On 2002.05.25 17:38 Rachel Carmichael wrote: > low cardinality -- good candidate for bitmap. little or no updates to > the column being indexed -- good candidate for bitmap > > "low" cardinality is relative -- 10,000 distinct values in a 300m row > table is low, 100 distinct values in a 10K row table might be high > > but the main criteria I have found is whether or not the column is > updated because when you update the values in a bitmap index, it's not > just the one row in the index that is updated but the entire segment > of > the bitmap. And THAT can lock the index on you. > > Rachel > Well, it doesn't exactly lock the whole index, it locks only the rows that have the same bitmap value as the row that is being updated. For a low cardinality column, that may mean locking down a significant portion of the index (and the underlying table, of course). That makes bitmap indexes (and indices) completely unsuitable for the OLTP environment. -- Mladen Gogala -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Mladen Gogala INET: [EMAIL PROTECTED]
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