Vacuum and index

Bonjour,


    Loading data (around 112000 record) to a newly created
    data-base, I am able to do select such as:

    select * from userreg where userid='cynthiaU';

    Where 'userid' is defined as:
    userid VARCHAR(30) UNIQUE,
    in the table.

    Everything work fine, as soon I run vacuum
    on this data-base some of the records "disappear",
    (say 10%) such the previous select is not responding, while

    select * from userreg where userid~*'cynthiaU';

    is still responding.
    Obviously there is an index problem, If drop and
    create the index related to the table, everything
    comeback fine....

    Questions:
    1) Why index become mis-aligned after a data-base
       vacuum?
    2) Is there a way to keep table and index aligned under
       vacuum?



A bientot
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