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Message Posted: Sat, 26 Apr 2003 @ 16:05:46 GMT


     
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Subj:   Join Index Space Usage Formula
 
From:   Ballinger, Carrie

Some one earlier in the week asked for a formula that could help assess how much perm space a join index will take. Here's one that a couple of people I know put together a few years back. I have not personally validated its accuracy, but I believe it will put you in the ballpark. It assumes the compressed format is being used.

Something else to keep in mind is the spool space required to build the join index. With multi-table JIs it can be up to double the size of the final JI structure.

Join Index Size = U * (F + O + (R * A)

F - Length of the fixed portion
R - Length of the single portion + 2 + number of presence bytes
A - Average number of repeated groups for a given fixed group
U - Number of unique occurrences of fixed portions (or number of physical rows)
O - Row overhead bytes (for the join index row)



     
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