确定具有数据偏斜或未排序行的表

以下查询标识数据分配不均匀（数据偏斜）或未排序行百分比较高的表。

skew 值较低表明表数据分配适当。如果表的 skew 值达到 4.00 或以上，可以考虑修改其数据分配方式。有关更多信息，请参阅 非最优数据分配。

如果表的 pct_unsorted 值大于 20%，可以考虑运行 VACUUM 命令。有关更多信息，请参阅 未排序或排序错乱的行。

还应检查每个表的 mbytes 和 pct_of_total 值。这些列标识表的大小及表占用的原始磁盘空间比例。原始磁盘空间包括 Amazon Redshift 保留供内部使用的空间，因此它大于名义磁盘容量（可供用户使用的磁盘空间量）。使用这些信息可验证可用磁盘空间等于最大表大小的 2.5 倍或以上。如果使用此空间，系统可以在处理复杂查询时将临时结果写入磁盘。

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;