Como listar todas as restrições de uma tabela no PostgreSQL?

As restrições podem ser recuperadas via pg_catalog.pg_constraint.

SELECT con.*
       FROM pg_catalog.pg_constraint con
            INNER JOIN pg_catalog.pg_class rel
                       ON rel.oid = con.conrelid
            INNER JOIN pg_catalog.pg_namespace nsp
                       ON nsp.oid = connamespace
       WHERE nsp.nspname = '<schema name>'
             AND rel.relname = '<table name>';

Substitua <schema name>pelo nome do seu esquema e <table name>pelo nome da sua tabela.

Na psqllinha de comando esta informação está na folha de tabela, obtida com o \d+comando. d+também informa sobre as NOT NULLrestrições, algo que não está presente na pg_catalog.pg_constrainttabela. Um exemplo:

# \d+ observations.stream   
                                                  Table "observations.stream"
 Column |       Type        | Collation | Nullable | Default | Storage  | Stats target |                 Description                 
--------+-------------------+-----------+----------+---------+----------+--------------+---------------------------------------------
 id     | integer           |           | not null |         | plain    |              | 
 name   | character varying |           | not null |         | extended |              | This should be a table in the import schema
 min_id | integer           |           | not null |         | plain    |              | 
 about  | character varying |           | not null |         | extended |              | 
Indexes:
    "stream_pkey" PRIMARY KEY, btree (id)
    "stream_name_key" UNIQUE CONSTRAINT, btree (name)
Check constraints:
    "stream_id_check" CHECK (id > 0)
Referenced by:
    TABLE "profile" CONSTRAINT "profile_id_stream_fkey" FOREIGN KEY (id_stream) REFERENCES stream(id)

A ressalva aqui é que você não obtém os nomes de todas as restrições dessa maneira.

Aqui está a resposta específica do PostgreSQL. Ele recuperará todas as colunas e seu relacionamento também:

select *FROM (
from (
    select
        pgc.contype as constraint_type,
        ccu.table_schema as table_schema,
        kcu.table_name as table_name,
        case when (pgc.contype = 'f') then kcu.column_name else ccu.column_name end as column_name, 
        case when (pgc.contype = 'f') then ccu.table_name else (null) end as reference_table,
        case when (pgc.contype = 'f') then ccu.column_name else (null) end as reference_col,
        case when (pgc.contype = 'p') then 'yes' else 'no' end as auto_inc,
        case when (pgc.contype = 'p') then 'no' else 'yes' end as is_nullable,
        'integer' as data_type,
        '0' as numeric_scale,
        '32' as numeric_precision
    from
        pg_constraint as pgc
        join pg_namespace nsp on nsp.oid = pgc.connamespace
        join pg_class cls on pgc.conrelid = cls.oid
        join information_schema.key_column_usage kcu on kcu.constraint_name = pgc.conname
        left join information_schema.constraint_column_usage ccu on pgc.conname = ccu.constraint_name 
        and nsp.nspname = ccu.constraint_schema
     union
        select 
            null as constraint_type ,
            table_schema,
            table_name,
            column_name, 
            null as refrence_table, 
            null as refrence_col, 
            'no' as auto_inc,
            is_nullable,
            data_type,
            numeric_scale,
            numeric_precision
        from information_schema.columns cols 
        where 
            table_schema = 'public'
            and concat(table_name, column_name) not in(
                select concat(kcu.table_name, kcu.column_name)
                from
                pg_constraint as pgc
                join pg_namespace nsp on nsp.oid = pgc.connamespace
                join pg_class cls on pgc.conrelid = cls.oid
                join information_schema.key_column_usage kcu on kcu.constraint_name = pgc.conname
                left join information_schema.constraint_column_usage ccu on pgc.conname = ccu.constraint_name 
                and nsp.nspname = ccu.constraint_schema
            )
    ) as foo
order by table_name asc, column_name