Slow procedure execution after re-optimised

We are running Online 7.23 on Dynix/ptx 4.1.3 and I have identified a peculiar problem. We make heavy use of Stored Procedures and have a performance problem with one in particular. The procedure queries a couple of 2million row tables and takes about 2.5 minutes. If I run an update statistics on the two tables the query comes down to 5 seconds. This might seem obvious at first. However if I then update statistics for the procedure the time goes back to 2.5minutes. This can be reproduced each time. The table statistics are obviously up to date but simply re-optimising the procedure slows it down again! In addition the fast runtime reduces to sub-second if it is repeated immeadiately - I assume because the data is now in buffers. The slow runtime can be repeated endlessly and will not perform any quicker, as if the data is either not in the buffers or online doesn't realise it is. These tests were performed on a 'quite' machine/instance with 62,500 buffers and included some tests after online re-starts to flush the buffers.OPTCOMPIND is 0 but other values don't seem to make any difference. Any ideas? Thanks in advance, Paul Tonkin DBA. -- ------------------------------------------ The views expressed here are mine and not those of my employer, Britannic Assurance ------------------------------------------