Recuperando todos os PK e FK

Você pode usar a função pg_get_constraintdef(constraint_oid)em uma consulta como a seguinte:

SELECT conrelid::regclass AS table_from
     , conname
     , pg_get_constraintdef(oid)
FROM   pg_constraint
WHERE  contype IN ('f', 'p ')
AND    connamespace = 'public'::regnamespace  -- your schema here
ORDER  BY conrelid::regclass::text, contype DESC;

Resultado:

 table_from | conname    | pg_get_constraintdef
------------+------------+----------------------
 tbl        | tbl_pkey   | PRIMARY KEY (tbl_id)
 tbl        | tbl_col_fk | FOREIGN KEY (col) REFERENCES tbl2(col) ON UPDATE CASCADE
...

Retorna todas as chaves primárias e estrangeiras para todas as tabelas no esquema fornecido, ordenadas por tablename, PKs primeiro.

O manual sobre pg_constraint.

O manual sobre tipos de identificadores de objetos ( regclass, regnamespace, ...).

Com base na solução Erwin:

SELECT conrelid::regclass AS "FK_Table"
      ,CASE WHEN pg_get_constraintdef(c.oid) LIKE 'FOREIGN KEY %' THEN substring(pg_get_constraintdef(c.oid), 14, position(')' in pg_get_constraintdef(c.oid))-14) END AS "FK_Column"
      ,CASE WHEN pg_get_constraintdef(c.oid) LIKE 'FOREIGN KEY %' THEN substring(pg_get_constraintdef(c.oid), position(' REFERENCES ' in pg_get_constraintdef(c.oid))+12, position('(' in substring(pg_get_constraintdef(c.oid), 14))-position(' REFERENCES ' in pg_get_constraintdef(c.oid))+1) END AS "PK_Table"
      ,CASE WHEN pg_get_constraintdef(c.oid) LIKE 'FOREIGN KEY %' THEN substring(pg_get_constraintdef(c.oid), position('(' in substring(pg_get_constraintdef(c.oid), 14))+14, position(')' in substring(pg_get_constraintdef(c.oid), position('(' in substring(pg_get_constraintdef(c.oid), 14))+14))-1) END AS "PK_Column"
FROM   pg_constraint c
JOIN   pg_namespace n ON n.oid = c.connamespace
WHERE  contype IN ('f', 'p ')
AND pg_get_constraintdef(c.oid) LIKE 'FOREIGN KEY %'
ORDER  BY pg_get_constraintdef(c.oid), conrelid::regclass::text, contype DESC;

Irá retornar uma tabela do formulário:

| FK_Table | FK_Column | PK_Table | PK_Column |

Não há necessidade de analisar pg_get_constraintdef(), basta usar as colunas da pg_constrainttabela para obter outros detalhes ( os docs ).

Aqui constraint_typepode ser:

• p - chave primária ,
• f - chave estrangeira ,
• u - único ,
• c - verificar restrição ,
• x - exclusão ,
• ...

Com base na resposta de Erwin :

SELECT c.conname                                 AS constraint_name,
   c.contype                                     AS constraint_type,
   sch.nspname                                   AS "self_schema",
   tbl.relname                                   AS "self_table",
   ARRAY_AGG(col.attname ORDER BY u.attposition) AS "self_columns",
   f_sch.nspname                                 AS "foreign_schema",
   f_tbl.relname                                 AS "foreign_table",
   ARRAY_AGG(f_col.attname ORDER BY f_u.attposition) AS "foreign_columns",
   pg_get_constraintdef(c.oid)                   AS definition
FROM pg_constraint c
       LEFT JOIN LATERAL UNNEST(c.conkey) WITH ORDINALITY AS u(attnum, attposition) ON TRUE
       LEFT JOIN LATERAL UNNEST(c.confkey) WITH ORDINALITY AS f_u(attnum, attposition) ON f_u.attposition = u.attposition
       JOIN pg_class tbl ON tbl.oid = c.conrelid
       JOIN pg_namespace sch ON sch.oid = tbl.relnamespace
       LEFT JOIN pg_attribute col ON (col.attrelid = tbl.oid AND col.attnum = u.attnum)
       LEFT JOIN pg_class f_tbl ON f_tbl.oid = c.confrelid
       LEFT JOIN pg_namespace f_sch ON f_sch.oid = f_tbl.relnamespace
       LEFT JOIN pg_attribute f_col ON (f_col.attrelid = f_tbl.oid AND f_col.attnum = f_u.attnum)
GROUP BY constraint_name, constraint_type, "self_schema", "self_table", definition, "foreign_schema", "foreign_table"
ORDER BY "self_schema", "self_table";

Os resultados são ordenados por schemae table.

Nota técnica: veja esta pergunta sobre with ordinality.

Recentemente, tive que implementar isso para uma camada de acesso a dados que cria utilitários CRUD com base no esquema de informações, acabou indo com isso.

SELECT

    current_schema() AS "schema",
    current_catalog AS "database",
    "pg_constraint".conrelid::regclass::text AS "primary_table_name",
    "pg_constraint".confrelid::regclass::text AS "foreign_table_name",

    (
        string_to_array(
            (
                string_to_array(
                    pg_get_constraintdef("pg_constraint".oid),
                    '('
                )
            )[2],
            ')'
        )
    )[1] AS "foreign_column_name",

    "pg_constraint".conindid::regclass::text AS "constraint_name",

    TRIM((
        string_to_array(
            pg_get_constraintdef("pg_constraint".oid),
            '('
        )
    )[1]) AS "constraint_type",

    pg_get_constraintdef("pg_constraint".oid) AS "constraint_definition"

FROM pg_constraint AS "pg_constraint"

JOIN pg_namespace AS "pg_namespace" ON "pg_namespace".oid = "pg_constraint".connamespace

WHERE

    "pg_constraint".contype IN ( 'f', 'p' )
    AND
    "pg_namespace".nspname = current_schema()
    AND
    "pg_constraint".conrelid::regclass::text IN ('whatever_table_name')