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Bitmap Indexes

Unique Bitmap Indexes Part II (You Can’t Do That)

As discussed in the previous post, a Bitmap index on a unique column will be larger than a corresponding Btree index due to the additional overheads associated with each index entry (such as the additional rowid, the additional column length bytes and the bitmap column itself). Oracle therefore attempts to protect you from explicitly creating such [...]

Unique Bitmap Indexes Part I (Unnatural Selection)

As I’ve discussed previously, a Bitmap index can be considered over a B-tree index (where concurrent DML is not an issue) even if there are potentially tens of millions of distinct values, in a table that has say hundreds of millions of rows.   However, if a column is unique or “approaches” uniqueness, then one [...]

Myth: Bitmap Indexes With High Distinct Columns (Supermassive Black Hole)

As discussed in my previous post, it’s a silly myth to suggest a bitmap index should only be used for so-called “low cardinality” columns else the resultant index would be “huge”. I thought it might be worth a quick refresh to see a simple little demonstration why such claims are a nonsense.  There is in fact no limit to [...]

Myth: Bitmap Indexes With High Distinct Columns (Blow Out)

I just couldn’t resist.   One of the great myths in Oracle is that bitmap indexes are only suitable and should only be used with columns that have so-called low cardinality (few distinct) values. A classic example of this myth being propagated is in this latest Burleson news item, “Oracle bitmap index maximum distinct values“, from Feb 16 [...]