library cache lock waits

Hi,
On my test database, I noticed that when I tried to compile a package while I'm adding a constraint (alter table add constraint) on a huge table XYZ (3GB), the package would not compile and waits on library cache lock. The package I'm compiling does not referenced table XYZ. The package compiles instantaneously after the constraint was added. I don't understand why I'm waiting on library cache lock? Anyone? Let me know if you need additional information.

DB - 9.2.0.4 on HPUX
db_cache_size=8GB
shared_pool_size=500MB

thanks.

geraldine

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