limit to default display in psql

In the following query (used by psql to show attribute defaults)

SELECT a.attname,
  pg_catalog.format_type(a.atttypid, a.atttypmod),
  (SELECT substring(pg_catalog.pg_get_expr(d.adbin, d.adrelid) for 128)
   FROM pg_catalog.pg_attrdef d
   WHERE d.adrelid = a.attrelid AND d.adnum = a.attnum AND a.atthasdef),
  a.attnotnull, a.attnum
FROM pg_catalog.pg_attribute a
WHERE a.attrelid = '32923' AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum

Is there some significance to the 128 other than simple formatting? 

Also, if anyone knows when pg_attrdef.adsrc became unreliable, that would save 
me some trouble. Thanks in advance. 

-- 
Robert Treat
Conjecture: http://www.xzilla.net
Consulting: http://www.omniti.com