query optimizator

Something strange happened to me (is not the first time) with Informix OnLine 7.2 on UnixWare 2.1.1 with dual Pentium 133. A query that we use frequently, suddendly stopped working. The query optimizer decided to change something, and the query now do not use anymore the right indexes. The query is like this: select d.x, d.y, sum(a.x) from a,b,c,d where <conditions> group by 1,2 order by 1,2;The tables a,c have been growing a lot each day adding new records. We perform weekly update statistics medium on all the fields of all the tables and high on ALL the fields involved in the conditions of tables a,c. This was OK for one year 'till yesterday. The query ought to start from table c to select the records and then apply the other conditions. Yesterday this changed: the query starts from table a with sequential scan. I must duplicate the conditions on table a to let the query use some index for this table. I was not able to make the query start again from table c. Is it possible that table c has grown so much that the optimizer finds more convenient starting from another table? (but I select only a few rows from table c). I also tried to check the indexes for table c and to drop and recreate them... Could it be a problem of too much levels in the index of table c? I think this is only a problem of the query optimizer. Is this true?? Apart from this specific case, Is it acceptable an optimizer that works this way? Is this a frequent case? I'm administring Informix since two years, and the optimization of the queries is the thing I found most difficult: there is nothing certain about the update statistics and the optimizer, even if you follow the directions of the manuals. If it's not so (and I hope this) please give me some suggest before my chief decides to change to Oracle..... Marco Traini CONSORZIO IDRICO PICENO alsiap@topnet.it 63100 ASCOLI PICENO ITALY