Physical Join Index Row Compression
Compression refers to a logical row compression in which multiple sets of nonrepeating column values are appended to a single set of repeating column values. This allows the system to store the repeating value set only once, while any nonrepeating column values are stored as logical segmental extensions of the base repeating set.
A physical join index row has:
When a column_2 list is specified, the columns specified in the column_1 list are the data that is compressed.
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.
Compression is only done on rows inserted by the same INSERT statement. 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 64KB row size limit.