You can use the REASSIGN OWNED command.

Synopsis:

REASSIGN OWNED BY old_role [, ...] TO new_role

This changes all objects owned by old_role to the new role. You don't have to think about what kind of objects that the user has, they will all be changed. Note that it only applies to objects inside a single database. It does not alter the owner of the database itself either.

It is available back to at least 8.2. Their online documentation only goes that far back.

This: is also a nice and fast solution, and works for multiple schemas in one database:

Tables

SELECT 'ALTER TABLE '|| schemaname || '."' || tablename ||'" OWNER TO my_new_owner;'
FROM pg_tables WHERE NOT schemaname IN ('pg_catalog', 'information_schema')
ORDER BY schemaname, tablename;

Sequences

SELECT 'ALTER SEQUENCE '|| sequence_schema || '."' || sequence_name ||'" OWNER TO my_new_owner;'
FROM information_schema.sequences WHERE NOT sequence_schema IN ('pg_catalog', 'information_schema')
ORDER BY sequence_schema, sequence_name;

Views

SELECT 'ALTER VIEW '|| table_schema || '."' || table_name ||'" OWNER TO my_new_owner;'
FROM information_schema.views WHERE NOT table_schema IN ('pg_catalog', 'information_schema')
ORDER BY table_schema, table_name;

Materialized Views

Based on this answer

SELECT 'ALTER TABLE '|| oid::regclass::text ||' OWNER TO my_new_owner;'
FROM pg_class WHERE relkind = 'm'
ORDER BY oid;

This generates all the required ALTER TABLE / ALTER SEQUENCE / ALTER VIEW statements, copy these and paste them back into plsql to run them.

Check your work in psql by doing:

\dt *.*
\ds *.*
\dv *.*

If you want to do it in one sql statement, you need to define an exec() function as mentioned in

CREATE FUNCTION exec(text) returns text language plpgsql volatile
  AS $f$
    BEGIN
      EXECUTE $1;
      RETURN $1;
    END;
$f$;

Then you can execute this query, it will change the owner of tables, sequences and views:

SELECT exec('ALTER TABLE ' || quote_ident(s.nspname) || '.' ||
            quote_ident(s.relname) || ' OWNER TO $NEWUSER')
  FROM (SELECT nspname, relname
          FROM pg_class c JOIN pg_namespace n ON (c.relnamespace = n.oid) 
         WHERE nspname NOT LIKE E'pg\\_%' AND 
               nspname <> 'information_schema' AND 
               relkind IN ('r','S','v') ORDER BY relkind = 'S') s;

$NEWUSER is the postgresql new name of the new owner.

In most circumstances you need to be superuser to execute this. You can avoid that by changing the owner from your own user to a role group you are a member of.

Thanks to RhodiumToad on #postgresql for helping out with this.

is very simple

1. su - postgres
2. psql
3. REASSIGN OWNED BY [old_user] TO [new_user];
4. \c [your database]
5. REASSIGN OWNED BY [old_user] TO [new_user];

done.

I like this one since it modifies tables, views, sequences and functions owner of a certain schema in one go (in one sql statement), without creating a function and you can use it directly in PgAdmin III and psql:

(Tested in PostgreSql v9.2)

DO $$DECLARE r record;
DECLARE
    v_schema varchar := 'public';
    v_new_owner varchar := '<NEW_OWNER>';
BEGIN
    FOR r IN 
        select 'ALTER TABLE "' || table_schema || '"."' || table_name || '" OWNER TO ' || v_new_owner || ';' as a from information_schema.tables where table_schema = v_schema
        union all
        select 'ALTER TABLE "' || sequence_schema || '"."' || sequence_name || '" OWNER TO ' || v_new_owner || ';' as a from information_schema.sequences where sequence_schema = v_schema
        union all
        select 'ALTER TABLE "' || table_schema || '"."' || table_name || '" OWNER TO ' || v_new_owner || ';' as a from information_schema.views where table_schema = v_schema
        union all
        select 'ALTER FUNCTION "'||nsp.nspname||'"."'||p.proname||'"('||pg_get_function_identity_arguments(p.oid)||') OWNER TO ' || v_new_owner || ';' as a from pg_proc p join pg_namespace nsp ON p.pronamespace = nsp.oid where nsp.nspname = v_schema
    LOOP
        EXECUTE r.a;
    END LOOP;
END$$;

Based on answers provided by @rkj, @AlannaRose, @SharoonThomas, @user3560574 and this answer by @a_horse_with_no_name

Thank's a lot.

Better yet: Also change database and schema owner.

DO $$DECLARE r record;
DECLARE
    v_schema varchar := 'public';
    v_new_owner varchar := 'admin_ctes';
BEGIN
    FOR r IN 
        select 'ALTER TABLE "' || table_schema || '"."' || table_name || '" OWNER TO ' || v_new_owner || ';' as a from information_schema.tables where table_schema = v_schema
        union all
        select 'ALTER TABLE "' || sequence_schema || '"."' || sequence_name || '" OWNER TO ' || v_new_owner || ';' as a from information_schema.sequences where sequence_schema = v_schema
        union all
        select 'ALTER TABLE "' || table_schema || '"."' || table_name || '" OWNER TO ' || v_new_owner || ';' as a from information_schema.views where table_schema = v_schema
        union all
        select 'ALTER FUNCTION "'||nsp.nspname||'"."'||p.proname||'"('||pg_get_function_identity_arguments(p.oid)||') OWNER TO ' || v_new_owner || ';' as a from pg_proc p join pg_namespace nsp ON p.pronamespace = nsp.oid where nsp.nspname = v_schema
        union all
        select 'ALTER SCHEMA "' || v_schema || '" OWNER TO ' || v_new_owner 
        union all
        select 'ALTER DATABASE "' || current_database() || '" OWNER TO ' || v_new_owner 
    LOOP
        EXECUTE r.a;
    END LOOP;
END$$;

I recently had to change the ownership of all objects in a database. Although tables, views, triggers and sequences were somewhat easily changed the above approach failed for functions as the signature is part of the function name. Granted, I have a MySQL background and am not that familiar with Postgres.

However, pg_dump allows you to dump just the schema and this contains the ALTER xxx OWNER TO yyy; statements you need. Here is my bit of shell magic on the topic

pg_dump -s YOUR_DB | grep -i 'owner to' | sed -e 's/OWNER TO .*;/OWNER TO NEW_OWNER;/i' | psqL YOUR_DB

very simple, try it...

 select 'ALTER TABLE ' || table_name || ' OWNER TO myuser;' from information_schema.tables where table_schema = 'public';

I had to change the ownership of tables, views and sequences and found the great solution posted by @rjk is working fine - despite one detail: If the object names are of mixed case (e.g. "TableName"), this will fail with an "not found"-error.
To circumvent this, wrap the object names with ' " ' like this: