If statistics are requested at export time and analyzer statistics are available for a table, Export will place the ANALYZE command to recalculate the statistics for the table into the dump file. In certain circumstances, Export will also write the precalculated optimizer statistics for tables, indexes, and columns to the dump file.
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 executing the ANALYZE statement saved by Export. However, in the following cases, Import will ignore the precomputed statistics because they are potentially unreliable:
Note: Specifying ROWS=N will not prevent the use of precomputed statistics. This feature allows plan generation for queries to be tuned in a nonproduction database using statistics from a production database.
In certain situations, the importer might want to always use ANALYZE statements rather than precomputed 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 may specifyRECALCULATE_STATISTICS=Y to force the recalculation of statistics.
If you do not want any statistics to be established by Import, you can specify ANALYZE=N, in which case, the RECALCULATE_STATISTICS parameter is ignored.
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