Teradata 中偏斜因子的错误数字是多少？

Question

我这样确定偏斜因子：

SELECT 
TABLENAME,
SUM(CURRENTPERM) /(1024*1024) AS CURRENTPERM, 
(100 - (AVG(CURRENTPERM)/MAX(CURRENTPERM)*100)) AS SKEWFACTOR 
FROM 
DBC.TABLESIZE 
WHERE DATABASENAME= <DATABASENAME> 
AND 
TABLENAME =<TABLENAME>  
GROUP BY 1;

对于一些大小为 ~600 Gb 的 table，偏斜系数为 30%。对于大小为 10 Gb 的 table，98% 相当高。上面的数字到底有多糟糕？官方有文章说要重新分配10%以上吗？我需要它来证实对市场开发人员的要求。我只找到 this

Answer 1

没有任何神奇的数字，但是 table 有 98% 的偏差意味着几乎所有数据都位于单个 AMP 中，这意味着 (1) 您正在失去 AMP 的性能优势并行数据库 (2) 您正在系统上创建不平衡的负载。

Answer 2

30 的偏斜因子意味着最大 AMP 上的数据比平均值多 40%。这可能仍然可以接受（当然 这取决于 ），与您的 DBSa 讨论他们通常认为太大的东西。

另一方面，98 意味着 max-AMP 上有 40 到 50 倍的数据，这真是太多了.

这比较了两种计算偏差的方法：

SELECT
   t.DatabaseName
   ,t.TableName

   -- currently used diskspace in GB
   ,SUM(t.CurrentPerm) / 1024**3 (DEC(9,2)) AS CurrentPermGB

   -- currently needed diskspace in GB to store this table as standalone (due to Skew)
   ,MAX(t.CurrentPerm) / 1024**3 * (HASHAMP() + 1) (DEC(9,2)) AS SkewedPermGB

   ,SkewedPermGB - CurrentPermGB  AS WastedPermGB

   -- AMP with higehst disk usage
   ,MAX(t.MaxPermAMP) AS SkewedAMP

   -- skew factor, 1 = even distribution, 1.1 = max AMP needs 10% more space than the average AMP
   ,MAX(t.CurrentPerm) / NULLIF(AVG(t.CurrentPerm),0) (DEC(5,2)) AS SkewFactor

   -- skew factor, between 0 and 99.  Same calculation as WinDDI/ TD Administrator
   ,(100 - (AVG(t.CurrentPerm) / NULLIF(MAX(t.CurrentPerm),0) * 100)) (DEC(3,0)) AS SkewFactor_WINDDI
FROM
 (
   SELECT
      DatabaseName,
      TableName,
      CurrentPerm,
      CASE WHEN CurrentPerm = MAX(CurrentPerm) OVER (PARTITION BY DatabaseName, TableName) THEN vproc END AS MaxPermAMP
   FROM dbc.TableSizeV
   WHERE DatabaseName = '???' --
) AS t
GROUP BY 1,2
HAVING SkewFactor > 1.1 -- or whatever
   AND SkewedPermGB > 10 -- or whatever
ORDER BY WastedPermGB DESC
;

Teradata 中偏斜因子的错误数字是多少？

What is bad numbers for skew factor in Teradata?

sql

optimization

teradata

skew