See the below, the VPX HIST table are tables for performance data, this clears all performance data from the database, which will then build backup as you use vcenter, if you do not want to clear all performance data (when you click on the performance tab in vcenter for CPU, memory etc) see below.
The stored procedure for VPX.EVENTS AND TASK retention will clear only information for tasks and events, see below if you wish to keep some performance data, I chose not to as this will build up over time anyway.
Warning: This procedure erases all historical data. If you want to retain some historical performance data instead of deleting all of it, see Purging old data from the database used by vCenter Server (1025914) or Purging old data from the database used by VirtualCenter 2.x (1000125).