Identifying tables with data skew or unsorted rows - Amazon Redshift
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Identifying tables with data skew or unsorted rows

The following query identifies tables that have uneven data distribution (data skew) or a high percentage of unsorted rows.

A low skew value indicates that table data is properly distributed. If a table has a skew value of 4.00 or higher, consider modifying its data distribution style. For more information, see Suboptimal data distribution.

If a table has a pct_unsorted value greater than 20 percent, consider running the VACUUM command. For more information, see Unsorted or missorted rows.

Also review the mbytes and pct_of_total values for each table. These columns identify the size of the table and what percentage of raw disk space the table consumes. The raw disk space includes space that is reserved by Amazon Redshift for internal use, so it is larger than the nominal disk capacity, which is the amount of disk space available to the user. Use this information to verify that you have free disk space equal to at least 2.5 times the size of your largest table. Having this space available enables the system to write intermediate results to disk when processing complex queries.

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 order by mbytes desc;