Bitmap indexes and cardinality

In my last two posts I talked a little bit about bitmap indexes, how they work and why they lock. I also mentioned that they should be used for low cardinality but that there is no cut-and-dried answer about what low cardinality is. I’m not starting here to talk about cardinality on bitmap indexes, actually Richard Foote did a much better job on that so if you interested in that check out the following blog posts from him:

• Myth: Bitmap Indexes with high distinct columns (Blow Out)
• Myth: Bitmap Indexes with high distinct columns (Supermassive Black Hole)
• So what is a good cardinality estimate for a bitmap index column?

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