Physical Join Index Row Compression
A physical join index row has two parts:
For example, if a logical join result has n rows with the same fixed part value, then there is one corresponding physical join index row that includes n repeated parts in the physical join index. A physical join index row represents one or more logical join index rows. The number of logical join index rows represented in the physical row depends on how many repeated values are stored.
Row compression is only done on rows inserted by the same INSERT request. This means that newly inserted rows are not added as logical rows to existing compressed rows.
When the number of repeated values associated with a given fixed value exceeds the system row size limit, the join index row is automatically split into multiple physical rows, each having the same fixed value but different lists of repeated values. Note that a given logical join index result row cannot exceed the system 64K row size limit.