Update statistics - Questions for guru's ?

Problem description. We currently run update statistics during the night ( 11 PM - 7 AM ) on the tables that have the biggest data distribution change through the day. The update statistics is controlled from a unix script that deploys each update statistics command one by one. Each table has it's statistics updated in the following way... Update statistics medium for table x resolution 1 0.99 distributionsonly Update statistics high for table x (y) distributions only Update statistics low for table x (z) One thing that occurs consistently is applications that run during the night can get -750 errors. I have documented below two cases that occur... ######################################################################## ######### CASE ONE. Code segment.. SET ISOLATION TO DIRTY READ SET LOCK MODE TO WAIT 120 SELECT MAX(activ_gsm_id) INTO v_activ_gsm_id FROM activ_gsm WHERE activ_gsm.actvn_id = work_rec.actvn_id Update statistics command running.. Update statistics medium for table activ_gsm resolution 1 0.99distributions only. Error statement report from application.. p_dirlist.4gl line 1033 3:08:39 SQL statement error number -750 Invalid distribution format found for actvn_id. General logfile.. 3:08:32 Update statistics statement started 3:08:39 -750 error reported by application. error message "Invalid distribution format found for actvn_id." ######################################################################## ############## CASE TWO. Code segment.. SET ISOLATION TO DIRTY READ SET LOCK MODE TO WAIT 120 DECLARE frn_cur CURSOR FOR SELECT after_disc_amt, cp_type, prod_id, eff_start_date, eff_end_date, tran_type_id FROM invoice_item_fnd WHERE cust_no = l_cust_no AND after_disc_amt <> 0 AND transit_date = end_date AND tran_type_id in (1,31,30) AND ((cp_type <> 6 AND transit_date >= mdy(1,27,1994)) OR transit_date < mdy(1,27,1994)) ORDER BY tran_type_id Update statistics command running.. Update statistics high for table invoice_item_fnd (transit_date)distributions only. Error statement report from application.. p_bill_min.4gl line 562 6:18:15 SQL statement error number -750 Invalid distribution format found for transit_date. General logfile.. 6:18:15 Update statistics statement completed 6:18:15 -750 error reported by application. error message "Invalid distribution format found for transit_date." ######################################################################## ############ My question are as follows...... 1) Why can I not run multiple update statistics statements at the same time ? 2) Why would the two cases documented above be causing problems ? 3) We run update statistics for all stored procedures at the completion of any update statistics scripts, Should we be running update statistics for our procedures more often ? Should we have the stored procedure update it's own statistics ? ######################################################################## ############# SITE INFORMATION. HP-UX 10.20 INFORMIX 7.22.UC1X2 INFORMIX 6.05.UC1 tools DB SIZE 50GB ######################################################################## ############# I look forward to any suggestions you can provide.... Cheers, Tony. #include <standard disclaimers>