Creating a secondary index causes the system to build a subtable to contain its index rows, thus adding another set of rows that requires updating each time a table row is inserted, deleted, or updated.
Secondary index subtables are also duplicated whenever a table is defined with FALLBACK.
When compression at the data block level is enabled for their primary table, secondary index subtables are not compressed. See Compression Types Supported by Vantage for more information about data block compression.