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I have a large table (6m records) containing data licensed from a vendor. The table contains an NVARCHAR2(1) column with Y/N values. I have created a view to filter out records with a value of ‘N’, and this view will be queried extensively. What is the best way to index the NVARCHAR2(1) column?
What is the Y/N ratio ?
Are records updated so they go from Y/N and/or N/Y ?
If so, does this happen regularly (eg in-stock/out-of-stock/in-stock/out-of-stock) or as a one off (unprocessed/processed) ?
If the entries are mixed around, and you’ve got an even-ish ratio then an index is unlikely to help.
You could use partitioning (if you have the licence) to split Y from N. Materialized views or making the table a UNION ALL view over two tables (maybe with INSTEAD OF triggers) could split Y from N too. All these will incur at a penalty to update processing.
A bitmap index may be appropriate if the column doesn’t get much update activity.
You could have a function based index on CASE WHEN flag = ‘Y’ then ‘Y’ end. That would exclude N values from the index making it a lot smaller.