You can find several tables in OpenX database that could be very large in comparation to other tables.

The database could keep growing constantly.

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These are the four largest tables in OpenX database:
 
ox_data_intermediate_ad
 
ox_data_raw_ad_impression
 
ox_data_summary_ad_hourly
 
ox_data_summary_zone_impression_history
 
You can delete records from the ox_data_summary_ad_hourly and ox_data_summary_zone_impression_history but in that case  all statistics data of the campaings will be lost.
 
Tables ox_data_raw_ad_impression and ox_data_intermediate_ad are temporary tables that keep records for the five days. After five days records will be transfered to ox_data_summary_ad_hourly table, so before you delete records from this table check whether data were transfered.

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