If statistics are requested at export time and analyzer statistics are available for a table, then Export will include the
ANALYZE statement used to recalculate the statistics for the table into the dump file. In most circumstances, Export will also write the precalculated optimizer statistics for tables, indexes, and columns to the dump file. See the description of the Import parameter "STATISTICS".
Because of the time it takes to perform an
ANALYZE statement, it is usually preferable for Import to use the precalculated optimizer statistics for a table (and its indexes and columns) rather than execute the
ANALYZE statement saved by Export. By default, Import will always use the precalculated statistics that are found in the export dump file.
The Export utility flags certain precalculated statistics as questionable. The importer might want to import only unquestionable statistics, not precalculated statistics, in the following situations:
Character set translations between the dump file and the import client and the import database could potentially change collating sequences that are implicit in the precalculated statistics.
Row errors occurred while importing the table.
A partition level import is performed (column statistics will no longer be accurate).
ROWS=n will not prevent the use of precalculated statistics. This feature allows plan generation for queries to be tuned in a nonproduction database using statistics from a production database. In these cases, the import should specify
In certain situations, the importer might want to always use
ANALYZE statements rather than precalculated statistics. For example, the statistics gathered from a fragmented database may not be relevant when the data is imported in a compressed form. In these cases, the importer should specify
STATISTICS=RECALCULATE to force the recalculation of statistics.
If you do not want any statistics to be established by Import, then you should specify