Index rebuilding

We have an index that we need to keep rebuilding in order for the CBO to use it. What happens is that we build the index and queries use the index. Then, after a couple of days the CBO reverts to full table scans/. Re-analysing the table and index does not change anything but re-building the index does.
The table holds transient data which is only ever about five days old before it is "archived" off to another table. Wach row might get about 2/3 updates during it's short life.

Why might this be? We're using 8.1.7.4 on W2K if that helps!

Cheers

Steve

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