Sybase 12.4.2 - Manual (Page 190)

Estimating the size of a join index
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These calculations should give you an idea of how much disk space you need
for the join index. The results include the segment size in bytes, and the number
of blocks. The procedure also tells you how long it will take to create the join
index.
If you want to know the actual size of an existing join index, you use a different
stored procedure,
sp_iqjoinindexsize .
See the Adaptive Server IQ Reference Manual for syntax details of all stored
procedures.