union all 的效率问题

select Substation.Substation_ID Substation_ID, Substation.NAME SubstationNAME, PowerTransformer.PowerTransformer_ID Equipment_ID, PowerTransformer.NAME EquipmentNAME, Substationuriandid.classid Substationuriandidclassid, Substationuriandid.uri Substationuriandiduri, Equipmenturiandid.classid Equipmenturiandidclassid, Equipmenturiandid.uri Equipmenturiandiduri from Substation join PowerTransformer on  exists (select 1 from CONTAINS_EQUI_MEMBEROF_EQUI where CONTAINS_EQUI_MEMBEROF_EQUI.MEMBEROF_EQUIPMENTCONTAINER_ID = Substation.Substation_ID and CONTAINS_EQUI_MEMBEROF_EQUI.CONTAINS_EQUIPMENTS_ID = PowerTransformer.PowerTransformer_ID) join uri_resourceid Substationuriandid on Substationuriandid.resource_id = Substation.Substation_ID join uri_resourceid Equipmenturiandid on Equipmenturiandid.resource_id = PowerTransformer.PowerTransformer_ID  
union all 
select Substation.Substation_ID Substation_ID, Substation.NAME SubstationNAME, Fuse.Fuse_ID Equipment_ID, Fuse.NAME EquipmentNAME, Substationuriandid.classid Substationuriandidclassid, Substationuriandid.uri Substationuriandiduri, Equipmenturiandid.classid Equipmenturiandidclassid, Equipmenturiandid.uri Equipmenturiandiduri from Substation join Fuse on  exists (select 1 from CONTAINS_EQUI_MEMBEROF_EQUI where CONTAINS_EQUI_MEMBEROF_EQUI.MEMBEROF_EQUIPMENTCONTAINER_ID = Substation.Substation_ID and CONTAINS_EQUI_MEMBEROF_EQUI.CONTAINS_EQUIPMENTS_ID = Fuse.Fuse_ID) join uri_resourceid Substationuriandid on Substationuriandid.resource_id = Substation.Substation_ID join uri_resourceid Equipmenturiandid on Equipmenturiandid.resource_id = Fuse.Fuse_ID union all ......union all ...
之后都是类似的查询语句.开始执行都很快，在1秒之内，到第13个union all的时候执行速度变成5秒，14个是20秒，再往后就等不到了，怎么解决啊?