The first important thing, we have to define Distribution key correctly which is a primary for distribution of data in Greenplum.

Greenplum is a base on MPP architecture where data equally distributes across the child segments.
Before creating a table, we should analyze the distribution logic and define distribution keys where data must be unique for equal distribution.

My suggestion: Once you distribute your data basis on defined distribution key, you should not alter the distribution key.

Because data redistribution requires on disk which can be resource intensive, but still I am sharing the steps to ALTER the distribution key.

Alter Distribution key:

ALTER TABLE table_name SET DISTRIBUTED BY (column_name);

For Random distribution:
For random distribution, you should do REORGANIZE=TRUE to remove unnecessary skewness of data.

ALTER TABLE table_name SET DISTRIBUTED RANDOMLY;
ALTER TABLE table_name SET WITH (REORGANIZE=TRUE);

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

In this post, I am sharing a script to find a list of distribution keys of a database. In the Greenplum MPP architecture, distribution keys are playing a primary role in selecting data.

If we define proper distribution key, we don’t require even table indexes.
‘ Using below script, Greenplum DBA can get the list of all distribution keys which further they can use for ad-hoc database reporting as well.

SELECT
	pgn.nspname as table_owner
	,pgc.relname as table_name
	,COALESCE(pga.attname,'DISTRIBUTED RANDOMLY') as distribution_keys
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
ORDER BY pgn.nspname, pgc.relname;