Primary Index and Skewed Row Distribution
The hash value of the PI for a row determines where the row is stored on the AMP.
In a normal environment, hash values are evenly distributed across the nodes and the
AMPs within a node.
The less unique the values are for the index, the less evenly the rows of that table
are distributed across the AMPs. If a table has a NUPI with thousands of instances
of a single value, the table can become skewed.
Performance Effects of a Primary Index
The more unique the PI, the more unique the row hash value.
The more unique the row hash value, the more even the data distribution across all
the AMP vprocs. Even data distribution enhances parallel efficiency on full table
UPIs generate the most even distribution of table rows across all AMP vprocs.
NUPIs generate even row distribution to the same degree that values for a column or
columns are unique. Rows with the same row hash always go to the same AMP, whether
they are from the same table or from different tables.
To determine the best PI for a table, factor in the:
Extent of update activity
Number of full table scans
Join activity against the PI definition
Frequency of PI as a selectivity column and, therefore, a potential access path
If a PI causes uneven data distribution, you should re-specify it according to the
primary index design best practices shown in Database Design.