Redshift Auto Vacuum - amazon-web-services

Question

I have recently implemented Distribution Styles and Sort Keys on a few Redshift Tables after doing the following analysis -
To find out the best candidate for Distribution Styles and Sort keys , I looked into the queries that run on these tables followed by shortlisting 1 column for Sort key & Distribution Key ( Instead of Style ) on the bases of Cardinality of the column ( Degree of Uniqueness of a columns data ) and also ran EXPLAIN query which clearly showed reduction in COST, Width & Row Scanned.
And on the basis of the above, I applied the sort key and dist key on few of the tables which produced the best performance.
Now After implementation , When I was looking into the pct_unsorted for these tables , then these are my observations -
Not All Tables were Auto Vacuumed by redshift , Some Had [BG] Vacuum run where as some others only had [BG] Vacuum Delete run on them. These were the tables with 100% Unsorted data -
Upon Looking into the Vacuum Status from stl_vacuum -
My Questions -
On What grounds does Redshift decide which kind of Vacuum is to be run ( Auto Vacuum )?
Why for some tables is redshift Running VACUUM and for others its running VACUUM DELETE ONLY ?
What is the most Ideal or best practice of VACUUM tables to get data sorted ?
Thanks in Advance.
Queries Run for the above shown results are -
select trim(pgn.nspname) as schema,
trim(a.name) as table, id as tableid,
decode(pgc.reldiststyle,0, 'even',1,det.distkey ,8,'all') as distkey, dist_ratio.ratio::decimal(10,4) as skew,
det.head_sort as "sortkey",
det.n_sortkeys as "#sks", b.mbytes,
decode(b.mbytes,0,0,((b.mbytes/part.total::decimal)*100)::decimal(5,2)) as pct_of_total,
decode(det.max_enc,0,'n','y') as enc, a.rows,
decode( det.n_sortkeys, 0, null, a.unsorted_rows ) as unsorted_rows ,
decode( det.n_sortkeys, 0, null, decode( a.rows,0,0, (a.unsorted_rows::decimal(32)/a.rows)*100) )::decimal(5,2) as pct_unsorted
from (select db_id, id, name, sum(rows) as rows,
sum(rows)-sum(sorted_rows) as unsorted_rows
from stv_tbl_perm a
group by db_id, id, name) as a
join pg_class as pgc on pgc.oid = a.id
join pg_namespace as pgn on pgn.oid = pgc.relnamespace
left outer join (select tbl, count(*) as mbytes
from stv_blocklist group by tbl) b on a.id=b.tbl
inner join (select attrelid,
min(case attisdistkey when 't' then attname else null end) as "distkey",
min(case attsortkeyord when 1 then attname else null end ) as head_sort ,
max(attsortkeyord) as n_sortkeys,
max(attencodingtype) as max_enc
from pg_attribute group by 1) as det
on det.attrelid = a.id
inner join ( select tbl, max(mbytes)::decimal(32)/min(mbytes) as ratio
from (select tbl, trim(name) as name, slice, count(*) as mbytes
from svv_diskusage group by tbl, name, slice )
group by tbl, name ) as dist_ratio on a.id = dist_ratio.tbl
join ( select sum(capacity) as total
from stv_partitions where part_begin=0 ) as part on 1=1
where mbytes is not null
and "table" in (<TABLE NAMES>)
order by mbytes desc
&
select xid, a.table_id,b."table", status, rows, sortedrows, blocks, eventtime
from stl_vacuum a
inner join (select table_id,"table" from svv_table_info where "schema" in ('touchpoint','activehealth')
and "table" in (<TABLE NAMES>)
) b on a.table_id = b. table_id
UPDATE -
One interesting observation is , in the svv_table_info ,
For these tables with 100% unsorted data, the vacuum_sort_benefit is 0.