[sql] Indexing nulls for fast searching on DB2



0 Answers

From where did you get the impression that DB2 doesn't index NULLs? I can't find anything in documentation or articles supporting the claim. And I just performed a query in a large table using a IS NULL restriction involving an indexed column containing a small fraction of NULLs; in this case, DB2 certainly used the index (verified by an EXPLAIN, and by observing that the database responded instantly instead of spending time to perform a table scan).

So: I claim that DB2 has no problem with NULLs in non-primary key indexes.

But as others have written: Your data may be composed in a way where DB2 thinks that using an index will not be quicker. Or the database's statistics aren't up-to-date for the involved table(s).

Question

It's my understanding that nulls are not indexable in DB2, so assuming we have a huge table (Sales) with a date column (sold_on) which is normally a date, but is occasionally (10% of the time) null.

Furthermore, let's assume that it's a legacy application that we can't change, so those nulls are staying there and mean something (let's say sales that were returned).

We can make the following query fast by putting an index on the sold_on and total columns

Select * from Sales 
where 
Sales.sold_on between date1 and date2
and Sales.total = 9.99

But an index won't make this query any faster:

Select * from Sales 
where 
Sales.sold_on is null
and Sales.total = 9.99

Because the indexing is done on the value.

Can I index nulls? Maybe by changing the index type? Indexing the indicator column?






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