A CTXCAT index comprises sub-indexes that you define as part of your index set. You create a sub-index on one or more columns to improve mixed query performance. However, adding sub-indexes to the index set has its costs. The time Oracle Text takes to create a CTXCAT index depends on its total size, and the total size of a CTXCAT index is directly related to the following factors:
Total text to be indexed
Number of sub-indexes in the index set
Number of columns in the base table that make up the sub-indexes
Having many component indexes in your index set also degrades DML performance, because more indexes must be updated.
Because of the added index time and disk space costs for creating a CTXCAT index, carefully consider the query performance benefit that each component index gives your application before adding it to your index set.
You can use I_ROWID_INDEX_CLAUSE of BASIC_STORAGE to speed up the creation of a CTXCAT index. This clause is described in Oracle Text Reference.