Oracle EXP-00091, Exporting Questionable Statistics

When performing an Oracle export, you may receive error, "EXP-00091: Exporting questionable statistics."

The first thing to do is gather a little more information with "oerr" on the command line:
$ oerr exp 91
00091, 00000, "Exporting questionable statistics."
// *Cause: Export was able export statistics, but the statistics may not be
// usuable. The statistics are questionable because one or more of
// the following happened during export: a row error occurred, client
// character set or NCHARSET does not match with the server, a query
// clause was specified on export, only certain partitions or
// subpartitions were exported, or a fatal error occurred while
// processing a table.
// *Action: To export non-questionable statistics, change the client character
// set or NCHARSET to match the server, export with no query clause,
// export complete tables. If desired, import parameters can be
// supplied so that only non-questionable statistics will be imported,
// and all questionable statistics will be recalculated.

Then search Oracle's documentation:
The precalculated optimizer statistics are flagged as questionable at export
time if:
* There are row errors while exporting
* The client character set or NCHAR character set does not match the server character set or NCHAR character set
* A QUERY clause is specified
* Only certain partitions or subpartitions are exported

The NCHAR setting often causes this message, and the message will often be avoided by setting the NLS_LANG environment variable to match the database setting.

Questionable statistics are ususally a minor inconvenience, because after the import it is easy to re-analyze. Additionally, you may set "statistics=safe" on the import to skip importing questionable statistics.