Обсуждение: how can I get the length of columns of a table by system tables/views

For example:
CREATE TABLE tb(name varchar(32));

According to pg_class, pg_attribute, pg_type, I can get the tablename,
column name, column type
however, how to get the length of columns of a table by system tables/views?
Thanks!

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For example:

CREATE TABLE tb(name varchar(32));

Raghavendra <raghavendra.rao@enterprisedb.com> writes:
> On Wed, Sep 7, 2011 at 8:50 PM, shuaixf <shuaixf@gmail.com> wrote:
>> According to pg_class, pg_attribute, pg_type, I can get the tablename,
>> column name, column type
>> however, how to get the length of columns of a table by system
>> tables/views?

> select pg_column_size(name) from tb;

That would provide the physical size of individual values.  I think what
the OP was after was how to get the declared length limit of a column.
That's encoded in pg_attribute.atttypmod.  Rather than hard-wiring
knowledge of the way it's encoded, it's best to rely on format_type(),
which knows the rules:

regression=# \d varchar_tbl
        Table "public.varchar_tbl"
 Column |         Type         | Modifiers
--------+----------------------+-----------
 f1     | character varying(4) |

regression=# select format_type(atttypid, atttypmod) from pg_attribute where attrelid = 'varchar_tbl'::regclass and
attname= 'f1'; 
     format_type
----------------------
 character varying(4)
(1 row)


            regards, tom lane

SELECT a.attname, pg_catalog.format_type(a.atttypid, a.atttypmod)
FROM pg_catalog.pg_attribute a
WHERE a.attrelid = (SELECT pg_class.oid
                    FROM pg_class INNER JOIN pg_namespace
                       ON (pg_class.relnamespace = pg_namespace.oid
                           AND lower(pg_namespace.nspname) = 'public')
                   WHERE pg_class.relname='tablename')
  AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum;

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