Any precautions before importing old statistic?

Question

Hello All,
On my Oracle 9i database I exported statistics of a schema in a table using following commands:
exec dbms_stats.create_stat_table(ownname => 'SCHEMA', stattab => 'STAT_TABLE');
exec dbms_stats.export_schema_stats(ownname => 'SCHEMA', stattab => 'STAT_TABLE');I need to import them back for the schema. I will use following steps:
exec dbms_stats.import_schema_stats(ownname => 'SCHEMA', stattab => 'STAT_TABLE');Do I need to delete existing STATs before I import old one? or I shuold above import command directly?
Thanks,

Answer 1

The precautions would really depend on when & why you exported the statistics. In general, I would probably delete the existing statistics first.
Be aware that when you import the statistics, Oracle is going to invalidate all your cached SQL plans, so the database is going to start hard parsing like crazy. Query plans should change back to the plans that were present with the old statistics, assuming the data was the same. If you have monotonically increasing columns with histograms (i.e. a create_date column with a histogram), old statistics with new data may generate new, bad query plans because queries would be passing in bind values greater than the histogram's top value. If there are new objects, new columns, or new histograms since the statistics were exported, those objects/ columns/ histograms would now be missing and would need to be collected again.
Justin