Parallel Queries on a Local Context Index

Parallel query refers to the parallelized processing of a local CONTEXT index. Based on the parallel degree of the index and various system attributes, Oracle determines the number of parallel query slaves to be spawned to process the index. Each parallel query slave processes one or more index partitions. This is the default query behavior for local indexes created in parallel.

However, for heavily loaded systems with high numbers of concurrent users, query throughput will generally be worse with parallel query because top-N hits can usually be satisfied by the first few partitions, if the query is run serially. For example, typical top-N text queries with an ORDER BY partition key column, such as:

select * from (
        select story_id from stories_tab where contains(...)>0 order by 
publication_date desc)
    where rownum <= 10;

will generally perform worse with a parallel query.

You can disable parallel querying after a parallel index operation with an ALTER INDEX statement as follows:

Alter index <text index name> NOPARALLEL;
Alter index <text index name> PARALLEL 1;

You can also enable or increase the parallel degree by specifying:

Alter index <text index name> parallel < parallel degree >;