SQL Query Optmizer

The following two queries QUERY 1 and QUERY 2 are the output of SET EXPLAIN ON. I do not understand why the query optimizer does a sequential scan on root.alujobsh in QUERY 2. The only difference in the two queries are these lines: QUERY 1: atkliquh.inv_num = '12316' and QUERY 2: atkliquh.inv_num >= '12316' and atkliquh.inv_num <= '12316' and QUERY 1 take 5 seconds to run while QUERY 2 may take 5 minutes to run. Why does adding the additional and cluase onto QUERY 2 cause the optmizer to do a sequential scan and not use the indexes being used in QUERY 1? QUERY 1: ------ select count(*) from ainvpost, strcustr, atkliquh, alujobsh, atkliqud whereatkliquh.inv_num = '12316' and atkliquh.inv_num = ainvpost.inv_num and ainvpost.c_num = strcustr.cust_code and atkliquh.job_num = alujobsh.job_num and atkliquh.tk_num = atkliqud.tk_num Estimated Cost: 29137 Estimated # of Rows Returned: 1 1) root.atkliquh: INDEX PATH (1) Index Keys: inv_num Lower Index Filter: root.atkliquh.inv_num = '12316' 2) root.ainvpost: INDEX PATH (1) Index Keys: inv_num Lower Index Filter: root.ainvpost.inv_num = root.atkliquh.inv_num 3) root.strcustr: INDEX PATH (1) Index Keys: cust_code Lower Index Filter: root.strcustr.cust_code = root.ainvpost.c_num 4) root.atkliqud: INDEX PATH (1) Index Keys: tk_num Lower Index Filter: root.atkliqud.tk_num = root.atkliquh.tk_num 5) root.alujobsh: INDEX PATH (1) Index Keys: job_num Lower Index Filter: root.alujobsh.job_num = root.atkliquh.job_num QUERY2 : ------ select count(*) from ainvpost, strcustr, atkliquh, alujobsh, atkliqud whereatkliquh.inv_num >= '12316' and atkliquh.inv_num <= '12316' and atkliquh.inv_num = ainvpost.inv_num and ainvpost.c_num = strcustr.cust_code and atkliquh.job_num = alujobsh.job_num and atkliquh.tk_num = atkliqud.tk_num Estimated Cost: 98119 Estimated # of Rows Returned: 1 1) root.alujobsh: SEQUENTIAL SCAN 2) root.atkliquh: INDEX PATH Filters: (root.atkliquh.inv_num >= '12316' AND root.atkliquh.inv_num <= '12316' ) (1) Index Keys: job_num Lower Index Filter: root.atkliquh.job_num = root.alujobsh.job_num 3) root.ainvpost: INDEX PATH (1) Index Keys: inv_num Lower Index Filter: root.ainvpost.inv_num = root.atkliquh.inv_num 4) root.strcustr: INDEX PATH (1) Index Keys: cust_code Lower Index Filter: root.strcustr.cust_code = root.ainvpost.c_num 5) root.atkliqud: INDEX PATH (1) Index Keys: tk_num Lower Index Filter: root.atkliqud.tk_num = root.atkliquh.tk_num -- Unix at Home perk machine Joseph A. Miele jam@perk.jpr.com Dir MIS Spectraserv, Inc. 76207.1365@compuserve.com