Talk:Bitmap index

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Something to note?

Should we note that bitmap indexes are good for equality operations in WHERE clauses, but not so good for less than or greater than operations? - Ta bu shi da yu 15:45, 10 November 2007 (UTC)[reply]

Huh, in what way aren't they "good" for < or >? -- intgr [talk] 23:20, 10 November 2007 (UTC)[reply]

"Bitmap indexes are also not suitable for columns that are primarily queried with less than or greater than comparisons. For example, a salary column that usually appears in WHERE clauses in a comparison to a certain value is better served with a B-tree index. Bitmapped indexes are only useful with equality queries, especially in combination with AND, OR, and NOT operators." Oracle Concepts document. Reread what they do and I'm sure you'll work out why this is the case. - Ta bu shi da yu 08:42, 11 November 2007 (UTC)[reply]

Bitmap indexes can be generated for any logical operation; e.g. you can have a bitmap index for the expression 'salary > 10000' and every time you query WHERE salary > 10000, this bitmap will be useful. I have no idea why Oracle's documentation singles out equality operations as the only use case for bitmap indexes — it isn't. -- intgr [talk] 21:12, 11 November 2007 (UTC)[reply]

I suppose that it's to do with a higher cardinality. My understanding of bitmap indexes is that the higher the cardinality (or percentage of unique elements) then the less efficient in storing the data in the index. Could be wrong. - Ta bu shi da yu 02:05, 12 November 2007 (UTC)[reply]