As Greenplum is a MPP architecture, so distribution of data in all segments is the first stuff.

You can distribute your table data using Distributed BY , and if you are not sure about a particular column, you can create your table using Distributed Randomly.

But tables which are distributed randomly, are not good for table performance because query optimizer will take more time for Randomly distributed tables.
Greenplum DBA can use below script to find randomly distributed tables.

with cte AS
(
	SELECT                 
		pgn.nspname as SchemaName                 
		,pgc.relname as TableName
		,pga.attname as DistributionType
	FROM
	(
		SELECT gdp.localoid,
			CASE
			WHEN ( Array_upper(gdp.attrnums, 1) > 0 ) THEN
			Unnest(gdp.attrnums)
			ELSE NULL
			END AS attnum
		FROM gp_distribution_policy gdp
		ORDER BY gdp.localoid
	) AS distrokey
	INNER JOIN pg_class AS pgc
		ON distrokey.localoid = pgc.oid
	INNER JOIN pg_namespace pgn
		ON pgc.relnamespace = pgn.oid
	LEFT OUTER JOIN pg_attribute pga
		ON distrokey.attnum = pga.attnum
	AND distrokey.localoid = pga.attrelid      
)
select
	SchemaName
	,TableName      
	,COALESCE(DistributionType,'DISTRIBUTED RANDOMLY') AS DistributionType
from cte
where COALESCE(DistributionType,'DISTRIBUTED RANDOMLY')='DISTRIBUTED RANDOMLY'
ORDER BY SchemaName,TableName