Question about cost estimated by Informix engine

Hi, All! I didn't receive any comments or suggestions about subj NVARCHAR posted by me sometime ago. Sorry... But OK, life continues. Next question. I have ODS for NT and simple query: SELECT a.id, a.code, a.birth, a.first, a.last, a.zip, b.name, c.name, d.name, e.name FROM person a, state b, family c, status d, sex e WHERE a.state = b.id AND a.family = c.id AND a.status = d.id AND a.sex = e.id AND a.id > 0 ORDER BY a.last; Table PERSON has 120000 rows and index on column LAST, table STATE has 250 rows, FAMILY - 10, STATUS - 10, SEX - 2 (of course). It is unpossible to get a result, after half an hour I geted error: "no free disk space for sort". What I saw in explain file: Estimated Cost: 123986 - Wow! Estimated # of Rows Returned: 80513 - ? Temporary Files Required For: Order By - What? WHY?! 1) e: SEQUENTIAL SCAN 2) c: SEQUENTIAL SCAN NESTED LOOP JOIN 3) a: INDEX PATH Filters: ( a.sex = e.id AND a.id > 0 ) (1) Index Keys: family Lower Index Filter: a.family = c.id NESTED LOOP JOIN 4) d: INDEX PATH (1) Index Keys: id Lower Index Filter: d.id = a.status NESTED LOOP JOIN 5) b: INDEX PATH (1) Index Keys: id Lower Index Filter: b.id = a.state NESTED LOOP JOIN Is there anybody who can explain me why server chose this execution plan? It must read table PERSON according to index LAST and then use nested loop join for other small tables, I think. Explain file must be approximately like this: Estimated Cost: Sorry, I don't know formula Estimated # of Rows Returned: Really there is only one row with id = -1 in the table PERSON 1) a: INDEX PATH Filters: a.id > 0 (1) Index Keys: last 2) b: INDEX PATH (1) Index Keys: id Lower Index Filter: b.id = a.state NESTED LOOP JOIN 3) c: INDEX PATH (1) Index Keys: id Lower Index Filter: c.id = a.family NESTED LOOP JOIN 4) d: INDEX PATH (1) Index Keys: id Lower Index Filter: d.id = a.status NESTED LOOP JOIN 5) e: INDEX PATH (1) Index Keys: id Lower Index Filter: e.id = a.sex NESTED LOOP JOIN With this plan time will be used only for reading and transfering data, first rows user will see momentary. Anybody knows where I can see exact algorithm how Informix calculates estimated cost? May be there is serious error in their approach. I can search (may be find) it? Best wishes. Leonid. ( Leonids.Voroncovs@dati.lv )