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Oracle Myths

Index Rebuild, the Need vs the Implications Support Note 989093.1 (Getting Better)

Once upon a time, Oracle Support had a note called Script: Lists All Indexes that Benefit from a Rebuild (Doc ID 122008.1) which lets just say I didn’t view in a particularly positive light :) Mainly because it gave dubious advice which included that indexes should be rebuilt if: Deleted entries represent 20% or more of […]

So What Is A Good Cardinality Estimate For A Bitmap Index Column ? (Song 2)

As I’ve discussed previously, using a Bitmap index on a unique column makes little sense as the underling index must be larger than a corresponding B-tree index due to the implicit additional overheads associated with Bitmap indexes. As such, Oracle doesn’t permit the use of a Bitmap Index on a declared unique column or to [...]

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 [...]

How Does An Execution Plan Suddenly Change When The Statistics (And Everything Else) Remains The Same ? (In Limbo)

I’ve slipped this post in as there have been a number of discussions recently on how execution plans have changed while nothing else appears to have changed in the database. How can an execution plan suddenly change when no one has made any changes to the database ?   By no changes, it means that there [...]