12. A system including memory and one or more processors operable to execute instructions, stored in the memory, including instructions to: receive a sequence of one or more characters entered into a search field on a computing device, wherein the sequence of characters represents a first partial query; provide a first list of query completions for the first partial query for display on the computing device; receive one or more additional characters entered into the search field upon display of the first list, wherein the sequence of characters and the additional characters cumulatively represent a second partial query; obtain a second list of query completions for the second partial query; identify, for each of one or more query completions in the second list, an initial ranking score for the query completion for the second partial query; identify one or more query completions in the second list that appear in the first list; calculate demotion scores for the identified query completions, wherein a demotion score for a particular identified query completion is a function of both: the initial ranking score for the particular identified query completion, and a value that is based on a period of time between when the first list was displayed and when the additional characters were entered; use the demotion scores to demote one or more of the identified query completions to a lesser position within the second list, thereby forming a refined second list; and provide the refined second list of query completions for the second list of query completions for display on the computing device.
12. A system including memory and one or more processors operable to execute instructions, stored in the memory, including instructions to: receive a sequence of one or more characters entered into a search field on a computing device, wherein the sequence of characters represents a first partial query; provide a first list of query completions for the first partial query for display on the computing device; receive one or more additional characters entered into the search field upon display of the first list, wherein the sequence of characters and the additional characters cumulatively represent a second partial query; obtain a second list of query completions for the second partial query; identify, for each of one or more query completions in the second list, an initial ranking score for the query completion for the second partial query; identify one or more query completions in the second list that appear in the first list; calculate demotion scores for the identified query completions, wherein a demotion score for a particular identified query completion is a function of both: the initial ranking score for the particular identified query completion, and a value that is based on a period of time between when the first list was displayed and when the additional characters were entered; use the demotion scores to demote one or more of the identified query completions to a lesser position within the second list, thereby forming a refined second list; and provide the refined second list of query completions for the second list of query completions for display on the computing device. 16. The system of claim 12 , wherein the demotion score for the particular query completion is based on whether the period of time that the first list was displayed exceeds a minimum threshold time.