Explain this

Sometimes the Informix "optimizer" baffles me. The following is the sqexplain file created upon running 3 slightly different queries which all return the same single row. The first query takes almost 2 minutes to return, while the other 2 return almost instantaneously. The optimizer has chosen the wrong key, but why? There is only 1 character difference between the first and second query and in the third query I just dropped the order by. Environment: INFORMIX-OnLine Version 5.10.UC1 on SCO UNIX 5.0.4 QUERY: ------ select fname,findex from folder where fmatter='22188.04000' andfname like '0001%' order by fname Estimated Cost: 1 Estimated # of Rows Returned: 1 1) informix.folder: INDEX PATH Filters: informix.folder.fmatter = '22188.04000' (1) Index Keys: fname findex Lower Index Filter: informix.folder.fname LIKE '0001%' QUERY: ------ select fname,findex from folder where fmatter='22188.04000' andfname like '000%' order by fname Estimated Cost: 3 Estimated # of Rows Returned: 1 Temporary Files Required For: Order By 1) informix.folder: INDEX PATH (1) Index Keys: fmatter fname findex (Key-Only) Lower Index Filter: (informix.folder.fmatter = '22188.04000' AND informix.folder.fname LIKE '000%' ) QUERY: ------ select fname,findex from folder where fmatter='22188.04000' andfname like '0001%' Estimated Cost: 1 Estimated # of Rows Returned: 1 1) informix.folder: INDEX PATH (1) Index Keys: fmatter fname findex (Key-Only) Lower Index Filter: (informix.folder.fmatter = '22188.04000' AND informix.folder.fname LIKE '0001%' ) <===========================> Larry Foote Footeware Software Systems