The Table is having too many Duplicate rows, The Skew table
will hadle the duplicate rows up to limitations,Once its 
cross the no of Rows it will effect on Performence.The Dba 
people will the SKEW tables.

I think this Answer helps u

The data distribution of table among AMPs is called Skew Factor

Generally For Non-Unique PI we get duplicate values so the
more duplicate vales we get more the data have same rowhash
so all the same data will come to same amp, it makes data
distribution inequality,

One amp will store more data and other amp stores less
amount of data, when we are accessing full table, The amp
which is having more data will take longer time and makes
other amps waiting which leads processing wastage 

In this situation (unequal distribution of data)we get Skew
Factor High

For this type of tables we should avoid full table scans

ex:
AMP0    AMP1
10000(10%)   9000000(90%)

in this situation skew factor is very high 90%

It is a number.It tells you how the data is distributed
among  Processors or Amps.Skew factor varies about 0-100.
If the data is distributed evenly among the processor the
skew factor is ZERO whether it is smaller or large table.
Skew factor doesn't depends on size of table but it only
depends on distribution of data.

If the skewfactor is more we have to access the table on
only primary index columns but not whole table.

Skew factor is distribution of rows of a table among the 
available no.of AMP's.
If your table has a chance of using unique primary index,it 
is always better to use UPI which ensures the skew factor 
around 0%.
If there is no chance of having unique values column in a 
table choose a column as PI(primary index) which has less 
duplicate values which inturn results in less skew factor.
That is the data will be distributed almost(not exactly 
equal percentage) equally to all AMP's.