Oracle 11G - Performance effect of indexing at insert

Question

Objective

Verify if it is true that insert records without PK/index plus create thme later is faster than insert with PK/Index.

Note

Answer 1

The current test case is probably good enough for you to overrule the "best practices". There are too many variables involved to make a blanket statement that "it's always best to leave the indexes enabled". But you're probably close enough to say it's true for your environment.

Below are some considerations for the test case. I've made this a community wiki in the hopes that others will add to the list.

1. Direct-path inserts. Direct-path writes use different mechanisms and may work completely differently. Direct-path inserts can often be significantly faster than regular inserts, although they have some complicated restrictions (for example, triggers must be disabled) and disadvantages (the data is not immediately backed-up). One particular way it affects this scenario is that NOLOGGING for indexes only applies during index creation. So even if a direct-path insert is used, an enabled index will always generate REDO and UNDO.
2. Parallelism. Large insert statements often benefit from parallel DML. Usually it's not worth worrying about the performance of bulk loads until it takes more than several seconds, which is when parallelism starts to be useful.
3. Bitmap indexes are not meant for large DML. Inserts or updates to a table with a bitmap index can lock the whole table and lead to disastrous performance. It might be helpful to limit the test case to b-tree indexes.
4. Add alter system switch logfile;? Log file switches can sometimes cause performance issues. The tests would be somewhat more consistent if they all started with empty logfiles.
5. Move data generation logic into a separate step. Hierarchical queries are useful for generating data but they can have their own performance issues. It might be better to create in intermediate table to hold the results, and then only test inserting the intermediate table into the final table.