
GRANT SELECT, UPDATE, INSERT, DELETE, REFERENCES, TRIGGER ON TABLE our_tables TO postgres;
GRANT SELECT ON TABLE our_tables TO public;

Then run:

SELECT tablename, description FROM our_tables WHERE schemaname = 'public'

The view is a modified version of the pg_tables view which adds in the description column. You could also monkey around with the view definition to make it a single query.

Answer 5

SELECT c.table_schema,c.table_name,c.column_name,pgd.description
FROM pg_catalog.pg_statio_all_tables as st
  inner join pg_catalog.pg_description pgd on (pgd.objoid=st.relid)
  inner join information_schema.columns c on (pgd.objsubid=c.ordinal_position
    and  c.table_schema=st.schemaname and c.table_name=st.relname);

Answer 6

Just to be here if somebody will need it.

There are many answers here, but none of them was as simple as I would like it to be. So, based on previous answers and current postgres 9.4, I have created this query:

SELECT 
    obj_description(format('%s.%s',isc.table_schema,isc.table_name)::regclass::oid, 'pg_class') as table_description,
    pg_catalog.col_description(format('%s.%s',isc.table_schema,isc.table_name)::regclass::oid,isc.ordinal_position) as column_description
FROM
    information_schema.columns isc

It fetches table and column descriptions, without any confusing joins and ugly string concatenations.