>>If an idx is faster than a cdx, and the index expression is the same, you must have some kind of data corruption. I suggest that you find the cause of this error instead of trying to fix the consequences of the error.
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>How could that be possible ?
>I have rebuilt the indexes (cdx) Just before running the tests
>Is it possible ?????

In that case, no. Maybe you simply should add more RAM? Since a CDX is bigger than an IDX, too little RAM can explain the difference.