To: Raghavendra <raghavendra.rao@enterprisedb.com>
cc: shuaixf <shuaixf@gmail.com>, pgsql-admin@postgresql.org
Subject: Re: how can I get the length of columns of a table by system
	tables/views
In-reply-to: 
 <CA+h6AhhcnXmvDPXk2XzqRY8W7J+WKmALSen_rMcXT_38=P3_Sw@mail.gmail.com>
References: <1315408840339-4778911.post@n5.nabble.com>
	<CA+h6AhhcnXmvDPXk2XzqRY8W7J+WKmALSen_rMcXT_38=P3_Sw@mail.gmail.com>
Comments: In-reply-to Raghavendra <raghavendra.rao@enterprisedb.com>
	message dated "Fri, 09 Sep 2011 13:11:06 +0530"
Date: Fri, 09 Sep 2011 09:57:10 -0400
Message-ID: <10512.1315576630@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

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