Find your widest tables (by column count)

SELECT
  n.nspname AS schema,
  c.relname AS table,
  count(*) FILTER (WHERE a.attnum > 0 AND NOT a.attisdropped) AS live_columns,
  count(*) FILTER (WHERE a.attisdropped) AS dropped_columns,
  count(*) FILTER (WHERE a.attnum > 0) AS total_attnums
FROM pg_attribute a
JOIN pg_class c      ON c.oid = a.attrelid
JOIN pg_namespace n  ON n.oid = c.relnamespace
WHERE c.relkind IN ('r','p')   -- r=table, p=partitioned table
  AND n.nspname NOT IN ('pg_catalog','information_schema')
GROUP BY 1,2
ORDER BY total_attnums DESC
LIMIT 50;

Identify “mostly NULL” columns (classic wide-table driver)

SELECT
  attname,
  null_frac,
  avg_width
FROM pg_stats
WHERE schemaname = 'public'
  AND tablename  = 'your_table'
ORDER BY null_frac DESC, avg_width DESC
LIMIT 30;

Get a rough “row width” estimate (planner stats)

SELECT
  relname,
  n_live_tup,
  n_dead_tup,
  pg_size_pretty(pg_relation_size(relid)) AS heap_size
FROM pg_stat_user_tables
ORDER BY pg_relation_size(relid) DESC
LIMIT 20;